QUANTUM FRONTIERS

The autumn of my sophomore year of college was mildly hellish. I took the equivalent of three semester-long computer-science and physics courses, atop other classwork; co-led a public-speaking self-help group; and coordinated a celebrity visit to campus. I lived at my desk and in office hours, always declining my flatmates’ invitations to watch The West Wing. PEEKING INTO THE WORLD OF QUANTUM INTELLIGENCE Intelligent beings have the ability to receive, process, store information, and based on the processed information, predict what would happen in the future and act accordingly. An illustration of receiving, processing, and storing information. Based on the processed information, one can make prediction about the future. We, as intelligent beings, receive, process, and

QUANTUM FRONTIERS

Imagine a line of ions trapped by lasers. Each ion contains the physical manifestation of a qubit—a quantum two-level system, the basic unit of quantum information. You can think of a qubit as having a quantum analogue of angular momentum, called spin. The spin has three components, one per direction of space. LEARNING ABOUT LEARNING The autumn of my sophomore year of college was mildly hellish. I took the equivalent of three semester-long computer-science and physics courses, atop other classwork; co-led a public-speaking self-help group; and coordinated a celebrity visit to campus. I lived at my desk and in office hours, always declining my flatmates’ invitations to

watch The West

LEARNING THEORY

Rigorously, one refers to a classical/quantum being as a classical/quantum model, algorithm, protocol, or procedure. This is because the actions of these classical/quantum beings are the center of the mathematical analysis. Formally, we consider the task of learning an unknown physical evolution described by a CPTP map that

takes in -qubit

THE GRAND TOUR OF QUANTUM THERMODYNAMICS The Grand Tour of quantum thermodynamics. Young noblemen used to undertake a “Grand Tour” during the 1600s and 1700s. Many of the tourists hailed from England, though well-to-do compatriots traveled from Scandinavia, Germany, and the United States. The men had just graduated from university—in many cases, Oxford or Cambridge.

THE SIGN PROBLEM(S)

The sign problem (s) The thirteen-month-old had mastered the word “dada” by the time I met her. Her parents were teaching her to communicate other concepts through sign language. Picture her, dark-haired and bibbed, in a high chair. Banana and mango slices litter the tray in front of her. More fruit litters the floor in front

of the tray.

QUANTUM INFORMATION IN QUANTUM COGNITION Some research topics, says conventional wisdom, a physics PhD student shouldn’t touch with an iron-tipped medieval lance: sinkholes in the foundations of quantum theory. Problems so hard, you’d have a snowball’s chance of achieving progress. Problems so obscure, you’d have a snowball’s chance of convincing anyone to care about

progress.

HSIN-YUAN HUANG (ROBERT) About Hsin-Yuan Huang (Robert) I am a third-year Caltech Ph.D. student (advised by John Preskill and Thomas Vidick). I think about fundamental questions to understand how quantum machines can improve our capability to learn about the world around us. TOP 10 QUESTIONS FOR YOUR POTENTIAL PHD ADVISER/GROUP Everyone in grad school has taken on the task of picking the perfect research group at some point. Then some among us had the dubious distinction of choosing the perfect research group twice. Luckily for me, a year of grad research taught me a lot and

QUANTUM FRONTIERS

The autumn of my sophomore year of college was mildly hellish. I took the equivalent of three semester-long computer-science and physics courses, atop other classwork; co-led a public-speaking self-help group; and coordinated a celebrity visit to campus. I lived at my desk and in office hours, always declining my flatmates’ invitations to watch The West Wing. PEEKING INTO THE WORLD OF QUANTUM INTELLIGENCE Intelligent beings have the ability to receive, process, store information, and based on the processed information, predict what would happen in the future and act accordingly. An illustration of receiving, processing, and storing information. Based on the processed information, one can make prediction about the future. We, as intelligent beings, receive, process, and

QUANTUM FRONTIERS

Imagine a line of ions trapped by lasers. Each ion contains the physical manifestation of a qubit—a quantum two-level system, the basic unit of quantum information. You can think of a qubit as having a quantum analogue of angular momentum, called spin. The spin has three components, one per direction of space. LEARNING ABOUT LEARNING The autumn of my sophomore year of college was mildly hellish. I took the equivalent of three semester-long computer-science and physics courses, atop other classwork; co-led a public-speaking self-help group; and coordinated a celebrity visit to campus. I lived at my desk and in office hours, always declining my flatmates’ invitations to

watch The West

LEARNING THEORY

Rigorously, one refers to a classical/quantum being as a classical/quantum model, algorithm, protocol, or procedure. This is because the actions of these classical/quantum beings are the center of the mathematical analysis. Formally, we consider the task of learning an unknown physical evolution described by a CPTP map that

takes in -qubit

THE GRAND TOUR OF QUANTUM THERMODYNAMICS The Grand Tour of quantum thermodynamics. Young noblemen used to undertake a “Grand Tour” during the 1600s and 1700s. Many of the tourists hailed from England, though well-to-do compatriots traveled from Scandinavia, Germany, and the United States. The men had just graduated from university—in many cases, Oxford or Cambridge.

THE SIGN PROBLEM(S)

The sign problem (s) The thirteen-month-old had mastered the word “dada” by the time I met her. Her parents were teaching her to communicate other concepts through sign language. Picture her, dark-haired and bibbed, in a high chair. Banana and mango slices litter the tray in front of her. More fruit litters the floor in front

of the tray.

QUANTUM INFORMATION IN QUANTUM COGNITION Some research topics, says conventional wisdom, a physics PhD student shouldn’t touch with an iron-tipped medieval lance: sinkholes in the foundations of quantum theory. Problems so hard, you’d have a snowball’s chance of achieving progress. Problems so obscure, you’d have a snowball’s chance of convincing anyone to care about

progress.

HSIN-YUAN HUANG (ROBERT) About Hsin-Yuan Huang (Robert) I am a third-year Caltech Ph.D. student (advised by John Preskill and Thomas Vidick). I think about fundamental questions to understand how quantum machines can improve our capability to learn about the world around us. TOP 10 QUESTIONS FOR YOUR POTENTIAL PHD ADVISER/GROUP Everyone in grad school has taken on the task of picking the perfect research group at some point. Then some among us had the dubious distinction of choosing the perfect research group twice. Luckily for me, a year of grad research taught me a lot and

QUANTUM FRONTIERS

Imagine a line of ions trapped by lasers. Each ion contains the physical manifestation of a qubit—a quantum two-level system, the basic unit of quantum information. You can think of a qubit as having a quantum analogue of angular momentum, called spin. The spin has three components, one per direction of space.

PROJECT ANT-MAN

Project Ant-Man. The craziest challenge I’ve undertaken hasn’t been skydiving; sailing the Amazon on a homemade raft; scaling Mt. Everest; or digging for artifacts atop a hill in a Middle Eastern desert, near midday, during high summer. 1 The craziest challenge has been to study the possibility that quantum phenomena affect cognition LIFE AMONG THE EXPERIMENTALISTS Life among the experimentalists. Posted on March 21, 2021 by Nicole Yunger Halpern. I used to catch lizards—brown anoles, as I learned to call them later—as a child. They were colored as their name suggests, were about as long as one of my hands, and resented my attention. But they frequented our back porch, and I had a butterfly

net.

PRESKILL | QUANTUM FRONTIERS John Preskill – There are two main classes of attempts. One is just to come up with a cryptographic protocol not so different conceptually from what’s done now, but based on a problem that’s hard for quantum computers. Craig Cannon – There you go. ONE IF BY LAND MINUS TWO IF BY SEA, OVER THE SQUARE-ROOT While laying plans, Revere instructs Newman: He said to his friend, “If the British march By land or sea from the town to-night, Hang a lantern aloft in the belfry-arch Of the North-Church-tower, as a signal light. Then comes one of the poem’s most famous lines: “One if by land, and two if by sea.”. The British could have left Boston

by

SHAUNMAGUIRE

The Science that made Stephen Hawking famous. Posted on November 5, 2014 by shaunmaguire. 8. In anticipation of The Theory of Everything which comes out today, and in the spirit of continuing with Quantum Frontiers’ current movie theme, I wanted to provide an overview of Stephen Hawking’s pathbreaking research. THE 10 BIGGEST BREAKTHROUGHS IN PHYSICS OVER THE PAST 25 The biggest breakthroughs of the past 25 years: *Neutrino Mass: surprisingly, neutrinos have a nonzero mass, which provides a window into particle physics beyond the standard model. THE STANDARD MODEL has been getting a lot of attention recently. This is well deserved in my opinion, considering that the vast majority of its predictions have come true, most of which were made by the

FERNANDO PASTAWSKI

Roughly speaking, tensor networks are ingenious ways of encoding (quantum) inputs into (quantum) outputs. In particular, if you enter some input at the boundary of your tensor network, the tensors do the work of processing that information throughout the network so that if you ask for an output at any one of the nodes in the bulk of the tensor network, you get the right encoded answer. THIS VIDEO OF SCIENTISTS SPLITTING AN ELECTRON WILL SHOCK by Jorge Cham. Ok, this is where things get weird. If quantum computers, femtometer motions or laser alligators weren't enough, let's throw in fractionalized electrons, topological surfaces and strings that go to the end of time. To be honest, the idea that an electron can't be split hadn't even occurred to me before my conversation with Gil and WE ARE ALL WILSONIANS NOW We are all Wilsonians now. Ken Wilson passed away on June 15 at age 77. He changed how we think about physics. Renormalization theory, first formulated systematically by Freeman Dyson in 1949, cured the flaws of quantum electrodynamics and turned it into a precise computational tool. But the subject seemed magical and mysterious.

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

PEEKING INTO THE WORLD OF QUANTUM INTELLIGENCE Peeking into the world of quantum intelligence. Intelligent beings have the ability to receive, process, store information, and based on the processed information, predict what would happen in the future and act accordingly. An illustration of receiving, processing, and storing information. Based on the processed information, one can make LEARNING ABOUT LEARNING The autumn of my sophomore year of college was mildly hellish. I took the equivalent of three semester-long computer-science and physics courses, atop other classwork; co-led a public-speaking self-help group; and coordinated a celebrity visit to campus. I lived at my desk and in office hours, always declining my flatmates’ invitations to

watch The West

THE GRAND TOUR OF QUANTUM THERMODYNAMICS The Grand Tour of quantum thermodynamics. Young noblemen used to undertake a “Grand Tour” during the 1600s and 1700s. Many of the tourists hailed from England, though well-to-do compatriots traveled from Scandinavia, Germany, and the United States. The men had just graduated from university—in many cases, Oxford or Cambridge.

THE SIGN PROBLEM(S)

The sign problem (s) The thirteen-month-old had mastered the word “dada” by the time I met her. Her parents were teaching her to communicate other concepts through sign language. Picture her, dark-haired and bibbed, in a high chair. Banana and mango slices litter the tray in front of her. More fruit litters the floor in front

of the tray.

QUANTUM INFORMATION IN QUANTUM COGNITION Some research topics, says conventional wisdom, a physics PhD student shouldn’t touch with an iron-tipped medieval lance: sinkholes in the foundations of quantum theory. Problems so hard, you’d have a snowball’s chance of achieving progress. Problems so obscure, you’d have a snowball’s chance of convincing anyone to care about

progress.

MERMIN SQUARE

Before telling you why quantum mechanics is contextual, let me give you an experiment that admits a simple non-contextual explanation. This story takes place in Flatland, a two-dimensional world inhabited by polygons.Our protagonist is a square who became famous after claiming that he met a TOP 10 QUESTIONS FOR YOUR POTENTIAL PHD ADVISER/GROUP Everyone in grad school has taken on the task of picking the perfect research group at some point. Then some among us had the dubious distinction of choosing the perfect research group twice. Luckily for me, a year of grad research taught me a lot and WE ARE ALL WILSONIANS NOW We are all Wilsonians now. Ken Wilson passed away on June 15 at age 77. He changed how we think about physics. Renormalization theory, first formulated systematically by Freeman Dyson in 1949, cured the flaws of quantum electrodynamics and turned it into a precise computational tool. But the subject seemed magical and mysterious. PAUL DIRAC AND POETRY Paul Dirac and poetry. In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in the case of poetry, it’s the exact opposite! I tacked Dirac’s quote onto the bulletin board above my desk,

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

PEEKING INTO THE WORLD OF QUANTUM INTELLIGENCE Peeking into the world of quantum intelligence. Intelligent beings have the ability to receive, process, store information, and based on the processed information, predict what would happen in the future and act accordingly. An illustration of receiving, processing, and storing information. Based on the processed information, one can make LEARNING ABOUT LEARNING The autumn of my sophomore year of college was mildly hellish. I took the equivalent of three semester-long computer-science and physics courses, atop other classwork; co-led a public-speaking self-help group; and coordinated a celebrity visit to campus. I lived at my desk and in office hours, always declining my flatmates’ invitations to

watch The West

THE GRAND TOUR OF QUANTUM THERMODYNAMICS The Grand Tour of quantum thermodynamics. Young noblemen used to undertake a “Grand Tour” during the 1600s and 1700s. Many of the tourists hailed from England, though well-to-do compatriots traveled from Scandinavia, Germany, and the United States. The men had just graduated from university—in many cases, Oxford or Cambridge.

THE SIGN PROBLEM(S)

The sign problem (s) The thirteen-month-old had mastered the word “dada” by the time I met her. Her parents were teaching her to communicate other concepts through sign language. Picture her, dark-haired and bibbed, in a high chair. Banana and mango slices litter the tray in front of her. More fruit litters the floor in front

of the tray.

QUANTUM INFORMATION IN QUANTUM COGNITION Some research topics, says conventional wisdom, a physics PhD student shouldn’t touch with an iron-tipped medieval lance: sinkholes in the foundations of quantum theory. Problems so hard, you’d have a snowball’s chance of achieving progress. Problems so obscure, you’d have a snowball’s chance of convincing anyone to care about

progress.

MERMIN SQUARE

Before telling you why quantum mechanics is contextual, let me give you an experiment that admits a simple non-contextual explanation. This story takes place in Flatland, a two-dimensional world inhabited by polygons.Our protagonist is a square who became famous after claiming that he met a TOP 10 QUESTIONS FOR YOUR POTENTIAL PHD ADVISER/GROUP Everyone in grad school has taken on the task of picking the perfect research group at some point. Then some among us had the dubious distinction of choosing the perfect research group twice. Luckily for me, a year of grad research taught me a lot and WE ARE ALL WILSONIANS NOW We are all Wilsonians now. Ken Wilson passed away on June 15 at age 77. He changed how we think about physics. Renormalization theory, first formulated systematically by Freeman Dyson in 1949, cured the flaws of quantum electrodynamics and turned it into a precise computational tool. But the subject seemed magical and mysterious. PAUL DIRAC AND POETRY Paul Dirac and poetry. In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in the case of poetry, it’s the exact opposite! I tacked Dirac’s quote onto the bulletin board above my desk,

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

ABOUT | QUANTUM FRONTIERS The Institute for Quantum Information and Matter (IQIM) at Caltech is the newest Physics Frontiers Center supported by the National Science Foundation and the Gordon and Betty Moore Foundation.Here at IQIM, we study physical systems in which the weirdness of the quantum world becomes manifest on macroscopic scales. Our work spans a wide range of cutting edge research, from superconductivity

LEARNING THEORY

Rigorously, one refers to a classical/quantum being as a classical/quantum model, algorithm, protocol, or procedure. This is because the actions of these classical/quantum beings are the center of the mathematical analysis. Formally, we consider the task of learning an unknown physical evolution described by a CPTP map that

takes in -qubit

PRESKILL | QUANTUM FRONTIERS John Preskill – There are two main classes of attempts. One is just to come up with a cryptographic protocol not so different conceptually from what’s done now, but based on a problem that’s hard for quantum computers. Craig Cannon – There you go. LIFE AMONG THE EXPERIMENTALISTS Life among the experimentalists. Posted on March 21, 2021 by Nicole Yunger Halpern. I used to catch lizards—brown anoles, as I learned to call them later—as a child. They were colored as their name suggests, were about as long as one of my hands, and resented my attention. But they frequented our back porch, and I had a butterfly

net.

WE ARE ALL WILSONIANS NOW We are all Wilsonians now. Ken Wilson passed away on June 15 at age 77. He changed how we think about physics. Renormalization theory, first formulated systematically by Freeman Dyson in 1949, cured the flaws of quantum electrodynamics and turned it into a precise computational tool. But the subject seemed magical and mysterious. ALWAYS LOOK ON THE BRIGHT SIDE…OF CPTP MAPS. CPTP maps represent processes undergone by quantum systems. Imagine preparing some system—an electron, a photon, a superconductor, etc.—in a state I’ll call “ “. Imagine turning on a magnetic field, or coupling one electron to another, or letting the superconductor sit untouched. A CPTP map, labeled as , represents every such evolution.

MATTHEW FISHER

Matthew Fisher is only 55, but reluctance to be seen as a crazy old guy might partially explain why he has kept pretty quiet about his passionate pursuit of neuroscience over the past three years. That changed two months ago when he posted a paper on the arXiv about

Quantum Cognition.

THIS VIDEO OF SCIENTISTS SPLITTING AN ELECTRON WILL SHOCK by Jorge Cham. Ok, this is where things get weird. If quantum computers, femtometer motions or laser alligators weren't enough, let's throw in fractionalized electrons, topological surfaces and strings that go to the end of time. To be honest, the idea that an electron can't be split hadn't even occurred to me before my conversation with Gil and SPIROS | QUANTUM FRONTIERS Posted on September 15, 2015 by spiros. 4. A blog on everything quantum is the perfect place to announce the launch of the 2015 Quantum Shorts competition. The contest encourages readers to create quantum-themed “flash fiction”: a short story of no more than 1000 words that is inspired by quantum physics.

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

PEEKING INTO THE WORLD OF QUANTUM INTELLIGENCE Peeking into the world of quantum intelligence. Intelligent beings have the ability to receive, process, store information, and based on the processed information, predict what would happen in the future and act accordingly. An illustration of receiving, processing, and storing information. Based on the processed information, one can make

THE SIGN PROBLEM(S)

The sign problem (s) The thirteen-month-old had mastered the word “dada” by the time I met her. Her parents were teaching her to communicate other concepts through sign language. Picture her, dark-haired and bibbed, in a high chair. Banana and mango slices litter the tray in front of her. More fruit litters the floor in front

of the tray.

THE GRAND TOUR OF QUANTUM THERMODYNAMICS The Grand Tour of quantum thermodynamics. Young noblemen used to undertake a “Grand Tour” during the 1600s and 1700s. Many of the tourists hailed from England, though well-to-do compatriots traveled from Scandinavia, Germany, and the United States. The men had just graduated from university—in many cases, Oxford or Cambridge. QUANTUM INFORMATION IN QUANTUM COGNITION Some research topics, says conventional wisdom, a physics PhD student shouldn’t touch with an iron-tipped medieval lance: sinkholes in the foundations of quantum theory. Problems so hard, you’d have a snowball’s chance of achieving progress. Problems so obscure, you’d have a snowball’s chance of convincing anyone to care about

progress.

RANDOM WALKS

On average, five minutes after arriving at the lamppost, he’s back at the lamppost. But, if we wait for a time , we have a decent chance of finding him a distance away. These characteristic typify a simple random walk. Random walks crop up across statistical physics. For instance, consider a grain of pollen dropped onto a thin film of

water.

PRESKILL | QUANTUM FRONTIERS John Preskill – There are two main classes of attempts. One is just to come up with a cryptographic protocol not so different conceptually from what’s done now, but based on a problem that’s hard for quantum computers. Craig Cannon – There you go.

FERNANDO PASTAWSKI

TOP 10 QUESTIONS FOR YOUR POTENTIAL PHD ADVISER/GROUP Everyone in grad school has taken on the task of picking the perfect research group at some point. Then some among us had the dubious distinction of choosing the perfect research group twice. Luckily for me, a year of grad research taught me a lot and MODERN PHYSICS EDUCATION? Being the physics department executive officer (on top of being a quantum physicist) makes me think a lot about our physics college program. It is exciting. We start with mechanics, and then go to electromagnetism (E&M) and relativity, then to quantum and statistical mechanics, and then to advanced mathematical methods, analytical mechanics and more E&M.

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

PEEKING INTO THE WORLD OF QUANTUM INTELLIGENCE Peeking into the world of quantum intelligence. Intelligent beings have the ability to receive, process, store information, and based on the processed information, predict what would happen in the future and act accordingly. An illustration of receiving, processing, and storing information. Based on the processed information, one can make

THE SIGN PROBLEM(S)

The sign problem (s) The thirteen-month-old had mastered the word “dada” by the time I met her. Her parents were teaching her to communicate other concepts through sign language. Picture her, dark-haired and bibbed, in a high chair. Banana and mango slices litter the tray in front of her. More fruit litters the floor in front

of the tray.

THE GRAND TOUR OF QUANTUM THERMODYNAMICS The Grand Tour of quantum thermodynamics. Young noblemen used to undertake a “Grand Tour” during the 1600s and 1700s. Many of the tourists hailed from England, though well-to-do compatriots traveled from Scandinavia, Germany, and the United States. The men had just graduated from university—in many cases, Oxford or Cambridge. QUANTUM INFORMATION IN QUANTUM COGNITION Some research topics, says conventional wisdom, a physics PhD student shouldn’t touch with an iron-tipped medieval lance: sinkholes in the foundations of quantum theory. Problems so hard, you’d have a snowball’s chance of achieving progress. Problems so obscure, you’d have a snowball’s chance of convincing anyone to care about

progress.

RANDOM WALKS

On average, five minutes after arriving at the lamppost, he’s back at the lamppost. But, if we wait for a time , we have a decent chance of finding him a distance away. These characteristic typify a simple random walk. Random walks crop up across statistical physics. For instance, consider a grain of pollen dropped onto a thin film of

water.

PRESKILL | QUANTUM FRONTIERS John Preskill – There are two main classes of attempts. One is just to come up with a cryptographic protocol not so different conceptually from what’s done now, but based on a problem that’s hard for quantum computers. Craig Cannon – There you go.

FERNANDO PASTAWSKI

TOP 10 QUESTIONS FOR YOUR POTENTIAL PHD ADVISER/GROUP Everyone in grad school has taken on the task of picking the perfect research group at some point. Then some among us had the dubious distinction of choosing the perfect research group twice. Luckily for me, a year of grad research taught me a lot and MODERN PHYSICS EDUCATION? Being the physics department executive officer (on top of being a quantum physicist) makes me think a lot about our physics college program. It is exciting. We start with mechanics, and then go to electromagnetism (E&M) and relativity, then to quantum and statistical mechanics, and then to advanced mathematical methods, analytical mechanics and more E&M.

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

LEARNING ABOUT LEARNING The autumn of my sophomore year of college was mildly hellish. I took the equivalent of three semester-long computer-science and physics courses, atop other classwork; co-led a public-speaking self-help group; and coordinated a celebrity visit to campus. I lived at my desk and in office hours, always declining my flatmates’ invitations to

watch The West

LEARNING THEORY

Rigorously, one refers to a classical/quantum being as a classical/quantum model, algorithm, protocol, or procedure. This is because the actions of these classical/quantum beings are the center of the mathematical analysis. Formally, we consider the task of learning an unknown physical evolution described by a CPTP map that

takes in -qubit

SEVEN REASONS WHY I CHOSE TO DO SCIENCE IN THE GOVERNMENT When I was in college, people asked me what I wanted to do with my life. I’d answer, “I want to be of use and to learn always.” The question resurfaced in grad school and at the beginning of my postdoc. I answered that I wanted to do extraordinary science that I’d steer.

Academia attracted

INTERACTION + ENTANGLEMENT = EFFICIENT PROOFS OF HALTING Interaction + Entanglement = Efficient Proofs of Halting. A couple weeks ago my co-authors Zhengfeng Ji (UTS Sydney), Heny Yuen (University of Toronto) and Anand Natarajan and John Wright (both at Caltech’s IQIM, with John soon moving to UT Austin) & I posted a manuscript on the arXiv preprint server entitled. LIFE AMONG THE EXPERIMENTALISTS Life among the experimentalists. Posted on March 21, 2021 by Nicole Yunger Halpern. I used to catch lizards—brown anoles, as I learned to call them later—as a child. They were colored as their name suggests, were about as long as one of my hands, and resented my attention. But they frequented our back porch, and I had a butterfly

net.

SHAUNMAGUIRE

The Science that made Stephen Hawking famous. Posted on November 5, 2014 by shaunmaguire. 8. In anticipation of The Theory of Everything which comes out today, and in the spirit of continuing with Quantum Frontiers’ current movie theme, I wanted to provide an overview of Stephen Hawking’s pathbreaking research. IN THE HOUR OF DARKNESS AND PERIL AND NEED A cry of defiance, and not of fear, A voice in the darkness, a knock at the door, And a word that shall echo forevermore! For, borne on the night-wind of the Past, Through all our history, to the last, In the hour of darkness and peril and need, The people will waken and listen

to hear.

QUANTUM CHESS

The Quantum Chess board begins in the same configuration as standard chess. All pawns move the same as they would in standard chess, but all other pieces get a choice of two movement types, standard or quantum. Standard moves act exactly as they would in standard chess. However, quantum moves, create superpositions. PAUL DIRAC AND POETRY Paul Dirac and poetry. In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in the case of poetry, it’s the exact opposite! I tacked Dirac’s quote onto the bulletin board above my desk,

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

PEEKING INTO THE WORLD OF QUANTUM INTELLIGENCE Peeking into the world of quantum intelligence. Intelligent beings have the ability to receive, process, store information, and based on the processed information, predict what would happen in the future and act accordingly. An illustration of receiving, processing, and storing information. Based on the processed information, one can make

THE SIGN PROBLEM(S)

The sign problem (s) The thirteen-month-old had mastered the word “dada” by the time I met her. Her parents were teaching her to communicate other concepts through sign language. Picture her, dark-haired and bibbed, in a high chair. Banana and mango slices litter the tray in front of her. More fruit litters the floor in front

of the tray.

THE GRAND TOUR OF QUANTUM THERMODYNAMICS The Grand Tour of quantum thermodynamics. Young noblemen used to undertake a “Grand Tour” during the 1600s and 1700s. Many of the tourists hailed from England, though well-to-do compatriots traveled from Scandinavia, Germany, and the United States. The men had just graduated from university—in many cases, Oxford or Cambridge. QUANTUM INFORMATION IN QUANTUM COGNITION Some research topics, says conventional wisdom, a physics PhD student shouldn’t touch with an iron-tipped medieval lance: sinkholes in the foundations of quantum theory. Problems so hard, you’d have a snowball’s chance of achieving progress. Problems so obscure, you’d have a snowball’s chance of convincing anyone to care about

progress.

RANDOM WALKS

On average, five minutes after arriving at the lamppost, he’s back at the lamppost. But, if we wait for a time , we have a decent chance of finding him a distance away. These characteristic typify a simple random walk. Random walks crop up across statistical physics. For instance, consider a grain of pollen dropped onto a thin film of

water.

PRESKILL | QUANTUM FRONTIERS John Preskill – There are two main classes of attempts. One is just to come up with a cryptographic protocol not so different conceptually from what’s done now, but based on a problem that’s hard for quantum computers. Craig Cannon – There you go.

FERNANDO PASTAWSKI

TOP 10 QUESTIONS FOR YOUR POTENTIAL PHD ADVISER/GROUPQUESTIONS TO ASK

PHD FACULTY

Everyone in grad school has taken on the task of picking the perfect research group at some point. Then some among us had the dubious distinction of choosing the perfect research group twice. Luckily for me, a year of grad research taught me a lot and MODERN PHYSICS EDUCATION? Being the physics department executive officer (on top of being a quantum physicist) makes me think a lot about our physics college program. It is exciting. We start with mechanics, and then go to electromagnetism (E&M) and relativity, then to quantum and statistical mechanics, and then to advanced mathematical methods, analytical mechanics and more E&M.

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

PEEKING INTO THE WORLD OF QUANTUM INTELLIGENCE Peeking into the world of quantum intelligence. Intelligent beings have the ability to receive, process, store information, and based on the processed information, predict what would happen in the future and act accordingly. An illustration of receiving, processing, and storing information. Based on the processed information, one can make

THE SIGN PROBLEM(S)

The sign problem (s) The thirteen-month-old had mastered the word “dada” by the time I met her. Her parents were teaching her to communicate other concepts through sign language. Picture her, dark-haired and bibbed, in a high chair. Banana and mango slices litter the tray in front of her. More fruit litters the floor in front

of the tray.

THE GRAND TOUR OF QUANTUM THERMODYNAMICS The Grand Tour of quantum thermodynamics. Young noblemen used to undertake a “Grand Tour” during the 1600s and 1700s. Many of the tourists hailed from England, though well-to-do compatriots traveled from Scandinavia, Germany, and the United States. The men had just graduated from university—in many cases, Oxford or Cambridge. QUANTUM INFORMATION IN QUANTUM COGNITION Some research topics, says conventional wisdom, a physics PhD student shouldn’t touch with an iron-tipped medieval lance: sinkholes in the foundations of quantum theory. Problems so hard, you’d have a snowball’s chance of achieving progress. Problems so obscure, you’d have a snowball’s chance of convincing anyone to care about

progress.

RANDOM WALKS

On average, five minutes after arriving at the lamppost, he’s back at the lamppost. But, if we wait for a time , we have a decent chance of finding him a distance away. These characteristic typify a simple random walk. Random walks crop up across statistical physics. For instance, consider a grain of pollen dropped onto a thin film of

water.

PRESKILL | QUANTUM FRONTIERS John Preskill – There are two main classes of attempts. One is just to come up with a cryptographic protocol not so different conceptually from what’s done now, but based on a problem that’s hard for quantum computers. Craig Cannon – There you go.

FERNANDO PASTAWSKI

TOP 10 QUESTIONS FOR YOUR POTENTIAL PHD ADVISER/GROUPQUESTIONS TO ASK

PHD FACULTY

Everyone in grad school has taken on the task of picking the perfect research group at some point. Then some among us had the dubious distinction of choosing the perfect research group twice. Luckily for me, a year of grad research taught me a lot and MODERN PHYSICS EDUCATION? Being the physics department executive officer (on top of being a quantum physicist) makes me think a lot about our physics college program. It is exciting. We start with mechanics, and then go to electromagnetism (E&M) and relativity, then to quantum and statistical mechanics, and then to advanced mathematical methods, analytical mechanics and more E&M.

QUANTUM FRONTIERS

The United States salutes word and whimsy in April, and Quantum Frontiers is continuing its tradition of celebrating. As a resident of Cambridge, Massachusetts and as a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”. Henry Wadsworth Longfellow wrote the poem, as well as others in the American

canon

LEARNING ABOUT LEARNING The autumn of my sophomore year of college was mildly hellish. I took the equivalent of three semester-long computer-science and physics courses, atop other classwork; co-led a public-speaking self-help group; and coordinated a celebrity visit to campus. I lived at my desk and in office hours, always declining my flatmates’ invitations to

watch The West

LEARNING THEORY

Rigorously, one refers to a classical/quantum being as a classical/quantum model, algorithm, protocol, or procedure. This is because the actions of these classical/quantum beings are the center of the mathematical analysis. Formally, we consider the task of learning an unknown physical evolution described by a CPTP map that

takes in -qubit

SEVEN REASONS WHY I CHOSE TO DO SCIENCE IN THE GOVERNMENT When I was in college, people asked me what I wanted to do with my life. I’d answer, “I want to be of use and to learn always.” The question resurfaced in grad school and at the beginning of my postdoc. I answered that I wanted to do extraordinary science that I’d steer.

Academia attracted

INTERACTION + ENTANGLEMENT = EFFICIENT PROOFS OF HALTING Interaction + Entanglement = Efficient Proofs of Halting. A couple weeks ago my co-authors Zhengfeng Ji (UTS Sydney), Heny Yuen (University of Toronto) and Anand Natarajan and John Wright (both at Caltech’s IQIM, with John soon moving to UT Austin) & I posted a manuscript on the arXiv preprint server entitled. LIFE AMONG THE EXPERIMENTALISTS Life among the experimentalists. Posted on March 21, 2021 by Nicole Yunger Halpern. I used to catch lizards—brown anoles, as I learned to call them later—as a child. They were colored as their name suggests, were about as long as one of my hands, and resented my attention. But they frequented our back porch, and I had a butterfly

net.

SHAUNMAGUIRE

The Science that made Stephen Hawking famous. Posted on November 5, 2014 by shaunmaguire. 8. In anticipation of The Theory of Everything which comes out today, and in the spirit of continuing with Quantum Frontiers’ current movie theme, I wanted to provide an overview of Stephen Hawking’s pathbreaking research. IN THE HOUR OF DARKNESS AND PERIL AND NEED A cry of defiance, and not of fear, A voice in the darkness, a knock at the door, And a word that shall echo forevermore! For, borne on the night-wind of the Past, Through all our history, to the last, In the hour of darkness and peril and need, The people will waken and listen

to hear.

QUANTUM CHESS

The Quantum Chess board begins in the same configuration as standard chess. All pawns move the same as they would in standard chess, but all other pieces get a choice of two movement types, standard or quantum. Standard moves act exactly as they would in standard chess. However, quantum moves, create superpositions. PAUL DIRAC AND POETRY Paul Dirac and poetry. In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in the case of poetry, it’s the exact opposite! I tacked Dirac’s quote onto the bulletin board above my desk,

QUANTUM FRONTIERS

A BLOG BY THE INSTITUTE FOR QUANTUM INFORMATION AND MATTER @ CALTECH

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SEVEN REASONS WHY I CHOSE TO DO SCIENCE IN THE GOVERNMENT Posted on October 25, 2020 by Nicole Yunger Halpern

2

When I was in college, people asked me what I wanted to do with my life. I’d answer, “I want to be of use and to learn always.” The question resurfaced in grad school and at the beginning of my postdoc. I answered that I wanted to do extraordinary science that I’d steer. Academia attracted me most, but I wouldn’t discount alternatives. Last spring, I accepted an offer to build my research group as a member of NIST, the National Institute for Standards and Technology in the U.S. government. My group will be headquartered on the University of Maryland campus, nestled amongst quantum and interdisciplinary institutes. I’m grateful to be joining NIST, and I’m surprised. I never envisioned myself working for the government. I could have accepted an assistant professorship (and I was extremely grateful for the offers), but NIST swept me off my feet. Here are seven reasons why, for other early-career researchers contemplating possibilities. 1) THE SCIENCE. One event illustrates this reason: The notice of my job offer came from NIST Maryland’s friendly neighborhood Nobel laureate. NIST and the university invested in quantum science years before everyone and her uncle began scrambling to create a quantum institute. That investment has flowered, including in reason (2). 2) THE RESEARCH ENVIRONMENT. I wouldn’t say that I have a love affair with the University of Maryland. But I’ve found myself visiting every few years (sometimes blogging about the experience). Why? Much of the quantum community passes through Maryland. Seminars fill the week, visitors fill many offices, and conferences happen once or twice a year. Theorists and experimentalists mingle over lunch and collaborate. The university shares two quantum institutes with NIST: QuICS (the Joint Center for Quantum Information and Computer Science) and the JQI (the Joint Quantum Institute). My group will be based at the former and affiliated with the latter. We’ll also belong to IPST (the university’s Institute for Physical Science and Technology), a hub for interdisciplinarity and thermodynamics. When visiting a university, I ask how much researchers collaborate across department lines. I usually hear an answer along the lines of “We value interdisciplinarity, and we wish that we had more of it, but we don’t have much.” Few universities ingrain interdisciplinarity into their bones by dedicating institutes to it. Maryland’s quantum community and thermodynamics communities bustle and produce. They grant NIST researchers an academic environment, independence to shape their research paths, and the freedom to participate in the broader scientific community. If weary of the three institutes mentioned above, one can explore the university’s Quantum Technology Center and Condensed-Matter-Theory Center

.

3) THE PEOPLE. The first Maryland quantum researcher I met was the friendly neighborhood Nobel laureate, Bill Phillips. Bill was presenting a keynote address at Dartmouth College’s physics department, where I’d earned my Bachelors. Bill said that he’d attended a small liberal-arts college before pursuing his PhD at MIT. During the question-and-answer session, I welcomed him back to a small liberal-arts college. How, I asked, had he benefited from the liberal arts? Juniata College, Bill said, had made him a good person. MIT had helped make him a good scientist. Since then, I’ve kept in occasional contact with Bill, we’ve attended talks of each other’s, and I’ve watched him exhibit the most curiosity I’ve seen in almost anyone. What more could one wish for in a colleague? An equality used across thermodynamics bears Chris Jarzynski’s last name, but he never calls the equality what everyone else does. I benefited from Chris’s mentorship during my PhD, despite our working on opposite sides of the country. His awards include not only membership in the National Academy of Sciences, but also an Outstanding Referee designation, for reviewing so many journal submissions in service to the scientific community. Chris calls IPST, the university’s interdisciplinary and thermodynamic institute, his intellectual home. That recommendation suffices for me. I’ve looked up to Alexey Gorshkov since beginning my PhD. I keep an eye out for Mohammad Hafezi’s and Pratyush Tiwari’s papers. A quantum researcher couldn’t ignore Chris Monroe’s papers if she tried. Postdoctoral and graduate fellowships stock the community with energetic young researchers. Three energetic researchers are joining QuICS as senior Fellows around the time I am. I’ll spare you the rest of my sources of inspiration. 4) THE TEACHING. Most faculty members at R1 research universities teach two to three courses per year. NIST members can teach once every other year. I value teaching and appreciate how teaching benefits not only students, but also instructors. I respect teachers and remain grateful for their influence. I’m grateful to have received reports that I teach well. Because I’ve acquired some skill at communicating, people tend to assume that I adore teaching. I adore presenting talks, but I don’t feel a calling to teach. Mentors have exhorted me to pursue what excites me most and what only I can accomplish. I feel called to do research and to mentor younger

researchers.

Furthermore, if I had to teach much, I wouldn’t have time for writing anything other than papers or grants, such as blog posts. Some of you readers have astonished me with accounts of what my writing means to you. You’ve approached me at conferences, buttonholed me after seminars, and emailed. I’m grateful (as I keep saying, but I mean what I say) for the opportunity to touch lives across the world. I hope to inspire students to take quantum, information-theory, and thermodynamics courses (including the quantum-thermodynamics course that I’d like to teach occasionally). Instructors teach quantum courses throughout the world. No one else writes about Egyptian sarcophagi and the second law of thermodynamics

,

to my knowledge, or the Russian writer Alexander Pushkin and reproductive science . Perhaps no one should. But, since no one else does, I have to.1 5) THE FUNDING. Faculty members complain that they do little apart from applying for grants. Grants fund students, postdocs, travel, summer salaries, equipment, visitors, and workshops. NIST provides primary investigators with research funding every year. Not all the funding that some groups need, but enough to free up time to undertake the research that primary investigators love. 6) THE LACK OF TENURE STRESS. Many junior faculty members fear that they won’t achieve tenure. The fear pushes them away from taking risks in their research programs. This month, I embarked upon a risk that I know I should take but that, had I been facing an assistant professorship, would have given me pause. 7) THE ACRONYMS. Above, I introduced NIST (the National Institute of Standards and Technology), UMD (the University of Maryland), QuICS (the Joint Center for Quantum Information and Computer Science), the JQI (the Joint Quantum Institute), and IPST (the Institute for Physical Science and Technology). I’ll also have an affiliation with UMIACS (the University of Maryland Institute for Advanced Computer Science). Where else can one acquire six acronyms? I adore collecting affiliations, which force me to cross intellectual borders. I also enjoy the opportunity to laugh at my CV. I’ve deferred joining NIST until summer 2021, to complete my postdoctoral fellowship at the Harvard-Smithsonian Institute for Theoretical Atomic, Molecular, and Optical Physics (an organization that needs its acronym, ITAMP, as much as “the Joint Center for Quantum Information and Computer Science” does). After then, please stop by. If you’d like to join my group, please email: I’m accepting applications for PhD and postdoctoral positions this fall. See you in Maryland next year. 1Also, blogging benefits my research. I’ll leave the explanation for

another post.

I credit my husband with the Nesquick-NIST/QuICS parallel.

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LOVE IN THE TIME OF THERMO Posted on September 20, 2020 by Nicole Yunger Halpern

2

An 81-year-old medical doctor has fallen off a ladder in his house. His pet bird hopped out of his reach, from branch to branch of a tree on the patio. The doctor followed via ladder and slipped. His servants cluster around him, the clamor grows, and he longs for his wife to join him before he dies. She arrives at last. He gazes at her face; utters, “Only God knows how much I loved you”; and expires. I set the book down on my lap and looked up. I was nestled in a wicker chair outside the Huntington Art Gallery in San Marino, California. Busts of long-dead Romans kept me company. The lawn in front of me unfurled below a sky that—unusually for San Marino—was partially obscured by clouds. My final summer at Caltech was unfurling. I’d walked to the Huntington, one weekend afternoon, with a novel from Caltech’s English library.1

What a novel.

You may have encountered the phrase “love in the time of corona.” Several times. Per week. Throughout the past six months. _Love in the Time of Cholera _predates the meme by 35 years. Nobel laureate Gabriel García Márquez captured the inhabitants, beliefs, architecture, mores, and spirit of a Colombian city around the turn of the 20th century. His work transcends its setting, spanning love, death, life, obsession, integrity, redemption, and eternity. A thermodynamicist couldn’t ask for more-fitting reading. _Love in the Time of Cholera_ centers on a love triangle. Fermina Daza, the only child of a wealthy man, excels in her studies. She holds herself with poise and self-assurance, and she spits fire whenever others try to control her. The girl dazzles Florentino Ariza, a poet, who restructures his life around his desire for her. Fermina Daza’s pride impresses Dr. Juvenal Urbino, a doctor renowned for exterminating a cholera epidemic. After rejecting both men, Fermina Daza marries Dr. Juvenal Urbino. The two personalities clash, and one betrays the other, but they cling together across the decades. Florentino Ariza retains his obsession with Fermina Daza, despite having countless affairs. Dr. Juvenal Urbino dies by ladder, whereupon Florentino Ariza swoops in to win Fermina Daza over. Throughout the book, characters mistake symptoms of love for symptoms of cholera; and lovers block out the world by claiming to have cholera and

self-quarantining.

As a thermodynamicist, I see the second law of thermodynamics in every chapter. The second law implies that time marches only forward

,

order decays

,

and randomness scatters information to the wind. García Márquez depicts his characters aging, aging more, and aging more. Many characters die. Florentino Ariza’s mother loses her memory to dementia or Alzheimer’s disease. A pawnbroker, she buys jewels from the elite whose fortunes have eroded. Forgetting the jewels’ value one day, she mistakes them for candies and distributes them to children. The second law bites most, to me, in the doctor’s final words, “Only God knows how much I loved you.” Later, the widow Fermina Daza sighs, “It is incredible how one can be happy for so many years in the midst of so many squabbles, so many problems, damn it, and not really know if it was love or not.” She doesn’t know how much her husband loved her, especially in light of the betrayal that rocked the couple and a rumor of another betrayal. Her husband could have affirmed his love with his dying breath, but he refused: He might have loved her with all his heart, and he might not have loved her; he kept the truth a secret to all but God. No one can retrieve the information

after he dies.2

_Love in the Time of Cholera_—and thermodynamics—must sound like a mouthful of horseradish. But each offers nourishment, an appetizer and an entrée. According to the first law of thermodynamics, the amount of energy in every closed, isolated system remains constant: Physics preserves something. Florentino Ariza preserves his love for decades, despite Fermina Daza’s marrying another man, despite her aging. The latter preservation can last only so long in the story: Florentino Ariza, being mortal, will die. He claims that his love will last “forever,” but he won’t last forever. At the end of the novel, he sails between two harbors—back and forth, back and forth—refusing to finish crossing a River Styx. I see this sailing as prethermalization : A few quantum systems resist thermalizing, or flowing to the physics analogue of death, for a while. But they succumb later. Florentino Ariza can’t evade the far bank forever, just as the second law of thermodynamics forbids his boat from functioning as a _perpetuum mobile_. Though mortal within his story, Florentino Ariza survives as a book character. The book survives. García Márquez wrote about a country I’d never visited, and an era decades before my birth, 33 years before I checked his book out of the library. But the book dazzled me. It pulsed with the vibrancy, color, emotion, and intellect—with the fullness—of life. The book gained another life when the coronavius hit. Thermodynamics dictates that people age and die, but the laws of thermodynamics remain.3 I hope and trust—with the caveat about humanity’s not destroying itself—that _Love in the Time of Cholera_ will pulse in 350 years. What’s not to love? 1Yes, Caltech has an English library. I found gems in it, and the librarians ordered more when I inquired about books they didn’t have. I commend it to everyone who has access. 2I googled “Only God knows how much I loved you” and was startled to see the line depicted as a hallmark of romance. Please tell your romantic partners how much you love them; don’t make them guess till the ends of their lives. 3Lee Smolin has proposed that the laws of physics change. If they do, the change seems to have to obey metalaws that remain constant.

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IF THE (QUANTUM-METROLOGY) KEY FITS… Posted on August 30, 2020 by Nicole Yunger Halpern

Reply

My maternal grandfather gave me an antique key when I was in middle school. I loved the workmanship: The handle consisted of intertwined loops. I loved the key’s gold color and how the key weighed on my palm. Even more, I loved the thought that the key opened something. I accompanied my mother to antique shops, where I tried unlocking chests, boxes, and drawers. My grandfather’s antique key I found myself holding another such key, metaphorically, during the autumn of 2018. MIT’s string theorists had requested a seminar, so I presented about quasiprobabilities. Quasiprobabilities

represent

quantum states

similarly

to how probabilities represent a swarm of classical particles. Consider the steam rising from asphalt on a summer day. Calculating every steam particle’s position and momentum would require too much computation for you or me to perform. But we can predict the probability that, if we measure every particle’s position and momentum, we’ll obtain such-and-such outcomes. Probabilities are real numbers between zero and one. Quasiprobabilities can assume negative and nonreal values. We call these values “nonclassical,” because they’re verboten to the probabilities that describe classical systems, such as steam. I’d defined a quasiprobability

,

with collaborators, to describe quantum chaos. David Arvidsson-Shukur was sitting in the audience. David is a postdoctoral fellow at the University of Cambridge and a visiting scholar in the other Cambridge (at MIT

).

He has a Swedish-and-southern-English accent that I’ve heard only once before and, I learned over the next two years, an academic intensity matched by his kindliness.1 Also, David has a name even longer than mine: David Roland Miran Arvidsson-Shukur. We didn’t know then, but we were destined to journey together, as postdoctoral knights-errant, on a quest for quantum truth. David studies the foundations of quantum theory: What distinguishes quantum theory from classical? David suspected that a variation on my quasiprobability could unlock a problem in metrology, the study of

measurements.

Suppose that you’ve built a quantum computer. It consists of gates—uses of, e.g., magnets or lasers to implement logical operations. A classical gate implements operations such as “add 11.” A quantum gate can implement an operation that involves some number more general than 11. You can try to build your gate correctly, but it might effect the wrong value. You need to

measure .

How? You prepare some quantum state and operate on it with the gate. imprints itself on the state, which becomes . Measure some observable . You repeat this protocol in each of many trials. The measurement yields different outcomes in different trials, according to quantum theory. The average amount of information that you learn about per trial is called the _Fisher information_. Let’s change this protocol. After operating with the gate, measure another observable, , and _postselect_: If the measurement yields a desirable outcome , measure . If the -measurement doesn’t yield the desirable outcome, abort the trial, and begin again. If you choose and adroitly, you’ll measure only when the trial will provide oodles of information about . You’ll save yourself many measurements that would have benefited you little.2 Why does postselection help us? We could understand easily if the system were classical: The postselection would effectively improve the input state. To illustrate, let’s suppose that (i) a magnetic field implemented the gate and (ii) the input were metal or rubber. The magnetic field wouldn’t affect the rubber; measuring in rubber trials would provide no information about the field. So you could spare yourself measurements by postselecting on the system’s consisting of metal. Postselection on a quantum system can defy this explanation. Consider optimizing your input state , beginning each trial with the quantum equivalent of metal. Postselection could still increase the average amount of information information provided about per trial. Postselection can enhance quantum metrology even when postselection can’t enhance the classical analogue. David suspected that he could prove this result, using, as a mathematical tool, the quasiprobability that collaborators and I had defined. We fulfilled his prediction, with Hugo Lepage, Aleks Lasek, Seth Lloyd, and Crispin Barnes. _Nature Communications_ published our

paper  last month

.

The work bridges the foundations of quantum theory with quantum metrology and quantum information theory—and, through that quasiprobability, string theory. David’s and my quantum quest continues, so keep an eye out for more theory from us, as well as a photonic experiment based on our first paper. I still have my grandfather’s antique key. I never found a drawer, chest, or box that it opened. But I don’t mind. I have other mysteries to help unlock. 1The morning after my wedding

this

June, my husband and I found a bouquet ordered by David on our

doorstep.

2Postselection has a catch: The measurement has a tiny probability of yielding the desirable outcome. But, sometimes, measuring costs more than preparing , performing the gate, and postselecting. For example, suppose that the system is a photon. A photodetector will measure . Some photodetectors have a _dead time_: After firing, they take a while to reset, to be able to fire again. The dead time can outweigh the cost of the beginning of the experiment.

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A QUANTUM WALK DOWN MEMORY LANE Posted on July 26, 2020 by Nicole Yunger Halpern

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In elementary and middle school, I felt an affinity for the class three years above mine. Five of my peers had siblings in that year. I carpooled with a student in that class, which partnered with mine in holiday activities and Grandparents’ Day revues. Two students in that class stood out. They won academic-achievement awards, represented our school in science fairs and speech competitions, and enrolled in rigorous high-school programs. Those students came to mind as I grew to know David Limmer. David

is an assistant

professor of chemistry at the University of California, Berkeley. He studies statistical mechanics far from equilibrium, using information theory. Though a theorist ardent about mathematics, he partners with experimentalists. He can pass as a physicist and keeps an eye on topics as far afield as black holes. According to his faculty page

, I discovered

while writing this article, he’s even three years older than I. I met David in the final year of my PhD. I was looking ahead to postdocking, as his postdoc fellowship was fading into memory. The more we talked, the more I thought, I’d like to be like him. I had the good fortune to collaborate with David on a paper

published

by _Physical Review A_ this spring (as an Editors’ Suggestion

!). The

project has featured in _Quantum Frontiers_ as the inspiration for a rewriting of “I’m a little teapot.” We studied a molecule prevalent across nature and technologies. Such molecules feature in your eyes, solar-fuel-storage devices, and more. The molecule has two clumps of atoms. One clump may rotate relative to the other if the molecule absorbs light. The rotation switches the molecule from a “closed” configuration to an “open”

configuration.

These molecular switches are small, quantum, and far from equilibrium; so modeling them is difficult. Making assumptions offers traction, but many of the assumptions disagreed with David. He wanted general, thermodynamic-style bounds on the probability that one of these molecular switches would switch. Then, he ran into me. I traffic in mathematical models, developed in quantum information theory, called _resource theories_

. We

use resource theories

to

calculate which states can transform into which in thermodynamics, as a dime can transform into ten pennies at a bank. David and I modeled his molecule in a resource theory, then bounded the molecule’s probability of switching from “closed” to “open.” I accidentally composed a theme song for the molecule; you can sing along with this post

.

That post didn’t mention what David and I discovered about quantum clocks. But what better backdrop for a mental trip to elementary school or to three years into the future? https://xkcd.com/255 I’ve blogged about autonomous quantum clocks (and ancient Assyria) before. Autonomous quantum clocks differ from quantum clocks of another type—the most precise clocks in the world

. Scientists

operate the latter clocks with lasers; autonomous quantum clocks need no operators. Autonomy benefits you if you want for a machine, such as a computer or a drone, to operate independently. An autonomous clock in the machine ensures that, say, the computer applies the right logical gate at the right time. What’s an autonomous quantum clock? First, what’s a clock? A clock has a degree of freedom (e.g., a pair of hands) that represents the time and that moves steadily. When the clock’s hands point to 12 PM, you’re preparing lunch; when the clock’s hands point to 6 PM, you’re reading _Quantum Frontiers_. An autonomous quantum clock has a degree of freedom that represents the time fairly accurately and moves fairly steadily. (The quantum uncertainty principle prevents a perfect quantum clock from existing.) Suppose that the autonomous quantum clock constitutes one part of a machine, such as a quantum computer, that the clock guides. When the clock is in one quantum state, the rest of the machine undergoes one operation, such as one quantum logical gate. (Experts: The rest of the machine evolves under one Hamiltonian

.)

When the clock is in another state, the rest of the machine undergoes another operation (evolves under another Hamiltonian). Physicists have been modeling quantum clocks using the resource theory with which David and I modeled our molecule. The math with which we represented our molecule, I realized, coincided with the math that represents an autonomous quantum clock. Think of the molecular switch as a machine that operates (mostly) independently and that contains an autonomous quantum clock. The rotating clump of atoms constitutes the clock hand. As a hand rotates down a clock face, so do the nuclei rotate downward. The hand effectively points to 12 PM when the switch occupies its “closed” position. The hand effectively points to 6 PM when the switch occupies its “open” position. The nuclei account for most of the molecule’s weight; electrons account for little. They flit about the landscape shaped by the atomic clumps’ positions. The landscape governs the electrons’ behavior. So the electrons form the rest of the quantum machine controlled by

the nuclear clock.

Experimentalists can create and manipulate these molecular switches easily. For instance, experimentalists can set the atomic clump moving—can “wind up” the clock—with ultrafast lasers. In contrast, the only other autonomous quantum clocks that I’d read about live in theory land. Can these molecules bridge theory to experiment? Reach out if you have ideas! And check out David’s theory lab on Berkeley’s website

and on Twitter

. We all need older siblings to look

up to.

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WHAT CAN YOU DO IN 48 HOURS? Posted on July 3, 2020 by Aleksander Kubica

1

Have you ever wondered what can be done in 48 hours? For instance, our heart beats around 200 000 times. One of the biggest supercomputers crunches petabytes (peta = 1015) of numbers to simulate an experiment that took Google’s quantum processor only 300 seconds to run. In 48 hours, one can also participate in the Sciathon with almost 500 young researchers from more than 80 countries! Two weeks ago I participated in a scientific marathon, the Sciathon. The structure of this event roughly resembled a hackathon. I am sure many readers are familiar with the idea of a hackathon from personal experience. For those unfamiliar — a hackathon is an intense collaborative event, usually organized over the weekend, during which people with different backgrounds work in groups to create prototypes of functioning software or hardware. For me, it was the very first time to have firsthand experience with a hackathon-like event! The Sciathon was organized by the Lindau Nobel Laureate Meetings (more about the meetings with Nobel laureates, which happen annually in the lovely German town of Lindau, in another blogpost, I promise!) This year, unfortunately, the face-to-face meeting in Lindau was postponed until the summer of 2021. Instead, the Lindau Nobel Laureate Meetings alumni and this year’s would-be attendees had an opportunity to gather for the Sciathon, as well as the Online Science Days earlier this week, during which the best Sciathon projects were presented. The participants of the Sciathon could choose to contribute new views, perspectives and solutions to three main topics: Lindau Guidelines, Communicating Climate Change and Capitalism After Corona. The first topic concerned an open, cooperative science community where data and knowledge are freely shared, the second — how scientists could show that the climate crisis is just as big a threat as the SARS-CoV-19 virus, and the last — how to remodel our current economic systems so that they are more robust to unexpected sudden crises. More detailed descriptions of each topic can be found on the official Sciathon

webpage .

My group of ten eager scientists, mostly physicists, from master students to postdoctoral researchers, focused on the first topic. In particular, our goal was to develop a method of familiarizing high school students with the basics of quantum information and computation. We envisioned creating an online notebook, where an engaging story would be intertwined with interactive blocks of Python code utilizing the open-source quantum computing toolkit Qiskit. This hands-on approach would enable students to play with quantum systems described in the story-line by simply running the pre-programmed commands with a click of the mouse and then observe how “experiment” matches “the theory”. We decided to work with a system comprising one or two qubits and explain such fundamental concepts in quantum physics as superposition, entanglement and measurement. The last missing part was a captivating story.

*

*

We prepared a two-minute video to illustrate our idea of explaining basic concepts of quantum physics as a children’s fairy tale intertwined with interactive blocks of Python code. The story we came up with involved two good friends from the lab, Miss Schrödinger and Miss Pauli, as well as their kittens, Alice and Bob. At first, Alice and Bob seemed to be ordinary cats, however whenever they sipped quantum milk, they would turn into quantum cats, or as quantum physicists would say — kets. Do I have to remind the reader that a quantum cat, unlike an ordinary one, could be both awake and asleep at the same time? Miss Schrödinger was a proud cat owner who not only loved her cat, but also would take hundreds of pictures of Alice and eagerly upload them on social media. Much to Miss Schrödinger’s surprise, none of the pictures showed Alice partly awake and partly asleep — the ket would always collapse to the cat awake or the cat asleep! Every now and then, Miss Pauli would come to visit Miss Schrödinger and bring her own cat Bob. While the good friends were chit-chatting over a cup of afternoon tea, the cats sipped a bit of quantum milk and started to play with a ball of wool, resulting in a cute mess of two kittens tangled up in wool. Every time after coming back home, Miss Pauli would take a picture of Bob and share it with Miss Schrödinger, who would obviously also take a picture of Alice. After a while, the young scientists started to notice some strange correlations between the states of their cats… The adventures of Miss Schrödinger and her cat continue! For those interested, you can watch a short video

about our project!

Overall, I can say that I had a lot of fun participating in the Sciathon. It was an intense yet extremely gratifying event. In addition to the obvious difficulty of racing against the clock, our group also had to struggle with coordinating video calls between group members scattered across three almost equidistant time zones — Eastern Australian, Central European and Central US! During the Sciathon I had a chance to interact with other science enthusiasts from different backgrounds and work on something from outside my area of expertise. I would strongly encourage anyone to participate in hackathon-like events to break the daily routine, particularly monotonous during the lockdown, and unleash one’s creative spirit. Such events can also be viewed as an opportunity to communicate science and scientific progress to the public. Lastly, I would like to thank other members of my team — collaborating with you during the Sciathon was a blast! During the Sciathon, we had many brainstorming sessions. You can see most of the members of my group in this video call (from left to right, top to bottom): Shuang, myself, Martin, Kyle, Hadewijch, Saskia, Michael and Bartłomiej. The team also included Ahmed and

Watcharaphol.

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ELEVEN RISKS OF MARRYING A QUANTUM INFORMATION SCIENTIST Posted on June 28, 2020 by Nicole Yunger Halpern

7

Some of you may have wondered whether I have a life. I do. He’s a computer scientist, and we got married earlier this month. Marrying a quantum information scientist comes with dangers not advertised in any _Brides_ magazine (I assume; I’ve never opened a copy of _Brides_ magazine). Never mind the perils of gathering together Auntie So-and-so and Cousin Such-and-such, who’ve quarreled since you were six; or spending tens of thousands of dollars on one day; or assembling two handfuls of humans during a pandemic. Beware the risks of marrying someone who unconsciously types “entropy” when trying to type “entry,” twice in a row. 1) SHE’LL INTRODUCE YOU TO FRIENDS AS “A CLASSICAL COMPUTER SCIENTIST.” They’d assume, otherwise, that he does quantum computer science. Of course. Wouldn’t you? 2) THE QUANTUM PUNNING WILL COMMENCE MONTHS BEFORE THE WEDDING. One colleague wrote, “Many congratulations! Now you know the true meaning of entanglement.” Quantum particles can share entanglement. If you measure entangled particles, your outcomes can exhibit correlations stronger than any produceable by classical particles. As a card from another colleague read, “May you stay forever entangled, with no decoherence.” I’d rather not dedicate much of a wedding article to decoherence, but suppose that two particles are maximally entangled (can generate the strongest correlations possible). Suppose that particle 2 heats up or suffers bombardment by other particles. The state of particle 2 _decoheres_ as the entanglement between 1 and 2 frays. Equivalently, particle 2 entangles with its environment, and particle 2 can entangle only so much: The more entanglement 2 shares with the environment, the less entanglement 2 can share with 1. Physicists call entanglement—ba-duh-_bum_—_monogamous_. The matron-of-honor toast featured another entanglement joke, as well as five more physics puns.1 (She isn’t a scientist, but she did her research.) She’ll be on Zoom till Thursday; try the virtual veal. 3) WHEN YOU ASK WHAT SORT OF ENGAGEMENT RING SHE’D LIKE, SHE’LL MENTION BLACK DIAMONDS. Experimentalists and engineers are building quantum computers from systems of many types, including diamond. Diamond consists of carbon atoms arranged in a lattice. Imagine expelling two neighboring carbon atoms and replacing one with a nitrogen atom. You’ll create a _nitrogen-vacancy center_ whose electrons you can control with light. Such centers color the diamond black but let you process quantum information. If I’d asked my fiancé for a quantum computer, we’d have had to wait 20 years to marry. He gave me an heirloom stone instead. 4) WHEN A WEDDING-GOWN SHOPKEEPER ASKS WHICH SORT OF TRAIN SHE’D PREFER, SHE’LL INQUIRE ABOUT MAGLEVS. I dislike shopping, as the best man knows better than most people. In middle school, while our classmates spent their weekends at the mall, we stayed home and read books. But I filled out gown shops’ questionnaires. “They want to know what kinds of material I like,” I told the best man over the phone, “and what styles, and what type of train. I had to pick from four types of train. I didn’t even know there were four

types of train!”

“Steam?” guessed the best man. “Diesel?” His suggestions appealed to me as a quantum thermodynamicist. Thermodynamics is the physics of energy, which engines process. Quantum thermodynamicists

study

how quantum phenomena, such as entanglement, can improve engines

.

“Get the Maglev train,” the best man added. “Low emissions.” “Ooh,” I said, “that’s superconducting.” Superconductors are quantum systems in which charge can flow forever, without dissipating.

Labs at Yale

, at

IBM , and

elsewhere are building quantum computers from superconductors. A superconductor consists of electrons that pair up with help from their positively charged surroundings—Cooper pairs. Separating Cooper-paired electrons requires an enormous amount of energy. What other type of train would better suit a wedding? I set down my phone more at ease. Later, pandemic-era business closures constrained me to wearing a knee-length dress that I’d worn at graduations. I didn’t mind dodging the train. 5) WHEN YOU ASK WHAT STYLE OF WEDDING DRESS SHE’LL WEAR, SHE’LL SAY THAT SHE LIKES HER CLOTHING AS SHE LIKES HER EQUATIONS. Elegant in

their simplicity.

6) YOU’LL PLAN YOUR WEDDING FOR WEDDING SEASON ONLY BECAUSE THE REST OF THE YEAR CONFLICTS WITH MORE SEMINARS, CONFERENCES, AND COLLOQUIA. The quantum-information-theory conference of the year takes place in January. We wanted to visit Australia in late summer, and Germany in autumn, for conferences. A quantum-thermodynamics conference takes place early in the spring, and the academic year ends in May. Happy is the June bride; happier is the June bride who isn’t preparing a

talk.

7) AN MIT CHAPLAIN WILL MARRY YOU. Who else would sanctify the union of a physicist and a computer scientist? 8) YOU’LL ACQUIRE MORE IN-LAWS THAN YOU BARGAINED FOR. Biological parents more than suffice for most spouses. My husband has to contend with academic in-laws, as my PhD supervisor is called my “academic

father.”

Academic in-law s of my husband’s attending the wedding via Zoom. 9) YOUR WEDDING CAN DOUBLE AS A CONFERENCE. Had our wedding taken place in person, collaborations would have flourished during the cocktail hour. Papers would have followed; their acknowledgements sections would have nodded at the wedding; and I’d have requested copies of all manuscripts for our records—which might have included

our wedding album.

10) YOU’LL HAVE TROUBLE IDENTIFYING A HONEYMOON DESTINATION WHERE SHE WON’T BE TEMPTED TO GIVE A SEMINAR. I thought that my then-fiancé would enjoy Vienna, but it boasts a quantum institute. So do Innsbruck and Delft. A colleague-friend works in Budapest, and I owe Berlin a professional visit. The list grew—or, rather, our options shrank. But he turned out not to mind my giving a seminar. The pandemic then cancelled our trip, so we’ll stay abroad for a week after some postpandemic European conference (hint hint). 11) YOUR WEDDING WILL FEATURE ON THE BLOG OF CALTECH’S INSTITUTE FOR QUANTUM INFORMATION AND MATTER. Never mind _The New York Times_. Where else would you expect to find a quantum information physicist? I feel fortunate to have found someone with whom I wouldn’t rather be

anywhere else.

1“I know that if Nicole picked him to stand by her side, he must be a FEYNMAN and not a BOZON.”

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UP WE GO! OR FROM ABSTRACT THEORY TO EXPERIMENTAL PROPOSAL Posted on May 31, 2020 by Nicole Yunger Halpern

3

Mr. Mole is trapped indoors, alone. Spring is awakening outside, but he’s confined to his burrow. Birds are twittering, and rabbits are chattering, but he has only himself for company.

Sound familiar?

Spring—crocuses, daffodils, and hyacinths budding; leaves unfurling; and birds warbling—burst upon Cambridge, Massachusetts last month. The city’s shutdown vied with the season’s vivaciousness. I relieved the tension by rereading _The Wind in the Willows_, which I’ve read every spring since 2017. Project Gutenberg offers free access to Kenneth Grahame’s 1908 novel

. He wrote

the book for children, but never mind that. Many masterpieces of literature happen to have been written for children. One line in the novel demanded, last year, that I memorize it. On page one, Mole is cleaning his house beneath the Earth’s surface. He’s been dusting and whitewashing for hours when the spring calls to him. Life is pulsating on the ground and in the air above him, and he can’t resist joining the party. Mole throws down his cleaning supplies and tunnels upward through the soil: “he scraped and scratched and scrabbled and scrooged, and then he scrooged again and scrabbled and scratched and scraped.” The quotation appealed to me not only because of its alliteration and chiasmus. Mole’s journey reminded me of research.

Take a paper

that

I published last month with Michael Beverland

of

Microsoft Research and Amir Kalev of the Joint Center for Quantum Information and Computer Science

(now

of the Information Sciences Institute at the University of Southern California). We translated a discovery from the abstract, mathematical language of quantum-information-theoretic thermodynamics

into

an experimental proposal. We had to scrabble, but we kept on

scrooging.

Over four years ago, other collaborators and I uncovered a thermodynamics problem

, as

did two other groups at the same time. Thermodynamicists often consider small systems that interact with large environments, like a magnolia flower releasing its perfume into the air. The two systems—magnolia flower and air—exchange things, such as energy and scent particles. The total amount of energy in the flower and the air remains constant, as does the total number of perfume particles. So we call the energy and the perfume-particle number _conserved_

_quantities_.

We represent quantum conserved quantities with matrices and . We nearly always assume that, in this thermodynamic problem, those matrices commute with each other: . Almost no one mentions this assumption; we make it without realizing. Eliminating this assumption invalidates a derivation of the state reached by the small system after a long time. But why assume that the matrices commute? Noncommutation typifies quantum physics and underlies quantum error correction and quantum cryptography. What if the little system exchanges with the large system thermodynamic quantities represented by matrices that don’t commute

with each other?

Colleagues and I began answering this question, four years ago. The small system, we argued, thermalizes to near a quantum state that contains noncommuting matrices. We termed that state, , _the non-Abelian thermal state_. The ’s represent conserved quantities, and the ’s resemble temperatures. The real number ensures that, if you measure any property of the state, you’ll obtain some outcome. Our arguments relied on abstract mathematics, resource

theories

, and

more quantum information theory. Over the past four years, noncommuting conserved quantities have propagated across quantum-information-theoretic thermodynamics.1 Watching the idea take root has been exhilarating, but the quantum information theory didn’t satisfy me. I wanted to see a real physical system thermalize to near the non-Abelian thermal state. Michael and Amir joined the mission to propose an experiment. We kept nosing toward a solution, then dislodging a rock that would shower dirt on us and block our path. But we scrabbled onward. Imagine a line of ions trapped by lasers. Each ion contains the physical manifestation of a qubit—a quantum two-level system, the basic unit of quantum information. You can think of a qubit as having a quantum analogue of angular momentum, called _spin_. The spin has three components, one per direction of space. These spin components are represented by matrices , , and that don’t commute with each other. A couple of qubits can form the small system, analogous to the magnolia flower. The rest of the qubits form the large system, analogous to the air. I constructed a Hamiltonian

—a

matrix that dictates how the qubits evolve—that transfers quanta of all the spin’s components between the small system and the large. (Experts: The Heisenberg Hamiltonian transfers quanta of all the spin components between two qubits while conserving .) The Hamiltonian led to our first scrape: I constructed an integrable Hamiltonian, by accident. Integrable Hamiltonians can’t thermalize systems. A system thermalizes by losing information about its initial conditions, evolving to a state with an exponential form, such as . We clawed at the dirt and uncovered a solution: My Hamiltonian coupled together nearest-neighbor qubits. If the Hamiltonian coupled also next-nearest-neighbor qubits, or if the ions formed a 2D or 3D array, the Hamiltonian would be

nonintegrable.

We had to scratch at every stage—while formulating the setup, preparation procedure, evolution, measurement, and prediction. But we managed; _Physical Review E_ published our paper last month. We showed how a quantum system can evolve to the non-Abelian thermal state. Trapped ions, ultracold atoms, and quantum dots can realize our experimental proposal. We imported noncommuting conserved quantities in thermodynamics from quantum information theory to condensed matter and atomic, molecular, and optical physics. As Grahame wrote, the Mole kept “working busily with his little paws and muttering to himself, ‘Up we go! Up we go!’ till at last, pop! his snout came out into the sunlight and he found himself rolling in the warm grass of a great meadow.” 1See our latest paper’s introduction for references. _https://journals.aps.org/pre/abstract/10.1103/PhysRevE.101.042117_

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QUANTUM STEAMPUNK INVADES SCIENTIFIC AMERICAN Posted on April 20, 2020 by Nicole Yunger Halpern

Reply

London, at an hour that made Rosalind glad she’d nicked her brother’s black cloak instead of wearing her scarlet one. The factory alongside her had quit belching smoke for the night, but it would start again soon. A noise caused her to draw back against the brick wall. Glancing up, she gasped. An oblong hulk was drifting across the sky. The darkness obscured the details, but she didn’t need to see; a brass-colored lock would be painted across the side. Mellator had launched his dirigible. A variation on the paragraph above began the article that I sent to _Scientific American_ last year. Clara Moskowitz , an editor, asked which novel I’d quoted the paragraph from. I’d made the text up, I

confessed.

Most of my publications, which wind up in physics journals, don’t read like novels. But I couldn’t resist when Clara invited me to write a feature about quantum steampunk , the confluence of quantum information and thermodynamics. _Quantum Frontiers_ regulars will anticipate paragraphs two and three of the article: Welcome to steampunk. This genre has expanded across literature, art and film over the past several decades. Its stories tend to take place near nascent factories and in grimy cities, in Industrial Age England and the Wild West—in real-life settings where technologies were burgeoning. Yet steampunk characters extend these inventions into futuristic technologies, including automata and time machines. The juxtaposition of old and new creates an atmosphere of romanticism and adventure. Little wonder that steampunk fans buy top hats and petticoats, adorn themselves in brass and glass, and flock to steampunk conventions. These fans dream the adventure. But physicists today who work at the intersection of three fields—quantum physics, information theory and thermodynamics—live it. Just as steampunk blends science-fiction technology with Victorian style, a modern field of physics that I call “quantum steampunk” unites 21st-century technology with 19th-century scientific principles. The _Scientific American_ graphics team dazzled me. For years, I’ve been hankering to work with artists on visualizing quantum steampunk. I had an opportunity after describing an example of quantum steampunk in the article. The example consists of a quantum many-body engine that I designed with members Christopher White, Sarang Gopalakrishnan, and Gil Refael of Caltech’s Institute for Quantum Information and

Matter. Our engine

is

a many-particle system ratcheted between two phases accessible to

quantum matter

,

analogous to liquid and solid. The engine can be realized with, e.g., ultracold atoms or trapped ions. Lasers would trap and control the particles. Clara, the artists, and I drew the engine, traded comments, and revised the figure tens of times. In early drafts, the lasers resembled the sketches in atomic physicists’ Powerpoints. Before the final draft, the lasers transformed into brass-and-glass beauties. They evoke the scientific instruments crafted through the early 1900s, before chunky gray aesthetics dulled labs. _Scientific American_ published the feature this month; you can read it in print or, here

,

online. Many thanks to Clara for the invitation, for shepherding the article into print, and for her enthusiasm. To repurpose the end of the article, “You’re_ _reading about this confluence of old and new on _Quantum Frontiers_. But you might as well be holding a novel by H. G. Wells or Jules Verne.” _Figures courtesy of the _Scientific American _graphics team._

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A NEW POSSIBILITY FOR QUANTUM NETWORKS Posted on April 19, 2020 by John G. Bartholomew

1

_It has been roughly 1 year since Dr Jon Kindem and I finished at Caltech (JK graduating with a PhD and myself – JB – graduating from my postdoc to take up a junior faculty position at the University of Sydney). During our three-and-a-half-year overlap in the IQIM we often told each other that we should write something for Quantum Frontiers. As two of the authors of a paper reporting a recent breakthrough for rare-earth ion spin qubits (Nature , 2020), it was now or never. Here we go…_ Throughout 2019, telecommunication companies began deploying 5th generation (5G ) network infrastructure to allow our wireless communication to be faster, more reliable, and cope with greater capacity. This roll out of 5G technology promises to support up to 10x the number of devices operating with speeds 10x faster than what is possible with 4th generation (4G) networks. If you stop and think about new opportunities 4G networks unlocked for working, shopping, connecting, and more, it is easy to see why some people are excited about the new world 5G networks might offer. Classical networks like 5G and fiber optic networks (the backbone of the internet ) share classical information: streams of bits (zeros and ones) that encode our conversations, tweets, music, podcasts, videos and anything else we communicate through our digital devices. Every improvement in the network hardware (for example an optical switch with less loss or a faster signal router) contributes to big changes in speed and capacity. The bottom line is that with enough advances, the network evolves to the point where things that were previously impossible (like downloading a movie in the late 90s) become instantaneous. _If you were using the internet in the 90s/00s then you would recognize this sound._ _ If you are Gen Z or Gen Alpha then you’ll probably need to google dial-up spectrogram. _

_.   _

Alongside the hype and advertising around 5G networks, we are part of the world-wide effort to develop a fundamentally different network (with a little less advertising, but similar amounts of hype). Rather than being a bigger, better version of 5G, this new network is trying to build a quantum internet: a set of technologies that will allows us to connect and share information at the quantum level. For an insight into the quantum internet origin story, read this post about the pioneering experiments that took place at Caltech in Prof. Jeff Kimble’s group. Quantum technologies operate using the counter-intuitive phenomena of quantum mechanics like superposition and entanglement

.

Quantum networks need to distribute this superposition and entanglement between different locations. This is a much harder task than distributing bits in a regular network because quantum information is extremely susceptible to loss and noise. If realized, this quantum internet could enable powerful quantum computing clusters, and create networks of quantum sensors that measure infinitesimally small fluctuations in their environment. At this point it is worth asking the question: _Does the world really need a quantum internet?_ This is an important question because a quantum internet is unlikely to improve any of the most common uses for the classical internet (internet facts and most popular searches

).

_2nd most viewed video on YouTube with almost 5 billion views as of

April 2020._

We think there are at least three reasons why a quantum network is

important:

* To build better quantum computers. The quantum internet will effectively transform small, isolated quantum processors into one much larger, more powerful computer. This could be a big boost in the race to scale-up quantum computing. * To build quantum-encrypted communication networks. The ability of quantum technology to make or break encryption is one of the earliest reasons why quantum technology was funded. A fully-fledged quantum computer should be very efficient at hacking commonly used encryption protocols, while ideal quantum encryption provides the basis for communications secured by the fundamental properties of physics. * To push the boundaries of quantum physics and measurement sensitivity by increasing the length scale and complexity of entangled systems. The quantum internet can help turn _thought experiments_ into

_real experiments_.

The next question is: _How do we build a quantum internet?_ The starting point for most long-distance quantum network strategies is to base them on the state-of-the-art technology for current classical networks: sending information using light. (But that doesn’t rule out microwave networks for local area networks, as recent work from ETH Zurich

has shown).

The technology that drives quantum networks is a set of interfaces that connect matter systems (like atoms) to photons at a quantum level. These interfaces need to efficiently exchange quantum information between matter and light, and the matter part needs to be able to store the information for a time that is much longer than the time it takes for the light to get to its destination in the network. We also need to be able to entangle the quantum matter systems to connect network links, and to process quantum information for error correction. This is a significant challenge that requires novel materials and unparalleled control of light to ultimately succeed. State-of-the-art quantum networks are still elementary links compared to the complexity and scale of modern telecommunication. One of the most advanced platforms that has demonstrated a quantum network link consists of two atomic defects in diamonds separated by 1.3 km

.

The defects act as the quantum light-matter interface allowing quantum information to be shared between the two remote devices. But these defects in diamond currently have limitations that prohibit the expansion of such a network. The central challenge is finding defects/emitters that are stable and robust to environmental fluctuations, while simultaneously efficiently connecting with light. While these emitters don’t have to be in solids, the allure of a scalable solid-state fabrication process akin to today’s semiconductor industry for integrated circuits is very appealing. This has motivated the research and development of a range of quantum light-matter interfaces in solids (for example, see recent work by

Harvard researchers

)

with the goal of meeting the simultaneous goals of efficiency and

stability.

The research group we were a part of at Caltech was Prof. Andrei Faraon’s group , which put forward an appealing alternative to other solid-state technologies. The team uses rare-earth atoms embedded in crystals commonly used for lasers. JK joined as the group’s 3rd graduate student in 2013, while I joined as a postdoc in 2016. _The rare-earth elements are found in the part of the periodic table that people often forget about. The elements from cerium (Ce) to ytterbium (Yb) are the most commonly used for quantum technologies._ Rare-earth atoms have long been of interest for quantum technologies such as quantum memories for light because they are very stable and are excellent at preserving quantum information. But compared to other emitters, they only interact very weakly with light, which means that one usually needs large crystals with billions of atoms all working in harmony to make useful quantum interfaces. To overcome this problem, research in the Faraon group pioneered coupling these ions to nanoscale optical cavities like these

ones:

* Nanophotonic cavities fabricated in the rare-earth host crystal. * Nanophotonic cavities fabricated in the rare-earth host crystal. * Nanophotonic cavities fabricated in the rare-earth host crystal. * Nanophotonic cavities fabricated in the rare-earth host crystal. Nanophotonic cavities fabricated in the rare-earth host crystal. These microscopic Toblerone-like structures are fabricated directly in the crystal that plays host to the rare-earth atoms. The periodic patterning effectively acts like two mirrors that form an optical cavity to confine light, which enhances the connection between light and the rare-earth atoms. In 2017, our group showed that the improved optical interaction in these cavities can be used to shrink down optical quantum memories by orders of magnitude compared to previous demonstrations, and ones manufactured on-chip. We have used this nanophotonic platform to open up new avenues for quantum networks based on single rare-earth atoms, a task that previously was exceptionally challenging because these atoms have very low brightness. We have worked with both neodymium and ytterbium atoms embedded in a commercially available laser crystal. Ytterbium looks particularly promising. Working with Prof. Rufus

Cone’s group

at Montana State University, we showed that these ytterbium atoms absorb and emit light better than most other rare-earth atoms and that they can store quantum information long enough for extended networks (>10 ms) when cooled down to a few Kelvin (-272 degrees Celsius)

.

By using the nanocavity to improve the brightness of these ytterbium atoms, we have now been able to identify and investigate their properties at the single atom level. We can precisely control the quantum state of the single atoms and measure them with high fidelity – both prerequisites for using these atoms in quantum information technologies. When combined with the long quantum information storage times, our work demonstrates important steps to using this system in a

quantum network.

The next milestone is forming an optical link between two individual rare-earth atoms to build an elementary quantum network. This goal is in our sights and we are already working on optimizing the light-matter interface stability and efficiency. A more ambitious milestone is to provide interconnects for other types of qubits – such as superconducting qubits – to join the network. This requires a quantum transducer to convert between microwave signals and light. Rare-earth atoms are promising for transducer technologies (see recent work from the Faraon group ), as are a number of other hybrid quantum systems (for example, optomechanical devices like the ones developed in the Painter group

at Caltech).

It took roughly 50 years from the first message sent over ARPANET to the roll out of 5G technology. _So, when are we going to see the quantum internet_? The technology and expertise needed to build quantum links between cities are developing rapidly with impressive progress made even

between 2018 and

2020

.

Basic quantum network capabilities will likely be up and running in the next decade, which will be an exciting time for breakthroughs in fundamental and applied quantum science. Using single rare-earth atoms is relatively new, but this technology is also advancing quickly (for example, our ytterbium material was largely unstudied just three years ago). Importantly, the discovery of new materials will continue to be important to push quantum technologies forward. You can read more about this work in this summary article and this synopsis written by lead author JK (Caltech PhD 2019), or dive into the full paper published in

Nature .

J. M. Kindem, A. Ruskuc, J. G. Bartholomew, J. Rochman, Y.-Q. Huan, and A. Faraon. Control and single-shot readout of an ion embedded in a nanophotonic cavity. _Nature_ (2020). Now is an especially exciting time for our field with the Thompson Lab at Princeton publishing a related paper on single rare-earth atom quantum state detection, in their case using erbium. Check out their

article here .

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ACHIEVING SUPERLUBRICITY WITH GRAPHENE Posted on March 24, 2020

by benphysics

1

Sometimes, experimental results spark enormous curiosity inspiring a myriad of questions and ideas for further experimentation. In 2004, Geim and Novoselov, from The University of Manchester, isolated a single layer of graphene from bulk graphite with the “Scotch Tape Method” for which they were awarded the 2010 Nobel Prize in Physics.  This one experimental result has branched out countless times serving as a source of inspiration in as many different fields.  We are now in the midst of an array of branching-out in graphene research, and one of those branches gaining attention is ultra low friction observed between graphene and other surface

materials.

Much has been learned about graphene in the past 15 years through an immense amount of research, most of which, in non-mechanical realms (e.g., electron transport measurements, thermal conductivity, pseudo magnetic fields in strain engineering).  However, superlubricity, a mechanical phenomenon, has become the focus among many research groups. Mechanical measurements have famously shown graphene’s tensile strength to be hundreds of times that of the strongest steel, indisputably placing it atop the list of construction materials best for a superhero suit.  Superlubricity is a tribological property of graphene and is, arguably, as equally impressive as graphene’s

tensile strength.

Tribology is the study of interacting surfaces during relative motion including sources of friction and methods for its reduction.  It’s not a recent discovery that coating a surface with graphite (many layers of graphene) can lower friction between two sliding surfaces. Current research studies the precise mechanisms and surfaces for which to minimize friction with single or several layers of graphene. Research published in _Nature Materials

_ in 2018

measures friction between surfaces under constant load and velocity. The experiment includes two groups; one consisting of two graphene surfaces (homogeneous junction), and another consisting of graphene and hexagonal boron nitride (heterogeneous junction).   The research group measures friction using Atomic Force Microscopy (AFM).  The hexagonal boron nitride (or graphene for a homogeneous junction) is fixed to the stage of the AFM while the graphene slides atop.  Loads are held constant at 20 𝜇N and sliding velocity constant at 200 nm/s. Ultra low friction is observed for homogeneous junctions when the underlying crystalline lattice structures of the surfaces are at a relative angle of 30 degrees.  However, this ultra low friction state is very unstable and upon sliding, the surfaces rotate towards a locked-in lattice alignment. Friction varies with respect to the relative angle between the two surface’s crystalline lattice structures. Minimum (ultra low) friction occurs at a relative angle of 30 degrees reaching a maximum when locked-in lattice alignment is realized upon sliding. While in a state of lattice alignment, shearing is rendered impossible with the experimental setup due to the relatively large amount of

friction.

Friction varies with respect to the relative angle of the crystalline lattice structures and is, therefore, anisotropic.  For example, the fact it takes less force to split wood when an axe blade is applied parallel to its grains than when applied perpendicularly illustrates the anisotropic nature of wood, as the force to split wood is dependent upon the direction along which the force is applied. Frictional anisotropy is greater in homogeneous junctions because the tendency to orient into a stuck, maximum friction alignment, is greater than with heterojunctions.  In fact, heterogeneous junctions experience frictional anisotropy three orders of magnitude less than homogeneous junctions. Heterogenous junctions display much less frictional anisotropy due to a lattice misalignment when the angle between the lattice vectors is at a minimum.  In other words, the graphene and hBN crystalline lattice structures are never parallel because the materials differ, therefore, never experience the impact of lattice alignment as do homogenous junctions. Hence, heterogeneous junctions do not become stuck in a high friction state that characterizes homogeneous ones, and experience ultra low friction during sliding at all relative crystalline lattice structure angles. Presumably, to increase applicability, upscaling to much larger loads will be necessary. A large scale cost effective method to dramatically reduce friction would undoubtedly have an enormous impact on a great number of industries.  Cost efficiency is a key component to the realization of graphene’s potential impact, not only as it applies to superlubricity, but in all areas of application.  As access to large amounts of affordable graphene increases, so will experiments in fabricating devices exploiting the extraordinary characteristics which have placed graphene and graphene based materials on the front lines of material research the past couple decades.

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