Showing papers in "Physical Review X in 2017"
TL;DR: This work proposes a variational method involving closely integrated classical and quantum coprocessors and finds that it is efficient and appears to be fundamentally more robust against error accumulation than a more conventional optimised Trotterisation technique.
Abstract: Quantum computers will need to be tolerant to errors introduced by noise, but current proposals estimate that the number of qubits required for error correction will be many orders of magnitude larger than the number needed for useful computation. A new proposal uses a classical-quantum hybrid scheme to implement simple error-tolerant quantum processors with relatively few resources.
558 citations
TL;DR: In this article, the authors show that encapsulation of monolayer MoS2 in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as they measure in photoluminescence and reflectivity a FWHM down to 2 meV at T=4
Abstract: The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS2 an exciting 2D material for fundamental physics and optoelectronics applications. But, so far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the better-characterized ML materials MoSe2 and WSe2. In this work, we show that encapsulation of ML MoS2 in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at T=4 K. Narrow optical transition linewidths are also observed in encapsulated WS2, WSe2, and MoSe2 MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.
540 citations
TL;DR: In this article, the authors study how entanglement grows with time in a many-body system, for example, after a quantum quench, and provide simple new heuristics for the entangler tsunami.
Abstract: Characterizing how entanglement grows with time in a many-body system, for example, after a quantum quench, is a key problem in nonequilibrium quantum physics. We study this problem for the case of random unitary dynamics, representing either Hamiltonian evolution with time-dependent noise or evolution by a random quantum circuit. Our results reveal a universal structure behind noisy entanglement growth, and also provide simple new heuristics for the “entanglement tsunami” in Hamiltonian systems without noise. In 1D, we show that noise causes the entanglement entropy across a cut to grow according to the celebrated Kardar-Parisi-Zhang (KPZ) equation. The mean entanglement grows linearly in time, while fluctuations grow like ðtimeÞ 1 = 3 and are spatially correlated over a distance ∝ ðtimeÞ 2 = 3 . We derive KPZ universal behavior in three complementary ways, by mapping random entanglement growth to (i) a stochastic model of a growing surface, (ii) a “minimal cut” picture, reminiscent of the Ryu-Takayanagi formula in holography, and (iii) a hydrodynamic problem involving the dynamical spreading of operators. We demonstrate KPZ universality in 1D numerically using simulations of random unitary circuits. Importantly, the leading-order time dependence of the entropy is deterministic even in the presence of noise, allowing us to propose a simple coarse grained minimal cut picture for the entanglement growth of generic Hamiltonians, even without noise, in arbitrary dimensionality. We clarify the meaning of the “velocity” of entanglement growth in the 1D entanglement tsunami. We show that in higher dimensions, noisy entanglement evolution maps to the well-studied problem of pinning of a membrane or domain wall by disorder.
516 citations
TL;DR: A brain-inspired computer made with optoelectronic parts runs faster thanks to a hardware redesign, recognizing simple speech at the rate of 1 million words per second.
Abstract: A brain-inspired computer made with optoelectronic parts runs faster thanks to a hardware redesign, recognizing simple speech at the rate of 1 million words per second
375 citations
TL;DR: In this article, a new method was proposed to categorize all topologically distinct electronic band structures in materials with only crystal symmetries for any number of physically relevant dimensions, including the dimension of the crystal lattice.
Abstract: The celebrated ``tenfold way'' provides a scheme for categorizing general topological states of matter, but it does not take into account the crystal symmetries that always exist in real materials. A new method extends this organization to allow the categorization of all topologically distinct electronic band structures in materials with only crystal symmetries for any number of physically relevant dimensions.
356 citations
TL;DR: In this paper, it was shown that in ordered atomic arrays in free space, subradiant states acquire an elegant interpretation in terms of optical modes that are guided by the array, which only emit due to scattering from the ends of the finite system.
Abstract: A central goal within quantum optics is to realize efficient, controlled interactions between photons and atomic media. A fundamental limit in nearly all applications based on such systems arises from spontaneous emission, in which photons are absorbed by atoms and then rescattered into undesired channels. In typical theoretical treatments of atomic ensembles, it is assumed that this rescattering occurs independently, and at a rate given by a single isolated atom, which in turn gives rise to standard limits of fidelity in applications such as quantum memories for light or photonic quantum gates. However, this assumption can in fact be dramatically violated. In particular, it has long been known that spontaneous emission of a collective atomic excitation can be significantly suppressed through strong interference in emission between atoms. While this concept of “subradiance” is not new, thus far the techniques to exploit the effect have not been well understood. In this work, we provide a comprehensive treatment of this problem. First, we show that in ordered atomic arrays in free space, subradiant states acquire an elegant interpretation in terms of optical modes that are guided by the array, which only emit due to scattering from the ends of the finite system. We also go beyond the typically studied regime of a single atomic excitation and elucidate the properties of subradiant states in the many-excitation limit. Finally, we introduce the new concept of “selective radiance.” Whereas subradiant states experience a reduced coupling to all optical modes, selectively radiant states are tailored to simultaneously radiate efficiently into a desired channel while scattering into undesired channels is suppressed, thus enabling an enhanced atom-light interface. We show that these states naturally appear in chains of atoms coupled to nanophotonic structures, and we analyze the performance of photon storage exploiting such states. We find numerically that selectively radiant states allow for a photon storage error that scales exponentially better with the number of atoms than previously known bounds.
308 citations
TL;DR: In this paper, an artificial neural network is trained to identify changes in the collective magnetic properties of electrons on a lattice and predict trends in the transition when some of the electrons are removed.
Abstract: Machine learning has strong potential as a tool for understanding how to classify phases in condensed matter physics. A new investigation shows that an artificial neural network can be trained to identify changes in the collective magnetic properties of electrons on a lattice and predict trends in the transition when some of the electrons are removed.
300 citations
TL;DR: In this paper, a new mathematical model was proposed to reconcile this behavior with other hallmarks of Brownian motion, such as the random movement of microscopic particles in a fluid and the Gaussian probability of finding a particle at a particular place at a specific time.
Abstract: Brownian motion---the random movement of microscopic particles in a fluid---usually gives rise to a Gaussian probability of finding a particle at a particular place at a specific time. But in some situations, this probability behaves differently. A new mathematical model shows how to reconcile this behavior with other hallmarks of Brownian motion.
299 citations
TL;DR: In this paper, nonequilibrium phase transitions are observed in a circuit quantum electrodynamics lattice, paving the way for greater insight into exotic materials, where the physical properties of a system change suddenly.
Abstract: Nonequilibrium phase transitions, where the physical properties of a system change suddenly, are of fundamental importance in condensed matter physics but are not well understood. Such phase transitions are now observed in a circuit quantum electrodynamics lattice, paving the way for greater insight into exotic materials.
292 citations
TL;DR: In this article, the authors proposed a unified nanoscale theory by showing how externally prepared systems (e.g., atoms in an optical cavity or DNA bases in an enzyme reaction) that interact with a nanoscopic device can be a source of nonequilbrium free energy.
Abstract: Nanomachines are subject to random thermal and quantum fluctuations that are not captured by traditional thermodynamic theory. A new theoretical investigation offers a step toward a unified nanoscale theory by showing how externally prepared systems (e.g., atoms in an optical cavity or DNA bases in an enzyme reaction) that interact with a nanoscopic device can be a source of nonequilbrium free energy.
284 citations
TL;DR: Stochastic thermodynamics as mentioned in this paper extends the traditional laws of thermodynamics to microscopic systems where thermal and quantum fluctuations cannot be ignored, and a review summarizes progress in this field with a look at several experimental and theoretical results and a look toward potential applications in biology and nanotechnology.
Abstract: Stochastic thermodynamics extends the traditional laws of thermodynamics to microscopic systems where thermal and quantum fluctuations cannot be ignored. This review summarizes progress in this field with a look at several experimental and theoretical results and a look toward potential applications in biology and nanotechnology.
TL;DR: A new entanglement analysis reveals crucial properties of the data structures that encode quantum states in a neural network, opening new inroads in applying machine learning to quantum many-body physics.
Abstract: Machine learning has recently gained attention as a possible way to understand phase transitions in many-body quantum systems. A new entanglement analysis reveals crucial properties of the data structures that encode quantum states in a neural network, opening new inroads in applying machine learning to quantum many-body physics.
TL;DR: Two experimental groups have taken a step towards observing the ''scrambling'' of information that occurs as a many-body quantum system thermalizes as discussed by the authors, but their experimental results were limited.
Abstract: Two experimental groups have taken a step towards observing the ``scrambling'' of information that occurs as a many-body quantum system thermalizes.
TL;DR: In this paper, a duality-based analysis of quantum electrodynamics with a deconfined quantum critical point is presented. But the analysis is limited to the case of quantum critical points.
Abstract: Different theories can be used to describe the same behavior in quantum matter, a concept known as duality. A new analysis uses duality to connect ideas in quantum electrodynamics with a type of quantum magnet known as a deconfined quantum critical point and reveal new properties of both.
TL;DR: In this article, a tiny engine can surpass the Carnot limit of efficiency when researchers engineer the thermal properties of the environment, such as temperature, humidity, and humidity of the air.
Abstract: A tiny engine can surpass the Carnot limit of efficiency when researchers engineer the thermal properties of the environment.
TL;DR: In this article, strong evidence is found that a particular class of exotic states of matter called a Dirac spin liquid is realized in a popular model of numerous magnets, which has long eluded condensed-matter physicists.
Abstract: Realizing an exotic phase of matter known as a quantum spin liquid has long eluded condensed-matter physicists. Now strong evidence is found that a particular class of exotic states of matter called a Dirac spin liquid is realized in a popular model of numerous magnets.
TL;DR: In this article, the authors experimentally explore the thermalization dynamics of a localized system in the presence of controlled dissipation and find that photon scattering results in a stretched exponential decay of an initial density pattern with a rate that depends linearly on the scattering rate.
Abstract: In the presence of disorder, an interacting closed quantum system can undergo many-body localization (MBL) and fail to thermalize. However, over long times, even weak couplings to any thermal environment will necessarily thermalize the system and erase all signatures of MBL. This presents a challenge for experimental investigations of MBL since no realistic system can ever be fully closed. In this work, we experimentally explore the thermalization dynamics of a localized system in the presence of controlled dissipation. Specifically, we find that photon scattering results in a stretched exponential decay of an initial density pattern with a rate that depends linearly on the scattering rate. We find that the resulting susceptibility increases significantly close to the phase transition point. In this regime, which is inaccessible to current numerical studies, we also find a strong dependence on interactions. Our work provides a basis for systematic studies of MBL in open systems and opens a route towards extrapolation of closed-system properties from experiments.
TL;DR: In this article, a many-body localized phase in a two-dimensional system and how a system transitions between this phase and a normal thermal phase were investigated in one-dimensional systems.
Abstract: While many-body localization is well understood in one-dimensional systems, its behavior in two or more dimensions is largely unknown. New experiments hint at a many-body localized phase in a two-dimensional system and provide insight into how a system transitions between this phase and a normal thermal phase.
TL;DR: In this paper, a quantum version of Reichenbach's principle is proposed for quantum causal models, in which the causal relationships among variables constrain the form of their joint probability distribution.
Abstract: Reichenbach’s principle asserts that if two observed variables are found to be correlated, then there should be a causal explanation of these correlations. Furthermore, if the explanation is in terms of a common cause, then the conditional probability distribution over the variables given the complete common cause should factorize. The principle is generalized by the formalism of causal models, in which the causal relationships among variables constrain the form of their joint probability distribution. In the quantum case, however, the observed correlations in Bell experiments cannot be explained in the manner Reichenbach’s principle would seem to demand. Motivated by this, we introduce a quantum counterpart to the principle. We demonstrate that under the assumption that quantum dynamics is fundamentally unitary, if a quantum channel with input
A
and outputs
B
and
C
is compatible with
A
being a complete common cause of
B
and
C
, then it must factorize in a particular way. Finally, we show how to generalize our quantum version of Reichenbach’s principle to a formalism for quantum causal models and provide examples of how the formalism works.
TL;DR: In this paper, a new theoretical analysis explores the transition between MBL and thermal phases and finds that the transition is driven by the growth of a network of quantum entanglement.
Abstract: Some quantum systems can enter a many-body localized (MBL) phase, where the particles do not settle into thermal equilibrium but remain stuck in some initial state. A new theoretical analysis explores the transition between MBL and thermal phases and finds that the transition is driven by the growth of a network of quantum entanglement.
TL;DR: In this article, a system combining a quantum dot and a superconducting cavity achieves the strongest light-matter coupling for this type of hybrid system, where the quantum dot is replaced by a superconductor.
Abstract: A system combining a quantum dot and a superconducting cavity achieves the strongest light-matter coupling for this type of hybrid system.
TL;DR: In this article, a comprehensive analysis of eleven glass-forming models is presented to demonstrate that both structural ordering and the dramatic increase of relaxation times at low temperatures can be efficiently tackled using carefully designed models of size polydisperse supercooled liquids together with an efficient Monte Carlo algorithm where translational particle displacements are complemented by swaps of particle pairs.
Abstract: Successful computer studies of glass-forming materials need to overcome both the natural tendency to structural ordering and the dramatic increase of relaxation times at low temperatures. We present a comprehensive analysis of eleven glass-forming models to demonstrate that both challenges can be efficiently tackled using carefully designed models of size polydisperse supercooled liquids together with an efficient Monte Carlo algorithm where translational particle displacements are complemented by swaps of particle pairs. We study a broad range of size polydispersities, using both discrete and continuous mixtures, and we systematically investigate the role of particle softness, attractivity and non-additivity of the interactions. Each system is characterized by its robustness against structural ordering and by the efficiency of the swap Monte Carlo algorithm. We show that the combined optimisation of the potential's softness, polydispersity and non-additivity leads to novel computer models with excellent glass-forming ability. For such models, we achieve over ten orders of magnitude gain in the equilibration timescale using the swap Monte Carlo algorithm, thus paving the way to computational studies of static and thermodynamic properties under experimental conditions. In addition, we provide microscopic insights into the performance of the swap algorithm which should help optimizing models and algorithms even further.
TL;DR: In this article, a variety of modern many-body methods are employed, with exhaustive cross-checks and validation, to reach the continuous space limit and the thermodynamic limit of an infinite chain of hydrogen atoms.
Abstract: We present numerical results for the equation of state of an infinite chain of hydrogen atoms. A variety of modern many-body methods are employed, with exhaustive cross-checks and validation. Approaches for reaching the continuous space limit and the thermodynamic limit are investigated, proposed, and tested. The detailed comparisons provide a benchmark for assessing the current state of the art in many-body computation, and for the development of new methods. The ground-state energy per atom in the linear chain is accurately determined versus bond length, with a confidence bound given on all uncertainties.
TL;DR: In this paper, a new framework uses informatics and phonon transport calculations to accelerate the design process and reveal nonintuitive structures that are more effective than their traditional counterparts, which is a key element in controlling solid heat conduction.
Abstract: Phonon transport---the movement of vibrational wave packets in a solid---in nanostructures is a key element in controlling solid heat conduction, but it remains a complex design challenge. A new framework uses informatics and phonon transport calculations to greatly accelerate the design process and reveals nonintuitive structures that are more effective than their traditional counterparts.
TL;DR: In this paper, angle-multiplexed metasurfaces, composed of reflective high-contrast dielectric U-shaped meta-atoms, whose response under illumination from different angles can be controlled independently.
Abstract: The angular response of thin diffractive optical elements is highly correlated. For example, the angles of incidence and diffraction of a grating are locked through the grating momentum determined by the grating period. Other diffractive devices, including conventional metasurfaces, have a similar angular behavior due to the fixed locations of the Fresnel zone boundaries and the weak angular sensitivity of the meta-atoms. To alter this fundamental property, we introduce angle-multiplexed metasurfaces, composed of reflective high-contrast dielectric U-shaped meta-atoms, whose response under illumination from different angles can be controlled independently. This enables flat optical devices that impose different and independent optical transformations when illuminated from different directions, a capability not previously available in diffractive optics.
TL;DR: In this article, the magnetization in Rydberg-dressed Ising spin chains of atoms trapped in an optical lattice is identified at up to about ten times the characteristic time scale set by the interactions.
Abstract: Coherent many-body quantum dynamics lies at the heart of quantum simulation and quantum computation. Both require coherent evolution in the exponentially large Hilbert space of an interacting many-body system. To date, trapped ions have defined the state of the art in terms of achievable coherence times in interacting spin chains. Here, we establish an alternative platform by reporting on the observation of coherent, fully interaction-driven quantum revivals of the magnetization in Rydberg-dressed Ising spin chains of atoms trapped in an optical lattice. We identify partialmany-body revivals at up to about ten times the characteristic time scale set by the interactions. At the same time, single-site-resolved correlation measurements link the magnetization dynamics with interspin correlations appearing at different distances during the evolution. These results mark an enabling step towards the implementation of Rydberg-atom-based quantum annealers, quantum simulations of higher-dimensional complex magnetic Hamiltonians, and itinerant long-range interacting quantum matter.
TL;DR: In this article, the axion-wind spin-precession frequencies of stored ultracold neutrons and 199Hg atoms were analyzed for axion induced oscillating electric dipole moment of the neutron and an axion wind spin precession effect.
Abstract: We report on a search for ultra-low-mass axion-like dark matter by analysing the ratio of the spinprecession frequencies of stored ultracold neutrons and 199Hg atoms for an axion-induced oscillating electric dipole moment of the neutron and an axion-wind spin-precession effect. No signal consistent with dark matter is observed for the axion mass range 1024 eV ma 10 17 eV. Our null result sets the first laboratory constraints on the coupling of axion dark matter to gluons, which improve on astrophysical limits by up to 3 orders of magnitude, and also improves on previous laboratory constraints on the axion coupling to nucleons by up to a factor of 40.
TL;DR: In this paper, a new analysis reveals the optimal method for implementing a VQA, which could lead to improvements in future quantum computing techniques, leading to improved quantum computing technique.
Abstract: Variational quantum algorithms (VQAs) mix quantum machines with classical optimizers to solve complex computational problems. A new analysis reveals the optimal method for implementing a VQA, which could lead to improvements in future quantum computing techniques.
TL;DR: An implementation of an invertible gate to bring out the key role of a three-terminal building block to enable the construction of correlated p-bit networks and establishes this result with examples including a 4-bit multiplier which in inverted mode functions as a factorizer.
Abstract: Digital electronics are based on deterministic units called bits that can have one of two values, 0 and 1. New theoretical work suggests that circuits built out of probabilistic units that fluctuate randomly in value between 0 and 1 can be used to perform multiple functions: A multiplier, for example, can also function as a factorizer.
TL;DR: In this article, a tunable, miniaturized Fabry-P\'erot microcavity was used to solve the problem of low rates of entanglement between the defect spin and the photons they produce hamper the mediation of long distance connections.
Abstract: Nitrogen-vacancy centers---a type of atom-sized defect in diamonds---have potential for use as quantum bits in quantum information technologies. However, low rates of entanglement between the defect spin and the photons they produce hamper the mediation of long-distance connections. A new experiment shows a way around this limitation by employing a tunable, miniaturized Fabry-P\'erot microcavity.