Showing papers in "Physical Review A in 2019"
TL;DR: This paper shows how gradients of expectation values of quantum measurements can be estimated using the same, or almost the same the architecture that executes the original circuit, and proposes recipes for the computation of gradients for continuous-variable circuits.
Abstract: An important application for near-term quantum computing lies in optimization tasks, with applications ranging from quantum chemistry and drug discovery to machine learning. In many settings, most prominently in so-called parametrized or variational algorithms, the objective function is a result of hybrid quantum-classical processing. To optimize the objective, it is useful to have access to exact gradients of quantum circuits with respect to gate parameters. This paper shows how gradients of expectation values of quantum measurements can be estimated using the same, or almost the same, architecture that executes the original circuit. It generalizes previous results for qubit-based platforms, and proposes recipes for the computation of gradients of continuous-variable circuits. Interestingly, in many important instances it is sufficient to run the original quantum circuit twice while shifting a single gate parameter to obtain the corresponding component of the gradient. More general cases can be solved by conditioning a single gate on an ancilla.
683 citations
IBM1
TL;DR: A single-number metric, quantum volume, that can be measured using a concrete protocol on near-term quantum computers of modest size, and measured on several state-of-the-art transmon devices, finding values as high as 16.5%.
Abstract: We introduce a single-number metric, quantum volume, that can be measured using a concrete protocol on near-term quantum computers of modest size ($n\ensuremath{\lesssim}50$), and measure it on several state-of-the-art transmon devices, finding values as high as 16. The quantum volume is linked to system error rates, and is empirically reduced by uncontrolled interactions within the system. It quantifies the largest random circuit of equal width and depth that the computer successfully implements. Quantum computing systems with high-fidelity operations, high connectivity, large calibrated gate sets, and circuit rewriting toolchains are expected to have higher quantum volumes. The quantum volume is a pragmatic way to measure and compare progress toward improved system-wide gate error rates for near-term quantum computation and error-correction experiments.
532 citations
TL;DR: Higgott et al. as discussed by the authors introduced a low depth, variational quantum algorithm to sequentially calculate the excited states of general Hamiltonians, and employed the low depth swap test to energetically penalize the ground state, and transform excited states into ground states of modified Hamiltonians.
Abstract: Calculating the energy spectrum of a quantum system is an important task, for example to analyze reaction rates in drug discovery and catalysis. There has been significant progress in developing algorithms to calculate the ground state energy of molecules on near-term quantum computers. However, calculating excited state energies has attracted comparatively less attention, and it is currently unclear what the optimal method is. We introduce a low depth, variational quantum algorithm to sequentially calculate the excited states of general Hamiltonians. Incorporating a recently proposed technique [O. Higgott, D. Wang, and S. Brierley, arXiv:1805.08138], we employ the low depth swap test to energetically penalize the ground state, and transform excited states into ground states of modified Hamiltonians. We use variational imaginary time evolution as a subroutine, which deterministically propagates toward the target eigenstate. We discuss how symmetry measurements can mitigate errors in the swap test step. We numerically test our algorithm on Hamiltonians which encode 3SAT optimization problems of up to 18 qubits, and the electronic structure of the lithium hydride molecule. As our algorithm uses only low depth circuits and variational algorithms, it is suitable for use on near-term quantum hardware.
237 citations
TL;DR: In this article, the breakdown of the bulk-boundary correspondence observed in non-Hermitian systems was addressed by considering the Hamiltonian's singular-value decomposition instead of its eigendecomposition, leading to a natural topological description in terms of a flattened singular decomposition.
Abstract: We address the breakdown of the bulk-boundary correspondence observed in non-Hermitian systems, where open and periodic systems can have distinct phase diagrams. The correspondence can be completely restored by considering the Hamiltonian's singular-value decomposition instead of its eigendecomposition. This leads to a natural topological description in terms of a flattened singular decomposition. This description is equivalent to the usual approach for Hermitian systems and coincides with a recent proposal for the classification of non-Hermitian systems. We generalize the notion of the entanglement spectrum to non-Hermitian systems, and show that the edge physics is indeed completely captured by the periodic bulk Hamiltonian. We exemplify our approach by considering the chiral non-Hermitian Su-Schrieffer-Heger and Chern insulator models. Our work advocates a different perspective on topological non-Hermitian Hamiltonians, paving the way to a better understanding of their entanglement structure.
173 citations
TL;DR: In this paper, Liouvillian superoperators are defined via degeneracies of non-Hermitian Hamiltonians such that at least two eigenfrequencies are identical and the corresponding eigenstates coalesce.
Abstract: Exceptional points (EPs) correspond to degeneracies of open systems. These are attracting much interest in optics, optoelectronics, plasmonics, and condensed matter physics. In the classical and semiclassical approaches, Hamiltonian EPs (HEPs) are usually defined as degeneracies of non-Hermitian Hamiltonians such that at least two eigenfrequencies are identical and the corresponding eigenstates coalesce. HEPs result from continuous, mostly slow, nonunitary evolution without quantum jumps. Clearly, quantum jumps should be included in a fully quantum approach to make it equivalent to, e.g., the Lindblad master equation approach. Thus, we suggest to define EPs via degeneracies of a Liouvillian superoperator (including the full Lindbladian term, LEPs), and we clarify the relations between HEPs and LEPs. We prove two main theorems: Theorem 1 proves that, in the quantum limit, LEPs and HEPs must have essentially different properties. Theorem 2 dictates a condition under which, in the ``semiclassical'' limit, LEPs and HEPs recover the same properties. In particular, we show the validity of Theorem 1 studying systems which have (1) an LEP but no HEPs and (2) both LEPs and HEPs but for shifted parameters. As for Theorem 2, (3) we show that these two types of EPs become essentially equivalent in the semiclassical limit. We introduce a series of mathematical techniques to unveil analogies and differences between the HEPs and LEPs. We analytically compare LEPs and HEPs for some quantum and semiclassical prototype models with loss and gain.
162 citations
TL;DR: In this article, a scheme to create squeezed states of magnons and phonons was proposed by exploring the strong coupling between magnons, and cavity-polaritons, as well as the magnetostrictive interaction.
Abstract: A ferrimagnetic yttrium-iron-garnet sphere placed inside a cavity provides a platform for investigating macroscopic quantum phenomena. By exploring the strong coupling between magnons and cavity-polaritons, as well as the magnetostrictive interaction, a scheme to create squeezed states of magnons and phonons is proposed.
152 citations
TL;DR: In this paper, a symmetry-protected BIC supported by metasurfaces composed of silicon nanodisks is studied and a sharp Fano resonance emerges and demonstrates the excitation of quasi-BIC.
Abstract: Symmetry-protected bound states in the continuum (BICs) are nonradiative states with infinite lifetime and perfect confinement of energy even though lying in the radiation continuum due to the symmetry incompatibility. Herein, we study the symmetry-protected BIC supported by metasurfaces composed of silicon nanodisks. Through adding or removing parts of the nanodisks from the edge, a sharp Fano resonance emerges and demonstrates the excitation of quasi-BIC. Their $Q$ factors exhibit the same dependence on the asymmetry degree with these two opposite operations. Furthermore, from both qualitative and quantitative perspectives, analysis on far-field contributions from multipole moments along different directions combining with near-field distributions explains the evolution from BIC to quasi-BIC. The dominant contributor to the quasi-BIC is illustrated to be the electric quadrupole in the $x\ensuremath{-}y$ plane. Finally, the topological charge carried by the BIC is calculated to be $\ensuremath{-}1$, demonstrating the topological characteristics of our design. Such metasurfaces are robust in nanofabrication. Our results may provide a route for resonators with better performance applied in sensing, switching, nonlinear optics, and so on.
148 citations
TL;DR: Three quantum repeater schemes are proposed and it is found that one of these schemes surpasses the capacity - the highest secret-key rate achievable with direct transmission - by a factor of seven, establishing it as a prime candidate for the first experimental realization of a quantum repeaters.
Abstract: Quantum channels enable the implementation of communication tasks inaccessible to their classical counterparts. The most famous example is the distribution of secret key. However, in the absence of quantum repeaters, the rate at which these tasks can be performed is dictated by the losses in the quantum channel. In practice, channel losses have limited the reach of quantum protocols to short distances. Quantum repeaters have the potential to significantly increase the rates and reach beyond the limits of direct transmission. However, no experimental implementation has overcome the direct transmission threshold. Here, we propose three quantum repeater schemes and assess their ability to generate secret key when implemented on a setup using nitrogen-vacancy (NV) centers in diamond with near-term experimental parameters. We find that one of these schemes - the so-called single-photon scheme, requiring no quantum storage - has the ability to surpass the capacity - the highest secret-key rate achievable with direct transmission - by a factor of 7 for a distance of approximately 9.2 km with near-term parameters, establishing it as a prime candidate for the first experimental realization of a quantum repeater.
136 citations
TL;DR: In this paper, the authors derive an expression that relates the probability to measure a specific photon output pattern from a Gaussian state to the Hafnian matrix function and use it to design a new Gaussian boson sampling protocol.
Abstract: Since the development of boson sampling, there has been a quest to construct more efficient and experimentally feasible protocols to test the computational complexity of sampling from photonic states. In this paper, we interpret and extend the results presented previously [Phys. Rev. Lett. 119, 170501 (2017)]. We derive an expression that relates the probability to measure a specific photon output pattern from a Gaussian state to the Hafnian matrix function and use it to design a Gaussian boson sampling protocol. Then, we discuss the advantages that this protocol has relative to other photonic protocols and the experimental requirements for Gaussian boson sampling. Finally, we relate it to the previously most general protocol, scattershot boson sampling [Phys. Rev. Lett. 113, 100502 (2014)].
129 citations
TL;DR: It is shown how one can optimize the tracking of errors in repeated noisy error correction for the GKP code, and the efficiency of a minimum-energy decoding strategy as a proxy for the path integral evaluation is demonstrated.
Abstract: We examine the performance of the single-mode Gottesman-Kitaev-Preskill (GKP) code and its concatenation with the toric code for a noise model of Gaussian shifts, or displacement errors. We show how one can optimize the tracking of errors in repeated noisy error correction for the GKP code. We do this by examining the maximum-likelihood problem for this setting and its mapping onto a 1D Euclidean path-integral modeling a particle in a random cosine potential. We demonstrate the efficiency of a minimum-energy decoding strategy as a proxy for the path integral evaluation. In the second part of this paper, we analyze and numerically assess the concatenation of the GKP code with the toric code. When toric code measurements and GKP error correction measurements are perfect, we find that by using GKP error information the toric code threshold improves from $10%$ to $14%$. When only the GKP error correction measurements are perfect we observe a threshold at $6%$. In the more realistic setting when all error information is noisy, we show how to represent the maximum likelihood decoding problem for the toric-GKP code as a 3D compact QED model in the presence of a quenched random gauge field, an extension of the random-plaquette gauge model for the toric code. We present a decoder for this problem which shows the existence of a noise threshold at shift-error standard deviation ${\ensuremath{\sigma}}_{0}\ensuremath{\approx}0.243$ for toric code measurements, data errors and GKP ancilla errors. If the errors only come from having imperfect GKP states, then this corresponds to states with just four photons or more. Our last result is a no-go result for linear oscillator codes, encoding oscillators into oscillators. For the Gaussian displacement error model, we prove that encoding corresponds to squeezing the shift errors. This shows that linear oscillator codes are useless for quantum information protection against Gaussian shift errors.
126 citations
TL;DR: In this paper, the authors used nonlinear Compton scattering in laser fields to illustrate how to overcome the problems in LCFA corrections, and derived an LCFA+ which showed an improvement over the LCFA across the whole photon emission spectrum.
Abstract: The locally constant field approximation (LCFA) has to date underpinned the numerical simulation of quantum processes in laser-plasma physics and astrophysics, but its validity has recently been questioned in the parameter regime of current laser experiments. While improvements are needed, literature corrections to the LCFA show inherent problems. Using nonlinear Compton scattering in laser fields to illustrate, we show here how to overcome the problems in LCFA corrections. We derive an LCFA+, which, compared with the full QED result, shows an improvement over the LCFA across the whole photon emission spectrum. We also demonstrate an implementation of our results in the type of numerical code used to design and analyze intense laser experiments.
TL;DR: In this paper, the authors use quantum detector tomography to characterize the qubit readout in terms of measurement positive operator-valued measures (POVMs) on IBM quantum computers IBM Q5 Tenerife and IBM Q 5 Yorktown.
Abstract: We use quantum detector tomography to characterize the qubit readout in terms of measurement positive operator-valued measures (POVMs) on IBM quantum computers IBM Q 5 Tenerife and IBM Q 5 Yorktown. Our results suggest that the characterized detector model deviates from the ideal projectors, ranging from 10 to 40%. This is mostly dominated by classical errors, evident from the shrinkage of arrows from the poles in the corresponding Bloch-vector representations. There are also small deviations that are not ``classical,'' of order 3% or less, represented by the tilt of the arrows from the $z$ axis. Further improvement on this characterization can be made by adopting two- or more-qubit detector models instead of independent single-qubit detectors for all the qubits in one device. We also find evidence indicating correlations in the detector behavior, i.e., the detector characterization is slightly altered (to a few percent) when other qubits and their detectors are in operation. Such peculiar behavior is consistent with characterization from the more sophisticated approach of the gate set tomography. We also discuss how the characterized detectors' POVMs, despite deviation from the ideal projectors, can be used to estimate the ideal detection distribution.
TL;DR: In this paper, the authors show how techniques associated with improved actions, which are heavily utilized in lattice QCD calculations to systematically reduce lattice-spacing artifacts, can be used to reduce the impact of the field digitization in the field theory.
Abstract: Qubit, operator, and gate resources required for the digitization of lattice $\ensuremath{\lambda}{\ensuremath{\phi}}^{4}$ scalar field theories onto quantum computers are considered, building upon the foundational work by Jordan et al. [Quantum Inf. Comput. 14, 1014 (2014); Science 336, 1130 (2012)], with a focus towards noisy intermediate-scale quantum devices. The Nyquist-Shannon sampling theorem, introduced in this context by Macridin et al. [Phys. Rev. A 98, 042312 (2018)] building on the work of Somma [Quantum Inf. Comput. 16, 1125 (2016)], provides a guide with which to evaluate the efficacy of two field-space bases, the eigenstates of the field operator, as used by Jordan et al., and eigenstates of a harmonic oscillator, to describe ($0+1$)- and ($d+1$)-dimensional scalar field theory. We show how techniques associated with improved actions, which are heavily utilized in lattice QCD calculations to systematically reduce lattice-spacing artifacts, can be used to reduce the impact of the field digitization in $\ensuremath{\lambda}{\ensuremath{\phi}}^{4}$, but are found to be inferior to a complete digitization improvement of the Hamiltonian using a quantum Fourier transform. When the Nyquist-Shannon sampling theorem is satisfied, digitization errors scale as $|log|log|{\ensuremath{\epsilon}}_{\mathrm{dig}}|||\ensuremath{\sim}{n}_{Q}$ (number of qubits describing the field at a given spatial site) for the low-lying states, leaving the familiar power-law lattice-spacing and finite-volume effects that scale as $|log|{\ensuremath{\epsilon}}_{\mathrm{latt}}||\ensuremath{\sim}{N}_{Q}$ (total number of qubits in the simulation). For localized (delocalized) field-space wave functions, it is found that ${n}_{Q}\ensuremath{\sim}4(7)$ qubits per spatial lattice site are sufficient to reduce theoretical digitization errors below error contributions associated with approximation of the time-evolution operator and noisy implementation on near-term quantum devices. Only classical computing resources have been used to obtain the results presented in this work.
TL;DR: In this article, the symmetry verification technique was applied to the experimental estimation of the ground-state energy and ground state of the hydrogen molecule in a variational quantum eigensolvers.
Abstract: Variational quantum eigensolvers offer a small-scale testbed to demonstrate the performance of error mitigation techniques with low experimental overhead. We present successful error mitigation by applying the recently proposed symmetry verification technique to the experimental estimation of the ground-state energy and ground state of the hydrogen molecule. A finely adjustable exchange interaction between two qubits in a circuit QED processor efficiently prepares variational ansatz states in the single-excitation subspace respecting the parity symmetry of the qubit-mapped Hamiltonian. Symmetry verification improves the energy and state estimates by mitigating the effects of qubit relaxation and residual qubit excitation, which violate the symmetry. A full-density-matrix simulation matching the experiment dissects the contribution of these mechanisms from other calibrated error sources. Enforcing positivity of the measured density matrix via scalable convex optimization correlates the energy and state estimate improvements when using symmetry verification, with interesting implications for determining system properties beyond the ground-state energy.
TL;DR: In this article, a general theoretical framework for measurement protocols employing statistical correlations of randomized measurements was developed for locally randomized measurements implemented with local random unitaries in quantum lattice models, and the quantum state tomography based on randomized measurements within this framework and the respective scaling of statistical errors with system size.
Abstract: We develop a general theoretical framework for measurement protocols employing statistical correlations of randomized measurements. We focus on locally randomized measurements implemented with local random unitaries in quantum lattice models. In particular, we discuss the theoretical details underlying the recent measurement of the second R\'enyi entropy of highly mixed quantum states consisting of up to ten qubits in a trapped-ion quantum simulator [Brydges et al., Science 364, 260 (2019)]. We generalize the protocol to access the overlap of quantum states, prepared sequentially in one experiment. Furthermore, we discuss proposals for quantum state tomography based on randomized measurements within our framework and the respective scaling of statistical errors with system size.
TL;DR: In this article, the authors analyzed the shortcomings of the local-constant-field approximation in nonlinear Compton scattering at low emitted photon energies for the case of a background plane-wave field.
Abstract: The local-constant-field approximation (LCFA) is an essential theoretical tool for investigating strong-field QED phenomena in background electromagnetic fields with complex spacetime structure. In our previous work [Phys. Rev. A 98, 012134 (2018)2469-992610.1103/PhysRevA.98.012134] we have analyzed the shortcomings of the LCFA in nonlinear Compton scattering at low emitted photon energies for the case of a background plane-wave field. Here, we generalize that analysis to background fields, which can feature a virtually arbitrary spacetime structure. In addition, we provide an explicit and simple implementation of an improved expression of the nonlinear Compton scattering differential probability that solves the main shortcomings of the standard LCFA in the infrared region and is suitable for background electromagnetic fields with arbitrary spacetime structure such as those occurring in particle-in-cell simulations. Finally, we carry out a systematic procedure to calculate the probability of nonlinear Compton scattering per unit of emitted photon light-cone energy and of nonlinear Breit-Wheeler pair production per unit of produced positron light-cone energy beyond the LCFA in a plane-wave background field, which allows us to identify the limits of validity of this approximation quantitatively.
TL;DR: In this article, the accessible entanglement was quantified in a model of interacting spinless fermions on a one-dimensional lattice via exact diagonalization and the density matrix renormalization group.
Abstract: For indistinguishable itinerant particles subject to a superselection rule fixing their total number, a portion of the entanglement entropy under a spatial bipartition of the ground state is due to particle fluctuations between subsystems and thus is inaccessible as a resource for quantum information processing. We quantify the remaining operationally accessible entanglement in a model of interacting spinless fermions on a one-dimensional lattice via exact diagonalization and the density matrix renormalization group. We find that the accessible entanglement exactly vanishes at the first-order phase transition between a Tomonaga-Luttinger liquid and phase separated solid for attractive interactions and is maximal at the transition to the charge density wave for repulsive interactions. Throughout the phase diagram, we discuss the connection between the accessible entanglement entropy and the variance of the probability distribution describing intrasubregion particle-number fluctuations.
TL;DR: In this article, the ground state energies of light nuclei including triton (H3), He3, and alpha particle (He4) are computed using an all-optical quantum frequency processor.
Abstract: Simulating complex many-body quantum phenomena is a major scientific impetus behind the development of quantum computing, and a range of technologies are being explored to address such systems. We present the results of the largest photonics-based simulation to date, applied in the context of subatomic physics. Using an all-optical quantum frequency processor, the ground-state energies of light nuclei including the triton (H3), He3, and the alpha particle (He4) are computed. Complementing these calculations and utilizing a 68-dimensional Hilbert space, our photonic simulator is used to perform subnucleon calculations of the two- and three-body forces between heavy mesons in the Schwinger model. This work is a first step in simulating subatomic many-body physics on quantum frequency processors - augmenting classical computations that bridge scales from quarks to nuclei.
TL;DR: Chomaz et al. as discussed by the authors performed numerical simulations of a dipolar Bose-Einstein condensate (BEC) in a tubular, periodic confinement at $T=0$ within density functional theory, where the beyond-mean-field correction to the ground-state energy is included in the local density approximation.
Abstract: Motivated by a recent experiment [L. Chomaz et al., Nat. Phys. 14, 442 (2018)], we perform numerical simulations of a dipolar Bose-Einstein condensate (BEC) in a tubular, periodic confinement at $T=0$ within density functional theory, where the beyond-mean-field correction to the ground-state energy is included in the local density approximation. We study the excitation spectrum of the system by solving the corresponding Bogoliubov--de Gennes equations. The calculated spectrum shows a roton minimum, and the roton gap decreases by reducing the effective scattering length. As the roton gap disappears, the system spontaneously develops a periodic linear structure formed by denser clusters of atomic dipoles immersed in a dilute superfluid background. This structure shows the hallmarks of a supersolid system, i.e., (i) a finite nonclassical translational inertia along the tube axis and (ii) the appearance of two gapless modes, i.e., a phonon mode associated with density fluctuations and resulting from the translational discrete symmetry of the system, and a Nambu-Goldstone gapless mode corresponding to phase fluctuations, resulting from the spontaneous breaking of the gauge symmetry. A further decrease in the scattering length eventually leads to the formation of a periodic linear array of self-bound droplets.
TL;DR: In this article, the authors considered a quantum Otto heat engine operating at finite time and observed an interference-like effect between the residual coherence and the coherence generated in the subsequent finite-time stroke.
Abstract: The working substance fueling a quantum heat engine may contain coherence in its energy basis, depending on the dynamics of the engine cycle. In some models of quantum Otto heat engines, energy coherence has been associated with entropy production and quantum friction. We considered a quantum Otto heat engine operating at finite time. Coherence is generated and the working substance does not reach thermal equilibrium after interacting with the hot heat reservoir, leaving the working substance in a state with residual energy coherence. We observe an interferencelike effect between the residual coherence (after the incomplete thermalization) and the coherence generated in the subsequent finite-time stroke. We introduce analytical expressions highlighting the role of coherence and examine how this dynamical interference effect influences the engine performance. Additionally, in this scenario in which coherence is present along the cycle, we argue that the careful tuning of the cycle parameters may exploit this interference effect and make coherence acts like a dynamical quantum lubricant. To illustrate this, we numerically consider an experimentally feasible example and compare the engine performance to the performance of a similar engine where the residual coherence is completely erased, ruling out the dynamical interference effect.
TL;DR: In this paper, the spatial and temporal properties of a pulsed optical beam (or wave packet) can be correlated with spatiotemporal spectral correlations, which can render the wave packet propagation invariant along the propagation axis.
Abstract: Introducing correlations between the spatial and temporal degrees of freedom of a pulsed optical beam (or wave packet) can profoundly alter its propagation in free space Indeed, appropriate spatiotemporal spectral correlations can render the wave packet propagation-invariant: the spatial and temporal profiles remain unchanged along the propagation axis The spatiotemporal spectral locus of any such wave packet lies at the intersection of the light cone with tilted spectral hyperplanes We investigate $(2+1)\mathrm{D}$ propagation-invariant ``space-time'' light sheets and identify ten classes categorized according to the magnitude and sign of their group velocity and the nature of their spatial spectrum---whether the low spatial frequencies are physically allowed or forbidden according to their compatibility with causal excitation and propagation We experimentally synthesize and characterize all ten classes using an experimental strategy capable of synthesizing space-time wave packets that incorporate arbitrary spatiotemporal spectral correlations
TL;DR: In this paper, a comprehensive Fisher information analysis is put forward to understand and achieve the limits in imaging resolution, and it is shown that for any incoherence sources, a 1D or 2D image can be precisely estimated up to its second moment.
Abstract: A comprehensive Fisher information analysis is put forward to understand and achieve the limits in imaging resolution. It is shown that for any incoherence sources, a 1D or 2D image can be precisely estimated up to its second moment.
TL;DR: In this paper, Harrow et al. showed that the quantum linear system algorithm can be applied to Gaussian process regression, leading to an exponential reduction in computation time in some instances.
Abstract: Gaussian processes (GPs) are a widely used model for regression problems in supervised machine learning. Implementation of GP regression typically requires $O({n}^{3})$ logic gates. We show that the quantum linear systems algorithm [Harrow et al., Phys. Rev. Lett. 103, 150502 (2009)] can be applied to Gaussian process regression (GPR), leading to an exponential reduction in computation time in some instances. We show that even in some cases not ideally suited to the quantum linear systems algorithm, a polynomial increase in efficiency still occurs.
TL;DR: In this article, correlated photon pair sources are used for the improved quantum-level detection of a target in the presence of a noise background, which can provide an enhanced signal-to-noise ratio when compared to a classical light source of the same intensity.
Abstract: We investigate the use of correlated photon pair sources for the improved quantum-level detection of a target in the presence of a noise background. Photon pairs are generated by spontaneous four-wave mixing, one photon from each pair (the herald) is measured locally while the other (the signal) is sent to illuminate the target. Following diffuse reflection from the target, the signal photons are detected by a receiver and nonclassical timing correlations between the signal and herald are measured in the presence of a configurable background noise source. Quantum correlations from the photon pair source can be used to provide an enhanced signal-to-noise ratio when compared to a classical light source of the same intensity.
TL;DR: In this article, the presence of collective excitations, i.e., collective excitation, is thoroughly analyzed in a nonperturbative fashion, showing the consequences of subradiance in the dynamics of such an open, many-body system.
Abstract: One possible route for improving the precision of optical clocks consists of using a set of them conveniently organized in an optical lattice. The presence of interaction, i.e., collective excitations, is thoroughly analyzed in a nonperturbative fashion, showing the consequences of subradiance in the dynamics of such an open, many-body system.
TL;DR: The proposed framework provides a method to search for the Gaussian circuit and measurement pattern that produces a target non-Gaussian state with optimal fidelity and success probability and has potential far-reaching implications for the generation of bosonic error-correction codes that require non- Gaussian states, resource states for the implementation of non-gaussian gates needed for universal quantum computation, among other applications requiring non-GAussianity.
Abstract: Generation of high-fidelity photonic non-Gaussian states is a crucial ingredient for universal quantum computation using continuous-variable platforms, yet it remains a challenge to do this efficiently. We present a general framework for a probabilistic production of multimode non-Gaussian states by measuring a few modes of multimode Gaussian states via photon-number-resolving detectors. We use Gaussian elements consisting of squeezed displaced vacuum states and interferometers, the only non-Gaussian elements consisting of photon-number-resolving detectors. We derive analytic expressions for the output Wigner function, and the probability of generating the states in terms of the mean and the covariance matrix of the Gaussian state and the photon detection pattern. We find that the output states can be written as a Fock-basis superposition state followed by a Gaussian gate, and we derive explicit expressions for these parameters. These analytic expressions show exactly what non-Gaussian states can be generated by this probabilistic scheme. Further, it provides a method to search for the Gaussian circuit and measurement pattern that produce a target non-Gaussian state with optimal fidelity and success probability. We present specific examples such as the generation of cat states, ON states, Gottesman-Kitaev-Preskill states, NOON states, and bosonic-code states. The proposed framework has potentially far-reaching implications for the generation of bosonic error-correction codes that require non-Gaussian states and resource states for the implementation of non-Gaussian gates needed for universal quantum computation, among other applications requiring non-Gaussianity. The tools developed here could also prove useful for the quantum resource theory of non-Gaussianity.
TL;DR: In this paper, the authors proposed a non-Gaussian continuous-variable quantum key distribution (CVQKD) by using quantum catalysis (QC), which is an intriguing nonGaussian operation in essence that can be implemented with current technologies.
Abstract: The non-Gaussian operation can be used not only to enhance and distill the entanglement between Gaussian entangled states, but also to improve the performance of quantum communications. In this paper, we propose a non-Gaussian continuous-variable quantum key distribution (CVQKD) by using quantum catalysis (QC), which is an intriguing non-Gaussian operation in essence that can be implemented with current technologies. We perform quantum catalysis on both ends of the Einstein-Podolsky-Rosen pair prepared by a sender, Alice, and find that for the single-photon QC-CVQKD, the bilateral symmetrical quantum catalysis performs better than the single-side quantum catalysis. Attributing to characteristics of an integral within an ordered product of operators, we find that the quantum-catalysis operation can improve the entanglement property of Gaussian entangled states by enhancing the success probability of non-Gaussian operation, leading to the improvement of the QC-CVQKD system. As a comparison, the QC-CVQKD system involving zero-photon and single-photon quantum catalysis outperforms the previous non-Gaussian CVQKD scheme via photon subtraction in terms of a secret key rate, maximal transmission distance, and tolerable excess noise.
TL;DR: This paper shows that this hard problem can be translated to a supervised machine learning task by thinking of the time-ordered quantum evolution as a layer-ordered neural network (NN), and opens up a door through which a family of robust control algorithms can be developed.
Abstract: Robust and high-precision quantum control is extremely important but challenging for the functionalization of scalable quantum computation. In this paper, we show that this hard problem can be translated to a supervised machine learning task by thinking of the time-ordered quantum evolution as a layer-ordered neural network (NN). The seeking of robust quantum controls is then equivalent to training a highly generalizable NN, to which numerous tuning skills matured in machine learning can be transferred. This opens up a door through which a family of robust control algorithms can be developed. We exemplify such potential by introducing the commonly used trick of batch-based optimization, and the resulting batch-based gradient algorithm is numerically shown to be able to remarkably enhance the control robustness while maintaining high fidelity.
TL;DR: In this paper, the authors present the characterization of thermorefractive noise in photonic chip-based silicon-nitride micro-resonators and show that the dominant thermal noise source is the dominant noise source in the platform.
Abstract: Thermodynamic noise places a fundamental limit on the frequency stability of dielectric optical resonators. Here, we present the characterization of thermorefractive noise in photonic-chip-based silicon-nitride (${\text{Si}}_{3}{\text{N}}_{4}$) microresonators and show that thermorefractive noise is the dominant thermal noise source in the platform. We employed balanced homodyne detection to measure the thermorefractive noise spectrum of microresonators of different diameters. The measurements are in good agreement with theoretical models and finite element method simulations. Our characterization sets quantitative bounds on the scaling and absolute magnitude of thermal noise in photonic-chip-based microresonators. An improved understanding of thermorefractive noise can prove valuable in the design considerations and performance limitations of future photonic integrated devices.
TL;DR: In this article, an experimental observation of an electromagnetic bound state in the radiation continuum in a one-dimensional array of dielectric particles is reported, where the authors demonstrate how a resonant state in near the center of the Brillouin zone turns into a symmetry-protected BIC with increase in the number of the disks.
Abstract: The existence of bound states in the continuum (BIC) manifests a general wave phenomenon first predicted in quantum mechanics by John von Neumann and Eugene Wigner [J. von Neumann and E. Wigner, Phys. Z. 30, 465 (1929)]. Today it is being actively explored in photonics, radiophysics, acoustics, and hydrodynamics. We report an experimental observation of an electromagnetic bound state in the radiation continuum in a one-dimensional array of dielectric particles. By measurement of the transmission spectra of the ceramic disk chain at GHz frequencies, we demonstrate how a resonant state in the vicinity of the center of the Brillouin zone turns into a symmetry-protected BIC with increase in the number of the disks. We estimate a number of the disks when the radiation losses become negligible in comparison to material absorption and, therefore, the chain could be considered practically as infinite. The presented analysis is supplemented by measurements of the near fields of the symmetry-protected BIC. All measurements are in a good agreement with the results of the numerical simulation and analytical model based on a tight-binding approximation. The obtained results provide useful guidelines for practical implementations of structures with bound states in the continuum that opens up horizons for the development of optical and radio-frequency metadevices.