Joint Quantum Institute

College Park, MD, United States

Joint Quantum Institute

College Park, MD, United States
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Bagci T.,Copenhagen University | Simonsen A.,Copenhagen University | Schmid S.,Technical University of Denmark | Villanueva L.G.,Technical University of Denmark | And 7 more authors.
Nature | Year: 2014

Low-loss transmission and sensitive recovery of weak radio-frequency and microwave signals is a ubiquitous challenge, crucial in radio astronomy, medical imaging, navigation, and classical and quantum communication. Efficient up-conversion of radio-frequency signals to an optical carrier would enable their transmission through optical fibres instead of through copper wires, drastically reducing losses, and would give access to the set of established quantum optical techniques that are routinely used in quantum-limited signal detection. Research in cavity optomechanics has shown that nanomechanical oscillators can couple strongly to either microwave or optical fields. Here we demonstrate a room-temperature optoelectromechanical transducer with both these functionalities, following a recent proposal using a high-quality nanomembrane. A voltage bias of less than 10 V is sufficient to induce strong coupling between the voltage fluctuations in a radio-frequency resonance circuit and the membrane's displacement, which is simultaneously coupled to light reflected off its surface. The radio-frequency signals are detected as an optical phase shift with quantum-limited sensitivity. The corresponding half-wave voltage is in the microvolt range, orders of magnitude less than that of standard optical modulators. The noise of the transducer-beyond the measured Johnson noise of the resonant circuit-consists of the quantum noise of light and thermal fluctuations of the membrane, dominating the noise floor in potential applications in radio astronomy and nuclear magnetic imaging. Each of these contributions is inferred to be when balanced by choosing an electromechanical cooperativity of with an optical power of 1 mW. The noise temperature of the membrane is divided by the cooperativity. For the highest observed cooperativity of, this leads to a projected noise temperature of 40 mK and a sensitivity limit of. Our approach to all-optical, ultralow-noise detection of classical electronic signals sets the stage for coherent up-conversion of low-frequency quantum signals to the optical domain. © 2014 Macmillan Publishers Limited.


News Article | March 15, 2016
Site: phys.org

While the method is not yet ready for commercialization, it reveals how an object's thermal energy—its heat—can be determined precisely by observing its physical properties at the quantum scale. While the initial demonstration has an absolute accuracy only within a few percentage points, the NIST approach works over a wide temperature range encompassing cryogenic and room temperatures. It is also accomplished with a small, nanofabricated photonic device, which opens up possible applications that are not practical with conventional temperature standards. The NIST team's approach arose from their efforts to observe the vibrations of a small transparent beam of silicon nitride using laser light. Thermal energy—often expressed as temperature—makes all objects vibrate; the warmer the object, the more pronounced the vibrations, though they are still on the order of just a picometer (trillionths of a meter) in size for the beam at room temperature. To observe these tiny perturbations, the team carved a small reflective cavity into the beam. When they shone a laser through the crystal, the light reflecting from the cavity experienced slight shifts in color or frequency due to the beam's temperature-induced vibrations, making the light's color change noticeably in time with the movement. But these were not the only vibrations the team members could see. The team also spotted the much more subtle vibrations that all objects possess due to a quantum-mechanical property called zero-point motion: Even at its lowest possible energy, the beam vibrates ever so slightly due to the inherent uncertainty at the heart of quantum mechanics. This motion is independent of temperature, and has a well-known amplitude fundamentally dictated by quantum mechanics. By comparing the relative size of the thermal vibration to the quantum motion, the absolute temperature can be determined. These intrinsic quantum fluctuations are thousands of times fainter and ordinarily get lost in the noise of the thermal energy-induced vibrations typical of ordinary temperatures, but the process of measuring the beam provides a method to distinguish quantum and thermal fluctuations. When photons from the laser bounce off the sides of the beam, they give it slight kicks, inducing correlations that make the quantum motion more pronounced. "Our technique allowed us to tease the quantum signals out from under the much larger thermal noise," says the team's Tom Purdy, a physicist at NIST's Physical Measurement Laboratory and at the Joint Quantum Institute. "Now we can directly connect temperature to the quantum mechanical fluctuations of a particle. It sets the stage for a new approach to primary thermometry." The power of this new method, when fully developed, will come when the beam is paired with other much more sensitive on-chip photonic thermometers also under development at NIST. Such devices offer the relative temperature sensitivity demanded by applications in pharmaceutical manufacturing, other high performance industrial applications, and climate monitoring, but require absolute calibration, and may drift over time. This new quantum thermometer will act as an integrated temperature standard, ready to keep the other thermometer on track over long periods of time. Purdy will present the team's results on March 16, 2016, at the American Physical Society March Meeting in Baltimore, Md. Explore further: Laser light used to cool object to quantum ground state


News Article | March 15, 2016
Site: www.rdmag.com

Better thermometers might be possible as a result of a discovery at the National Institute of Standards and Technology (NIST), where physicists have found a way to calibrate temperature measurements by monitoring the tiny motions of a nanomechanical system that are governed by the often counterintuitive rules of quantum mechanics. While the method is not yet ready for commercialization, it reveals how an object’s thermal energy—its heat—can be determined precisely by observing its physical properties at the quantum scale. While the initial demonstration has an absolute accuracy only within a few percentage points, the NIST approach works over a wide temperature range encompassing cryogenic and room temperatures. It is also accomplished with a small, nanofabricated photonic device, which opens up possible applications that are not practical with conventional temperature standards. The NIST team’s approach arose from their efforts to observe the vibrations of a small transparent beam of silicon nitride using laser light. Thermal energy—often expressed as temperature—makes all objects vibrate; the warmer the object, the more pronounced the vibrations, though they are still on the order of just a picometer (trillionths of a meter) in size for the beam at room temperature. To observe these tiny perturbations, the team carved a small reflective cavity into the beam. When they shone a laser through the crystal, the light reflecting from the cavity experienced slight shifts in color or frequency due to the beam’s temperature-induced vibrations, making the light’s color change noticeably in time with the movement. But these were not the only vibrations the team members could see. The team also spotted the much more subtle vibrations that all objects possess due to a quantum-mechanical property called zero-point motion: Even at its lowest possible energy, the beam vibrates ever so slightly due to the inherent uncertainty at the heart of quantum mechanics. This motion is independent of temperature, and has a well-known amplitude fundamentally dictated by quantum mechanics. By comparing the relative size of the thermal vibration to the quantum motion, the absolute temperature can be determined. These intrinsic quantum fluctuations are thousands of times fainter and ordinarily get lost in the noise of the thermal energy-induced vibrations typical of ordinary temperatures, but the process of measuring the beam provides a method to distinguish quantum and thermal fluctuations. When photons from the laser bounce off the sides of the beam, they give it slight kicks, inducing correlations that make the quantum motion more pronounced. “Our technique allowed us to tease the quantum signals out from under the much larger thermal noise,” says the team’s Tom Purdy, a physicist at NIST’s Physical Measurement Laboratory and at the Joint Quantum Institute. “Now we can directly connect temperature to the quantum mechanical fluctuations of a particle. It sets the stage for a new approach to primary thermometry.” The power of this new method, when fully developed, will come when the beam is paired with other much more sensitive on-chip photonic thermometers also under development at NIST. Such devices offer the relative temperature sensitivity demanded by applications in pharmaceutical manufacturing, other high performance industrial applications, and climate monitoring, but require absolute calibration, and may drift over time. This new quantum thermometer will act as an integrated temperature standard, ready to keep the other thermometer on track over long periods of time. Purdy will present the team’s results on March 16, 2016, at the American Physical Society March Meeting in Baltimore, MD.


News Article | February 27, 2017
Site: phys.org

Solitons can arise in the quantum world as well. At most temperatures, gas atoms bounce around like billiard balls, colliding with each other and rocketing off into random directions, following the rules of classical physics. Near absolute zero, however, certain kinds of atoms suddenly start behaving according to the very different rules of quantum mechanics, and begin a kind of coordinated dance. Under pristine conditions, solitons can emerge inside these ultracold quantum fluids, surviving for several seconds. Curious about how solitons behave in less than pristine conditions, scientists at NIST's Physical Measurement Laboratory, in collaboration with researchers at the Joint Quantum Institute (JQI), have added some stress to a soliton's life. They began by cooling down a cloud of rubidium atoms. Right before the gas could take on uniform properties and become a homogenous quantum fluid, a radio-frequency magnetic field coaxed a handful of these atoms into retaining their classical, billiard ball-like state. Those atoms are, in effect, "impurities" in the atomic mix. The scientists then used laser light to push apart atoms in one region of the fluid, creating a solitary wave of low density—a "dark" soliton. In the absence of impurities, this low-density region stably pulses through the ultracold fluid. But when atomic impurities are present, the dark soliton behaves as if it were a heavy particle, with lightweight impurity atoms bouncing off of it. These collisions make the dark soliton's movement more random. This effect is reminiscent of Einstein's 1905 predictions about randomized particle movement, dubbed Brownian motion. Guided by this framework, the scientists also expected the impurities to act like friction and slow down the soliton. But surprisingly, dark solitons do not completely follow Einstein's rules. Instead of dragging down the soliton, collisions accelerated it to a point of destabilization. The soliton's speed limit is set by the speed of sound in the quantum fluid, and upon exceeding that limit it exploded into a puff of sound waves. This behavior made sense only after researchers changed their mathematical perspective and remembered to treat the soliton as though it has a negative mass. This is a quirky phenomenon that arises for certain collective behaviors of many-particle systems. Here the negative mass is manifested by the soliton's darkness—it is a dip in the quantum fluid rather than a tall tsunami-like pulse. Particles with negative mass respond to friction forces opposite to their ordinary cousins, speeding up instead of slowing down. "All those assumptions about Brownian motion ended up going out the window. None of it applied," says Hilary Hurst, a graduate student at JQI and lead theorist on the paper. "But at the end we had a theory that described this behavior very well, which is really nice." Lauren Aycock, lead author on the paper, lauded what she saw as particularly strong feedback between theory and experiment, adding that "it's satisfying to have this kind of successful collaboration, where measurement informs theory, which then explains experimental results." Solitons in the land of ultracold atoms are intriguing, say Aycock and Hurst, because they are as close as you can get to observing the interface between quantum effects and the ordinary physics of everyday life. Experiments like this may help answer a deep physics riddle: where is the boundary between classical and quantum? In addition, this result may cast light on a similar problem with solitons in optical fibers, where random noise can disrupt the precise timing needed for communication over long distances. Explore further: Ultra-cold atoms may wade through quantum friction


News Article | February 27, 2017
Site: phys.org

Solitons can arise in the quantum world as well. At most temperatures, gas atoms bounce around like billiard balls, colliding with each other and rocketing off into random directions. Near absolute zero, however, certain kinds of atoms suddenly start behaving according to the very different rules of quantum mechanics, and begin a kind of coordinated dance. Under pristine conditions, solitons can emerge inside these ultracold quantum fluids, surviving for several seconds. Curious about how solitons behave in less than pristine conditions, scientists at NIST's Physical Measurement Laboratory, in collaboration with researchers at the Joint Quantum Institute (JQI), have added some stress to a soliton's life. They began by cooling down a cloud of rubidium atoms. Right before the gas became a homogenous quantum fluid, a radio-frequency magnetic field coaxed a handful of these atoms into retaining their classical, billiard ball-like state. Those atoms are, in effect, impurities in the atomic mix. The scientists then used laser light to push apart atoms in one region of the fluid, creating a solitary wave of low density—a "dark" soliton. In the absence of impurities, this low-density region stably pulses through the ultracold fluid. But when atomic impurities are present, the dark soliton behaves as if it were a heavy particle, with lightweight impurity atoms bouncing off of it. These collisions make the dark soliton's movement more random. This effect is reminiscent of Einstein's 1905 predictions about randomized particle movement, dubbed Brownian motion. Guided by this framework, the scientists also expected the impurities to act like friction and slow down the soliton. But surprisingly, dark solitons do not completely follow Einstein's rules. Instead of dragging down the soliton, collisions accelerated it to a point of destabilization. The soliton's speed limit is set by the speed of sound in the quantum fluid, and upon exceeding that limit it exploded into a puff of sound waves. This behavior made sense only after researchers changed their mathematical perspective and treated the soliton as though it has a negative mass. This is a quirky phenomenon that arises for certain collective behaviors of many-particle systems. Here the negative mass is manifested by the soliton's darkness—it is a dip in the quantum fluid rather than a tall tsunami-like pulse.  Particles with negative mass respond to friction forces opposite to their ordinary cousins, speeding up instead of slowing down. "All those assumptions about Brownian motion ended up going out the window—none of it applied," says Hilary Hurst, a graduate student at JQI and lead theorist on the paper. "But at the end we had a theory that described this behavior very well, which is really nice." Lauren Aycock, lead author on the paper, lauded what she saw as particularly strong feedback between theory and experiment, adding that "it's satisfying to have this kind of successful collaboration, where measurement informs theory, which then explains experimental results." Solitons in the land of ultracold atoms are intriguing, say Aycock and Hurst, because they are as close as you can get to observing the interface between quantum effects and the ordinary physics of everyday life. Experiments like this may help answer a deep physics riddle: where is the boundary between classical and quantum? In addition, this result may cast light on a similar problem with solitons in optical fibers, where random noise can disrupt the precise timing needed for communication over long distances. Explore further: Ultra-cold atoms may wade through quantum friction More information: Lauren M. Aycock et al. Brownian motion of solitons in a Bose–Einstein condensate, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1615004114


News Article | January 6, 2016
Site: phys.org

The Intelligence Advanced Research Projects Activity (IARPA) invests in high-risk, high-payoff research programs to tackle some of the most difficult challenges in the intelligence community. One of these challenges is dealing with encryption. Codes considered unbreakable by today's best supercomputers could be handled in a matter of hours by quantum computers. The basic building blocks of a quantum device are qubits. These are the quantum mechanical analogue of a traditional logical bit, which can be in a "1" or a "0" state. Quantum physics allows for qubits to take on multiple configurations simultaneously (e.g. an equally-weighted superposition of 0 and 1), which is forbidden in conventional computing. When scientists in the 1990s proved that this strange property could be harnessed for solving certain tasks, such as decryption, the quantum information revolution began. While researchers have proven that robust qubits can be built, scaling them into large networks while detecting and correcting errors remains a challenge. IARPA has selected the Duke/Maryland/Georgia Tech partnership as one of the awardees in its program dubbed LogiQ. Their goal is to bring together a large number of atomic qubits to realize modular "super-qubits" that can be scaled up while correcting for errors. This major multi-year award is led by Jungsang Kim (Duke University), Christopher Monroe (University of Maryland and the Joint Quantum Institute) and Ken Brown (Georgia Tech). The effort also includes industry partners AOSense, Inc. (Sunnyvale, California), ColdQuanta, Inc. (Boulder, Colorado) and Harris Corporation (Melbourne, Florida), as well as theoretical support from Andrew Childs (University of Maryland and the Joint Center for Quantum Information and Computer Science) and Luming Duan (University of Michigan). "Our ion trapping approach is one of the leading technologies that can accomplish this goal," said Jungsang Kim, professor of electrical and computer engineering, computer science, and physics at Duke University, and the principal investigator on the project. "We're excited that IARPA sees our group as one of the leaders in the field and has entrusted this important task to us." Quantum systems are fundamentally delicate, and superpositions collapse if they are observed. In other words, before useful information can be extracted, computational resources can be compromised and even destroyed by interactions with the environment. But this obstacle is not insurmountable. One of the first steps is to construct an extremely robust physical realization of a qubit. In this regard, physicists have demonstrated that trapped atomic ions have quantum staying power. In this system, each qubit is stored in the internal energy levels of a single atomic ion—the same states that are used in atomic clocks. Such states boast coherence times unmatched in any other physical system. The qubits are manipulated through laser and microwave radiation to form quantum logic gates and extended circuits for calculations. Ion trappers have become quite adept at controlling a handful of individual qubits. This collaboration has previously proposed and performed demonstrations that their approach is scalable and modular, a necessity because many qubits are needed for useful quantum computation. "Atomic ion qubits are fundamentally scalable, because they can be replicated with virtually identical characteristics: an isolated ytterbium atom is exactly the same in Washington, D.C. as it is in Los Angeles," said Christopher Monroe, professor of physics at the University of Maryland and the Joint Quantum Institute, and co-leader on the project. "Quantum computing is going through the same research and design processes that conventional computing went through decades ago," said Kim. "Just as the first digital computer was constructed once we had reliable switching devices, we are ready to explore the construction of more complex quantum circuits based on the robust multi-qubit manipulation that is possible in trapped ions." But all potential quantum computing technologies still face the same problem that is central to the new IARPA LogiQ program: quantum error correction. "We know how to build a quantum computer with 50-100 qubits with trapped ions right now," said Monroe. "This is a big enough system that we cannot simulate what happens, even with all the conventional computers in the world. But some killer applications of quantum computing require thousands or millions of qubits, and error correction will be crucial to getting there." As with a conventional computer, scientists can encode quantum systems in a way that corrects for errors that happen along the way, such as an accidental bit flip where a 1 becomes 0, or vice versa. Even in record-breaking pristine ion-trapping systems, errors grow fairly rapidly as qubits are added. Because of that sneaky rule that makes quantum systems collapse due to measurement—intentional or otherwise—simply interrogating the qubits directly and fixing the broken ones destroys the quantum computation. The idea of a modular super-qubit or logical qubit begins to address this problem. In this system, the information stored in a logical qubit is encoded into specialized quantum states comprising multiple physical qubits. Distributing the information in this way not only adds protection—it allows for errors to be detected and corrected, all without actually knowing (or needing to know) the exact details of the quantum state as a whole. "The engineering required to achieve the goal of stopping qubits from degrading through error correction will go a long way toward making quantum computers practically viable," said Kim. Explore further: Researchers find qubits based on trapped ions offer a promising scalable platform for quantum computing


News Article | November 15, 2016
Site: www.rdmag.com

When is a traffic jam not a traffic jam? When it's a quantum traffic jam, of course. Only in quantum physics can traffic be standing still and moving at the same time. A new theoretical paper from scientists at the National Institute of Standards and Technology (NIST) and the University of Maryland suggests that intentionally creating just such a traffic jam out of a ring of several thousand ultracold atoms could enable precise measurements of motion. If implemented with the right experimental setup, the atoms could provide a measurement of gravity, possibly even at distances as short as 10 micrometers - about a tenth of a human hair's width. While the authors stress that a great deal of work remains to show that such a measurement would be attainable, the potential payoff would be a clarification of gravity's pull at very short length scales. Anomalies could provide major clues on gravity's behavior, including why our universe appears to be expanding at an accelerating rate. In addition to potentially answering deep fundamental questions, these atom rings may have practical applications, too. They could lead to motion sensors far more precise than previously possible, or serve as switches for quantum computers, with 0 represented by atomic gridlock and 1 by moving atom traffic. The authors of the paper are affiliated with the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science, both of which are partnerships between NIST and the University of Maryland. Over the past two decades, physicists have explored an exotic state of matter called a Bose-Einstein condensate (BEC), which exists when atoms overlap one another at frigid temperatures a smidgen of a degree away from absolute zero. Under these conditions, a tiny cloud of atoms can essentially become one large quantum "superatom," allowing scientists to explore potentially useful properties like superconductivity and superfluidity more easily. Theoretical physicists Stephen Ragole and Jake Taylor, the paper's authors, have now suggested that a variation on the BEC idea could be used to sense rotation or even explore gravity over short distances, where other forces such as electromagnetism generally overwhelm gravity's effects. The idea is to use laser beams - already commonly used to manipulate cold atoms - to string together a few thousand atoms into a ring 10 to 20 micrometers in diameter. Once the ring is formed, the lasers would gently stir it into motion, making the atoms circulate around it like cars traveling one after another down a single-lane beltway. And just as car tires spin as they travel along the pavement, the atoms' properties would pick up the influence of the world around them - including the effects of gravity from masses just a few micrometers away. The ring would take advantage of one of quantum mechanics' counterintuitive behaviors to help scientists actually measure what its atoms pick up about gravity. The lasers could stir the atoms into what is called a "superposition," meaning in effect they would be both circulating about the ring and simultaneously at a standstill. This superposition of flow and gridlock would help maintain the relationships among the ring's atoms for a few crucial milliseconds after removing their laser constraints, enough time to measure their properties before they scatter. Not only might this quantum traffic jam overcome a difficult gravity measurement challenge, but it might help physicists discard some of the many competing theories about the universe - potentially helping clear up a longstanding traffic jam of ideas. One of the great mysteries of the cosmos is why it is expanding at an apparently accelerating rate. Physicists have suggested an outward force, dubbed "dark energy," causes this expansion, but they have yet to discover its origin. One among many theories is that in the vacuum of space, short-lived virtual particles constantly appear and wink out of existence, and their mutual repulsion creates dark energy's effects. While it's a reasonable enough explanation on some levels, physicists calculate that these particles would create so much repulsive force that it would immediately blow the universe apart. So how can they reconcile observations with the virtual particle idea? "One possibility is that the basic fabric of spacetime only responds to virtual particles that are more than a few micrometers apart," Taylor said, "and that's just the sort of separation we could explore with this ring of cold atoms. So if it turns out you can ignore the effect of particles that operate over these short length scales, you can account for a lot of this unobserved repulsive energy. It would be there, it just wouldn't be affecting anything on a cosmic scale." The research appears in the journal Physical Review Letters.


News Article | November 14, 2016
Site: www.eurekalert.org

When is a traffic jam not a traffic jam? When it's a quantum traffic jam, of course. Only in quantum physics can traffic be standing still and moving at the same time. A new theoretical paper from scientists at the National Institute of Standards and Technology (NIST) and the University of Maryland suggests that intentionally creating just such a traffic jam out of a ring of several thousand ultracold atoms could enable precise measurements of motion. If implemented with the right experimental setup, the atoms could provide a measurement of gravity, possibly even at distances as short as 10 micrometers - about a tenth of a human hair's width. While the authors stress that a great deal of work remains to show that such a measurement would be attainable, the potential payoff would be a clarification of gravity's pull at very short length scales. Anomalies could provide major clues on gravity's behavior, including why our universe appears to be expanding at an accelerating rate. In addition to potentially answering deep fundamental questions, these atom rings may have practical applications, too. They could lead to motion sensors far more precise than previously possible, or serve as switches for quantum computers, with 0 represented by atomic gridlock and 1 by moving atom traffic. The authors of the paper are affiliated with the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science, both of which are partnerships between NIST and the University of Maryland. Over the past two decades, physicists have explored an exotic state of matter called a Bose-Einstein condensate (BEC), which exists when atoms overlap one another at frigid temperatures a smidgen of a degree away from absolute zero. Under these conditions, a tiny cloud of atoms can essentially become one large quantum "superatom," allowing scientists to explore potentially useful properties like superconductivity and superfluidity more easily. Theoretical physicists Stephen Ragole and Jake Taylor, the paper's authors, have now suggested that a variation on the BEC idea could be used to sense rotation or even explore gravity over short distances, where other forces such as electromagnetism generally overwhelm gravity's effects. The idea is to use laser beams - already commonly used to manipulate cold atoms - to string together a few thousand atoms into a ring 10 to 20 micrometers in diameter. Once the ring is formed, the lasers would gently stir it into motion, making the atoms circulate around it like cars traveling one after another down a single-lane beltway. And just as car tires spin as they travel along the pavement, the atoms' properties would pick up the influence of the world around them - including the effects of gravity from masses just a few micrometers away. The ring would take advantage of one of quantum mechanics' counterintuitive behaviors to help scientists actually measure what its atoms pick up about gravity. The lasers could stir the atoms into what is called a "superposition," meaning in effect they would be both circulating about the ring and simultaneously at a standstill. This superposition of flow and gridlock would help maintain the relationships among the ring's atoms for a few crucial milliseconds after removing their laser constraints, enough time to measure their properties before they scatter. Not only might this quantum traffic jam overcome a difficult gravity measurement challenge, but it might help physicists discard some of the many competing theories about the universe - potentially helping clear up a longstanding traffic jam of ideas. One of the great mysteries of the cosmos is why it is expanding at an apparently accelerating rate. Physicists have suggested an outward force, dubbed "dark energy," causes this expansion, but they have yet to discover its origin. One among many theories is that in the vacuum of space, short-lived virtual particles constantly appear and wink out of existence, and their mutual repulsion creates dark energy's effects. While it's a reasonable enough explanation on some levels, physicists calculate that these particles would create so much repulsive force that it would immediately blow the universe apart. So how can they reconcile observations with the virtual particle idea? "One possibility is that the basic fabric of spacetime only responds to virtual particles that are more than a few micrometers apart," Taylor said, "and that's just the sort of separation we could explore with this ring of cold atoms. So if it turns out you can ignore the effect of particles that operate over these short length scales, you can account for a lot of this unobserved repulsive energy. It would be there, it just wouldn't be affecting anything on a cosmic scale." The research appears in the journal Physical Review Letters.


News Article | November 14, 2016
Site: phys.org

A new theoretical paper from scientists at the National Institute of Standards and Technology (NIST) and the University of Maryland suggests that intentionally creating just such a traffic jam out of a ring of several thousand ultracold atoms could enable precise measurements of motion. If implemented with the right experimental setup, the atoms could provide a measurement of gravity, possibly even at distances as short as 10 micrometers - about a tenth of a human hair's width. While the authors stress that a great deal of work remains to show that such a measurement would be attainable, the potential payoff would be a clarification of gravity's pull at very short length scales. Anomalies could provide major clues on gravity's behavior, including why our universe appears to be expanding at an accelerating rate. In addition to potentially answering deep fundamental questions, these atom rings may have practical applications, too. They could lead to motion sensors far more precise than previously possible, or serve as switches for quantum computers, with 0 represented by atomic gridlock and 1 by moving atom traffic. The authors of the paper are affiliated with the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science, both of which are partnerships between NIST and the University of Maryland. Over the past two decades, physicists have explored an exotic state of matter called a Bose-Einstein condensate (BEC), which exists when atoms overlap one another at frigid temperatures a smidgen of a degree away from absolute zero. Under these conditions, a tiny cloud of atoms can essentially become one large quantum "superatom," allowing scientists to explore potentially useful properties like superconductivity and superfluidity more easily. Theoretical physicists Stephen Ragole and Jake Taylor, the paper's authors, have now suggested that a variation on the BEC idea could be used to sense rotation or even explore gravity over short distances, where other forces such as electromagnetism generally overwhelm gravity's effects. The idea is to use laser beams - already commonly used to manipulate cold atoms - to string together a few thousand atoms into a ring 10 to 20 micrometers in diameter. Once the ring is formed, the lasers would gently stir it into motion, making the atoms circulate around it like cars traveling one after another down a single-lane beltway. And just as car tires spin as they travel along the pavement, the atoms' properties would pick up the influence of the world around them - including the effects of gravity from masses just a few micrometers away. The ring would take advantage of one of quantum mechanics' counterintuitive behaviors to help scientists actually measure what its atoms pick up about gravity. The lasers could stir the atoms into what is called a "superposition," meaning in effect they would be both circulating about the ring and simultaneously at a standstill. This superposition of flow and gridlock would help maintain the relationships among the ring's atoms for a few crucial milliseconds after removing their laser constraints, enough time to measure their properties before they scatter. Not only might this quantum traffic jam overcome a difficult gravity measurement challenge, but it might help physicists discard some of the many competing theories about the universe - potentially helping clear up a longstanding traffic jam of ideas. One of the great mysteries of the cosmos is why it is expanding at an apparently accelerating rate. Physicists have suggested an outward force, dubbed "dark energy," causes this expansion, but they have yet to discover its origin. One among many theories is that in the vacuum of space, short-lived virtual particles constantly appear and wink out of existence, and their mutual repulsion creates dark energy's effects. While it's a reasonable enough explanation on some levels, physicists calculate that these particles would create so much repulsive force that it would immediately blow the universe apart. So how can they reconcile observations with the virtual particle idea? "One possibility is that the basic fabric of spacetime only responds to virtual particles that are more than a few micrometers apart," Taylor said, "and that's just the sort of separation we could explore with this ring of cold atoms. So if it turns out you can ignore the effect of particles that operate over these short length scales, you can account for a lot of this unobserved repulsive energy. It would be there, it just wouldn't be affecting anything on a cosmic scale." The research appears in the journal Physical Review Letters. Explore further: What is Nothing? More information: Stephen Ragole et al, Interacting Atomic Interferometry for Rotation Sensing Approaching the Heisenberg Limit, Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.117.203002


Home > Press > Semiconductor-inspired superconducting quantum computing devices: A microwave-free approach to superconducting quantum computing uses design principles gleaned from semiconductor spin qubits Abstract: Builders of future superconducting quantum computers could learn a thing or two from semiconductors, according to a report in Nature Communications this week. By leveraging the good ideas of the natural world and the semiconductor community, researchers may be able to greatly simplify the operation of quantum devices built from superconductors. They call this a "semiconductor-inspired" approach and suggest that it can provide a useful guide to improving superconducting quantum circuits. Superconducting quantum bits, or qubits, are circuits made from superconducting components--such as wires, capacitors or non-linear inductors--that have zero resistance to electrical current. Designing these circuits from scratch offers tremendous flexibility, and has gone a long way toward realizing a full-scale quantum computer. On the other hand, qubits found in semiconductor materials like ultra-pure silicon offer good properties for quantum computing, like long quantum memory times and fast two-qubit gates. These benefits come with constraints, but those constraints have led to creative solutions from the semiconductor community. Yun-Pil Shim and Charles Tahan at the Laboratory for Physical Sciences (1) and the University of Maryland in College Park are exploring whether ideas gleaned from semiconductor qubits may be useful in designing better approaches to superconducting quantum computers. As a first step, they considered applying novel control approaches to state-of-the-art superconducting qubits. They found that they could eliminate one of the most costly overheads for control--microwave sources--by using a solution developed in the semiconductor qubit community. Notably, they found an even more efficient implementation in superconducting qubits, making the approach easier to realize than the semiconductor original. "If the community could mimic the great properties of semiconductor qubits in man-made superconducting circuits, they might be able to have the best of both worlds," Tahan says. "In a large sea of parameters sometimes the best guide is nature." Qubits can be realized in many different physical platforms, such as a superconducting circuit or an electron's spin. Spin is a quantum property of particles that physicists often think of as a small magnet that will point along the direction of an applied magnetic field. A spin can point up or down, corresponding the the 0 or 1 of conventional bits, but it can also point horizontally. This results in a quantum "superposition" of 0 and 1, a key feature of qubits. In some systems, these spin qubits can carry quantum information robustly because they are unaffected by electrical charge, a common source of noise. Spins and superconducting qubits are controlled in similar ways. In both, microwave radiation can drive transitions between the two levels of the qubit allowing for quantum logic gates. But semiconductor spin qubits are also different. They often have weak coupling to the environment, leading to long memory times but slow quantum gates. Additionally, spin qubits are quite small, making them susceptible to inadvertent crosstalk from nearby spins. The semiconductor community has dealt with both problems by developing "all-electrical" approaches to quantum computation that represent one qubit with multiple physical spins. Operations on this "encoded" qubit are performed by pairwise interactions between the physical spins. This requires at least three spins per encoded qubit and a large number of physical pulses to achieve a single encoded gate--a costly overhead for quantum computing, especially when pulses aren't perfect. Shim and Tahan show that an encoded qubit approach can work even better with superconducting qubits. In fact, they show that modern superconducting qubits called transmons or fluxmons, which can be tuned individually, require only two physical qubits per encoded qubit. More importantly, the encoded gate time and gate error don't change much. For example, while a controlled-NOT gate may take roughly 20 qubit-qubit interactions to accomplish in semiconductor spins, Shim and Tahan show that a similar two-qubit gate can be accomplished using only one two-qubit pulse. This means that all quantum logic gates can be performed with fast DC pulses instead of relying on microwave-driven qubit rotations. The authors claim that their scheme can be implemented with current superconducting qubits and control methods, but there are still open questions. In the encoded scheme, initializing qubits may be noisy. And ubiquitous "transmon" qubits maybe be outperformed by newer qubit types like the "fluxmon" or "fluxonium." Quantum computers must preserve qubits from outside interference for as long as a calculation proceeds. Despite rapid progress in the quality of superconducting qubits (qubit lifetimes now surpass 100 microseconds, up from tens of nanoseconds a decade ago), qubit gate error rates are still limited by loss in the metals, insulators, substrates and interfaces that make up these devices. These limitations will also limit the performance of the encoded scheme as proposed, and more progress on these fundamental device issues is still needed. A major goal on the path to a full-scale quantum computer is the demonstration of "fault-tolerant" quantum error correction, where the error of physical quantum gates is reduced by repeated error correction on a "logical" qubit consisting of many physical qubits. Removing the need for microwave control, along with the other benefits of the encoded qubit proposal, could make realizing a logical qubit with superconducting qubits easier. While the authors believe that this work represents an advance, they suggest that additional progress can be made by looking closer still at spin qubits. ### (1) The Laboratory for Physical Sciences is affiliated with the Joint Quantum Institute (JQI). For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

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