Showing papers in "Physical review applied in 2022"

Experimental High-Dimensional Greenberger-Horne-Zeilinger Entanglement with Superconducting Transmon Qutrits

Abstract: Multipartite entanglement is one of the core concepts in quantum information science with broad applications that span from condensed matter physics to quantum physics foundation tests. Although its most studied and tested forms encompass two-dimensional systems, current quantum platforms technically allow the manipulation of additional quantum levels. We report the experimental demonstration and certification of a high-dimensional multipartite entangled state in a superconducting quantum processor. We generate the three-qutrit Greenberger-Horne-Zeilinger state by designing the necessary pulses to perform high-dimensional quantum operations. We obtain the fidelity of $76\mathrm{%}\ifmmode\pm\else\textpm\fi{}1\mathrm{%}$, proving the generation of a genuine three-partite and three-dimensional entangled state. To this date, only photonic devices have been able to create and certify the entanglement of these high-dimensional states. Our work demonstrates that another platform, superconducting systems, is ready to exploit genuine high-dimensional entanglement and that a programmable quantum device accessed on the cloud can be used to design and execute experiments beyond binary quantum computation.

Phase-Binarized Spin Hall Nano-Oscillator Arrays: Towards Spin Hall Ising Machines

Abstract: Ising machines (IMs) are physical systems designed to find solutions to combinatorial optimization (CO) problems mapped onto the IM via the coupling strengths between its binary spins. Using its intrinsic dynamics and different annealing schemes, the IM relaxes over time to its lowest-energy state, which is the solution to the CO problem. IMs have been implemented on different platforms, and interacting nonlinear oscillators are particularly promising candidates. Here we demonstrate a pathway towards an oscillator-based IM using arrays of nanoconstriction spin Hall nano-oscillators (SHNOs). We show how SHNOs can be readily phase binarized and how their resulting microwave power corresponds to well-defined global phase states. To distinguish between degenerate states, we use phase-resolved Brillouin-light-scattering microscopy and directly observe the individual phase of each nanoconstriction. Micromagnetic simulations corroborate our experiments and confirm that our proposed IM platform can solve CO problems, showcased by how the phase states of a $2\ifmmode\times\else\texttimes\fi{}2$ SHNO array are solutions to a modified max-cut problem. Compared with the commercially available D-Wave ${\mathrm{Advantage}}^{\mathrm{TM}}$, our architecture holds significant promise for faster sampling, substantially reduced power consumption, and a dramatically smaller footprint.

Enhanced High-Temperature Thermoelectric Performance by Strain Engineering in BiOCl

Abstract: Semiconductor BiOCl has a layered structure with ultralow lattice thermal conductivity [Q.D. Gibson et al., Science 373, 1017--1022 (2021)] and has potential applications in the field of thermoelectric materials. In the present study, the thermoelectric properties of BiOCl crystals are accurately predicted using the first-principles calculation combined with Boltzmann transport theory. The dimensionless figure of merit ($ZT$) of $p$-type BiOCl is found to be 0.2 at room temperature, reaching 1.1 at 800 K. In addition, applying in-plane biaxial tensile strain ${ϵ}_{xy}$ can lead to a further increase in the value of $ZT$ to 1.9 at 800 K. This means that $p$ BiOCl is an excellent high-temperature thermoelectric material. And this is due to the fact that biaxial strain drastically reduces the lattice thermal conductivity and the shift of the valence band toward the Fermi level can optimize the carrier concentration. Thus, the present work paves a way for the design of adjustable high-temperature thermoelectric materials.

Robust Far-Field Optical Image Transmission with Structured Random Light Beams

Abstract: The ability to overcome the adverse effect induced by the obstacles within the transmission link is a central challenge in long-distance optical image transmission and is significantly crucial in free-space optical communication. In this work, we introduce an efficient protocol to realize the robust far-field optical image-signal transmission by modulating the spatial-coherence structure of a partially coherent random light source. The image information encoded in the spatial-coherence structure can be stably transmitted to the far field and can resist the influence of obstructions within the communication link. This is due to the self-reconstruction property of the spatial-coherence structure embedded with the cross phase in the far field. We demonstrate experimentally that the image information can be recovered well by measuring the second-order spatial-coherence structure of the obstructed random light in the far field. Our findings open a door for robust optical signal transmission through the complex environment and may find application in optical communication through a turbulent atmosphere.

Spiderweb Array: A Sparse Spin-Qubit Array

Abstract: One of the main bottlenecks in the pursuit of a large-scale--chip-based quantum computer is the large number of control signals needed to operate qubit systems. As system sizes scale up, the number of terminals required to connect to off-chip control electronics quickly becomes unmanageable. Here, we discuss a quantum-dot spin-qubit architecture that integrates on-chip control electronics, allowing for a significant reduction in the number of signal connections at the chip boundary. By arranging the qubits in a two-dimensional (2D) array with $\sim$12 $\mu$m pitch, we create space to implement locally integrated sample-and-hold circuits. This allows to offset the inhomogeneities in the potential landscape across the array and to globally share the majority of the control signals for qubit operations. We make use of advanced circuit modeling software to go beyond conceptual drawings of the component layout, to assess the feasibility of the scheme through a concrete floor plan, including estimates of footprints for quantum and classical electronics, as well as routing of signal lines across the chip using different interconnect layers. We make use of local demultiplexing circuits to achieve an efficient signal-connection scaling leading to a Rent's exponent as low as $p = 0.43$. Furthermore, we use available data from state-of-the-art spin qubit and microelectronics technology development, as well as circuit models and simulations, to estimate the operation frequencies and power consumption of a million-qubit processor. This work presents a novel and complementary approach to previously proposed architectures, focusing on a feasible scheme to integrating quantum and classical hardware, and significantly closing the gap towards a fully CMOS-compatible quantum computer implementation.

Magnetic Imaging with Spin Defects in Hexagonal Boron Nitride

Abstract: Optically-active spin defects hosted in hexagonal boron nitride (hBN) are promising candidates for the development of a two-dimensional (2D) quantum sensing unit. Here, we demonstrate quantitative magnetic imaging with hBN flakes doped with negatively-charged boron-vacancy (V − B ) centers through neutron irradiation. As a proof-of-concept, we image the magnetic field produced by CrTe 2 , a van der Waals ferromagnet with a Curie temperature slightly above 300 K. Compared to other quantum sensors embedded in 3D materials, the advantages of the hBN-based magnetic sensor described in this work are its ease of use, high flexibility and, more importantly, its ability to be placed in close proximity to a target sample. Such a sensing unit will likely find numerous applications in 2D materials research by offering a simple way to probe the physics of van der Waals heterostructures.

Field-Free Switching of Perpendicular Magnetization Induced by Longitudinal Spin-Orbit-Torque Gradient

Abstract: Spin-orbit-torque (SOT) symmetry in a heavy-metal--ferromagnet bilayer forbids the deterministic switching of perpendicular magnetization. Traditionally, an external magnetic field-exchange bias-tilted magnetic anisotropy is introduced to break the symmetry, which is not suitable for practical applications. Here, we propose to break the SOT symmetry by introducing a longitudinal composition gradient in $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Pt}$ heavy metal. We demonstrate the deterministic switching of a perpendicularly magnetized $\mathrm{Co}/\mathrm{Ni}$ multilayer with good endurance when the electric current flows along the direction of the composition gradient. No field-free switching is observed when the current is applied transverse to the gradient or the gradient is removed. The composition gradient of $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Pt}$ leads to a longitudinal gradient of SOT, which results in the field-free switching of perpendicular magnetization. Our work offers a promising approach for lowering the symmetry of the material system for developing spintronic applications.

Degenerate Parametric Amplification via Three-Wave Mixing Using Kinetic Inductance

Abstract: Microwave parametric amplifiers operating at the quantum noise limit have become indispensable tools for a range of cryogenic quantum technologies. These amplifiers are typically constructed from nonlinear Josephson junctions, which limit the ability to amplify high-power signals. This study reports a device based instead on the weakly nonlinear kinetic inductance intrinsic to a superconducting film of niobium titanium nitride. The amplifier offers large phase-sensitive gain and high power handling, plus a simple design and fabrication process. As it contains no junctions, it is robust to electrostatic discharge and potentially operable under high temperatures and large magnetic fields.

Reflective Metasurfaces with Multiple Elastic Mode Conversions for Broadband Underwater Sound Absorption

Abstract: Unlike their electromagnetic and acoustic counterparts, elastic waves involve different wave modes. The interplay and the coupling among them increase the complexity of the problem while also offering a larger space for wave manipulation. Elastic bulk wave conversion in an elastic metamaterial has recently shown great promise in medical ultrasound and nondestructive testing. Unlike the transmission-type conversion, however, reflective elastic mode conversion has been explored less in terms of analysis and design, despite the enormous possibilities that it might offer for energy trapping and dissipation. In this work, we develop a theoretical framework for constructing elastic anisotropic metasurfaces that can enable reflective longitudinal-to-transverse (L-to-T) and transverse-to-longitudinal (T-to-L) wave conversions. We capitalize on the mechanism of multiple reflective mode conversion to achieve broadband, subwavelength, and near perfect sound absorption in the underwater environment. The reflective scattering properties of the metasurfaces are systematically exploited for incident longitudinal or transverse waves. The conversion mechanism is rooted in reflective Fabry-Perot (FP) resonance, whose occurrence conditions and features are predicted for prescribed effective parameters of the metasurface. We then establish an inverse-design framework for conceiving an underwater coating system formed by a viscoelastic rubber layer and the metasurface. A series of metasurfaces allowing for customized mode conversions are realized for delivering broadband low-frequency and high-efficiency underwater sound absorption. Specifically, an ultrathin rubber-metasurface layer in which the metasurface with a thickness of approximately \ensuremath{\lambda}/70 can lead to nearly 100% sound absorption. Furthermore, we demonstrate that a persistently high absorption (over 80%) can be obtained in a rather robust manner within a wide range of wave incidence angle from \ensuremath{-}60\ifmmode^\circ\else\textdegree\fi{} to 60\ifmmode^\circ\else\textdegree\fi{}. More importantly, high-efficiency acoustic absorption exceeding 75% can be readily achieved through multiple mode conversions within the ultrabroadband range featuring a relative bandwidth of 119%. We reveal the combined FP resonance mechanism of underwater sound absorption, i.e., the FP resonance of the metaconverter, which determines the L-to-T and T-to-L conversion ratio, and the FP resonance of the rubber-metasurface layers, which enhances the wave attenuation inside the rubber. The proposed reflective multiple mode-conversion mechanism and metasurface design methodology open a route towards a class of elastic-wave-based devices with promising potential for underwater applications.

Steadily Decreasing Power Loss of a Double Schottky Barrier Originating from the Dynamics of Mobile Ions with Stable Interface States

Abstract: A general aging model of the double Schottky barrier was proposed to unveil the long-term aging behaviors of $\mathrm{Zn}\mathrm{O}$ varistor ceramics, especially for those ones with steadily decreasing power loss. For those samples, the barrier height and electrical properties were even enhanced rather than commonly deteriorated, which were beyond the classic ion migration model. In this paper, inspired by the unique reversible aging of them, interface states are proposed to remain stable in those samples. The major mobile ions, which have been in debate, are further identified to be ${\mathrm{Zn}}_{i}^{\ensuremath{\cdot}}$ ions. Based on these assumptions, a quantitative dynamic ion migration-diffusion model is proposed. The calculated power loss steadily decreases with aging time, which well supported our proposal. When the interface states are not combined with those mobile ions, the formation of a ``U-shape'' ion spatial distribution in depletion layers is found to be responsible for the unique aging phenomena, i.e., a reduction in the depletion layer and interfacial charge, a rise in the depletion layer width, and an increase in the barrier height. However, continuously increasing power loss would be generated if the mobile ions combined with the interface states. Therefore, a general mechanism on the aging of the double Schottky barrier is unveiled that it is a competition process between consumption of the interface states and the dynamics of mobile ions in depletion layers.

On-Chip Coherent Transduction between Magnons and Acoustic Phonons in Cavity Magnomechanics

Abstract: Magnons, namely spin waves, are collective spin excitations in ferromagnets, and their control through coupling with other excitations is a key technology for future hybrid spintronic devices. Although strong coupling has been demonstrated with microwave photonic structures, an alternative approach permitting high density integration and minimized electromagnetic crosstalk is required. Here we report a planar cavity magnomechanical system, where the cavity of surface acoustic waves enhances the spatial and spectral power density to thus implement magnon-phonon coupling at room temperature. Excitation of spin-wave resonance involves significant acoustic power absorption, whereas the collective spin motion reversely exerts a back-action force on the cavity dynamics. The cavity frequency and quality-factor are significantly modified by the back-action effect, and the resultant cooperativity exceeds unity, suggesting coherent interaction between magnons and phonons. The demonstration of a chip-scale magnomechanical system paves the way to the development of novel spin-acoustic technologies for classical and quantum applications.

Programmable Rainbow Trapping and Band-Gap Enhancement via Spatial Group-Velocity Tailoring in Elastic Metamaterials

Abstract: Stretch here, not there: Programmable tailoring of a wave's group velocity in space, achieved via synthetic impedance circuitry with digital signal processing, enables substantial design flexibility for devices. Here researchers demonstrate the programming of ``rainbow trapping'' (slowing waves to a temporary stop, based on frequency) in an elastic waveguide for chosen spatial grading profiles. The use of spatially graded resonators also significantly enhances the band gap's bandwidth, beyond that for a uniform resonator. This class of metamaterials enables simple on-demand programming of elastic wave trapping, spatial filtering, and attenuation through a digital interface.

Resonant Tunneling Diode Nano-Optoelectronic Excitable Nodes for Neuromorphic Spike-Based Information Processing

Abstract: In this work, we introduce an interconnected nano-optoelectronic spiking artificial neuron emitter-receiver system capable of operating at ultrafast rates (about $100\phantom{\rule{0.2em}{0ex}}\mathrm{ps}/$optical spike) and with low-energy consumption ( pJ/spike). The proposed system combines an excitable resonant tunneling diode (RTD) element exhibiting negative differential conductance, coupled to a nanoscale light source (forming a master node) or a photodetector (forming a receiver node). We study numerically the spiking dynamical responses and information propagation functionality of an interconnected master-receiver RTD node system. Using the key functionality of pulse thresholding and integration, we utilize a single node to classify sequential pulse patterns and perform convolutional functionality for image feature (edge) recognition. We also demonstrate an optically interconnected spiking neural network model for processing of spatiotemporal data at over 10 Gbit/s with high inference accuracy. Finally, we demonstrate an off-chip supervised learning approach utilizing spike-timing-dependent plasticity for the RTD-enabled photonic spiking neural network. These results demonstrate the potential and viability of RTD spiking nodes for low footprint, low-energy, high-speed optoelectronic realization of spike-based neuromorphic hardware.

Nonreciprocal Thermal Photonics for Energy Conversion and Radiative Heat Transfer

Abstract: Nonreciprocal thermal photonics is an emerging topic in thermal radiation control. Recent advances in using this approach for energy harvesting, thermal management, and even communication have stimulated substantial interest in the subject. The authors review recent developments, challenges, and opportunities in the unidirectional transfer of heat energy via light, to provide a snapshot of the current status of the field, and to advance future research in it.

Quantum Secure Direct Communication with Private Dense Coding Using a General Preshared Quantum State

Abstract: We study quantum secure direct communication by using a general preshared quantum state and a generalization of dense coding. In this scenario, Alice is allowed to apply a unitary on the preshared state to encode her message, and the set of allowed unitaries forms a group. To decode the message, Bob is allowed to apply a measurement across his own system and the system he receives. In the worst scenario, we guarantee that Eve obtains no information for the message even when Eve access the joint system between the system that she intercepts and her original system of the preshared state. For a practical application, we propose a concrete protocol and derive an upper bound of information leakage in the finite-length setting. We also discuss how to apply our scenario to the case with discrete Weyl-Heisenberg representation when the preshared state is unknown.

Two-Qutrit Quantum Algorithms on a Programmable Superconducting Processor

Abstract: Processing quantum information using quantum three-level systems or qutrits as the fundamental unit is an alternative to contemporary qubit-based architectures with the potential to provide significant computational advantages. We demonstrate a fully programmable two-qutrit quantum processor by utilizing the third energy eigenstates of two transmons. We develop a parametric coupler to achieve excellent connectivity in the nine-dimensional Hilbert space enabling efficient implementations of two-qutrit gates. We characterize our processor by realizing several algorithms like Deutsch-Jozsa, Bernstein-Vazirani, and Grover's search. Our efficient ancilla-free protocols allow us to show that two stages of Grover's amplification can improve the success rates of an unstructured search with quantum advantage. Our results pave the way for building fully programmable ternary quantum processors using transmons as building blocks for a universal quantum computer.

Liquid-Nitrogen-Cooled <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Ca</mml:mi></mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msup><mml:mi /><mml:mo>+</mml:mo></mml:msup></mml:math>

Abstract: We present a liquid-nitrogen-cooled $\mathrm{Ca}$${}^{+}$ optical clock with an overall systematic uncertainty of $3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$. In contrast to the room-temperature $\mathrm{Ca}$${}^{+}$ optical clock that we have reported previously, the cryogenic black-body radiation (BBR) shield in vacuum is cooled to $82\ifmmode\pm\else\textpm\fi{}5$ K using liquid nitrogen. We also implement an ion trap with a reduced heating rate and improved laser cooling. This allows the ion temperature to fall to the Doppler-cooling limit during the clock operation and the systematic uncertainty associated with the secular (thermal) motion of the ion is reduced to $<1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$. The uncertainty arising from the probe laser light shift and the servo error is also reduced to $<1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ and $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ with the hyper-Ramsey method and the higher-order servo algorithm, respectively. By comparing the output frequency of the cryogenic clock to that of a room-temperature clock, the differential BBR shift between the two is determined with a fractional statistical uncertainty of $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$. The differential BBR shift is used to calculate the static differential polarizability and the result is found to be in excellent agreement with our previous measurement using a different method. This work suggests that the BBR shift of optical clocks can be suppressed well in a liquid-nitrogen environment. Systems similar to what is presented here can also be used to suppress the BBR shift significantly in other types of optical clocks, such as $\mathrm{Yb}$${}^{+}$, $\mathrm{Sr}$${}^{+}$, $\mathrm{Yb}$, Sr, etc.

Intrinsic Ultralow Lattice Thermal Conductivity in the Full-Heusler Compound <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>Ba</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mi>Ag</mml:mi><mml:mi>Sb</mml:mi></mml:mrow></mml:math>

Abstract: Full-Heusler thermoelectric materials have intrinsically low lattice thermal conductivity. Our first-principles calculations show that ${\mathrm{Ba}}_{2}\mathrm{Ag}\mathrm{Sb}$ is a semiconductor with an indirect band gap of 0.49 eV. The electronic band degeneracy and pockets near the Fermi level facilitate electron transport. The short phonon relaxation time, small group velocity (1.89 km ${\mathrm{s}}^{\ensuremath{-}1}$), and large phonon scattering space reflect the intense phonon-phonon scattering. The large Gr\"uneisen parameter (1.44) accounts for the strong phonon anharmonicity, thus the low lattice thermal conductivity of $0.5\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 800 K. The isotropic figure of merit with a maximum value of 4.7 at 750 K is comparable to that of reported materials. The distribution of phonon momentum uncovers the important role of $\mathrm{Ag}$ in resisting thermal transport. The analysis of symmetry-based phonon-phonon scattering routes reveals the significance of symmetry on phonon scattering. The crystal structure of ${\mathrm{Ba}}_{2}\mathrm{Ag}\mathrm{Sb}$ can be used to regulate chemical elements to build high-performance thermoelectric materials. Our calculations provide an effective way to design thermoelectric materials, stimulating the study of full-Heusler materials.

Broadband Coding Metasurfaces with 2-bit Manipulations

Abstract: Due to their limited number of units but outstanding ability to control rather complex wave-propagation phenomena, acoustic coding metasurfaces (ACMs) as two-dimensional metamaterials show a stronger competitiveness in metamaterial applications. However, hindered by their narrow-band modulation capability, the previously reported ACMs do not exhibit a broadband applicability. To address the frequency-based coding capability, in this paper we report broadband acoustic coding metasurfaces (BACMs) whose units are designed by the bottom-up topology optimization method. Subsequently, by utilizing our optimization strategy, we design the 1-bit coding units ``0'' and ``1'' with out-of-phase responses and the 2-bit coding units ``00'', ``01'', ``10,'' and ``11'' with four different phase shifts of 0\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, 180\ifmmode^\circ\else\textdegree\fi{}, and 270\ifmmode^\circ\else\textdegree\fi{}. The topological features show constant phase differences, and in the following analysis, we attempt to explain this phenomenon by the related mechanisms of the internal resonances and the bianisotropy effect. The optimized BACMs are beneficial to improve the functions of a fixed coding metasurface in the frequency range. This idea provides an inspiration for fabricating fast-sector-scanning sound antenna. In combination with the convolution method, we also present the design strategy for the broadband fast-scanning multiple-beam antenna. The design of the acoustic antenna is a fixed coding sequence, and its scanning and echolocation mode does not depend on the reconfiguration of the coding units. Furthermore, we also demonstrate a pair of opposite functions, namely the broadband sound focusing and diffusion-scattering characteristics, which have potential applications in ultrasonic therapy, low-scattering target design and noise control, and so on.

Multicore Quantum Computing

Abstract: Any architecture for practical quantum computing must be scalable. An attractive approach is to create multiple cores , computing regions of fixed size that are well-spaced but interlinked with communication channels. This exploded architecture can relax the demands associated with a single monolithic device: the complexity of control, cooling and power infrastructure as well as the difficulties of cross-talk suppression and near-perfect component yield. Here we explore interlinked multicore architectures through analytic and numerical modelling. While elements of our analysis are relevant to diverse platforms, our focus is on semiconductor electron spin systems in which numerous cores may exist on a single chip within a single fridge. We model shuttling and microwave-based interlinks and estimate the achievable fidelities, finding values that are encouraging but markedly inferior to intra-core operations. We therefore introduce optimised entanglement purification to enable high-fidelity communication, finding that 99 . 5% is a very realistic goal. We then assess the prospects for quantum advantage using such devices in the NISQ-era and beyond: we simulate recently proposed exponentially-powerful error mitigation schemes in the multicore environment and conclude that these techniques impressively suppress imperfections in both the inter- and intra-core operations.

QAOA-in-QAOA: Solving Large-Scale MaxCut Problems on Small Quantum Machines

Abstract: The design of fast algorithms for combinatorial optimization greatly contributes to a plethora of domains such as logistics, finance, and chemistry. Quantum approximate optimization algorithms (QAOAs), which utilize the power of quantum machines and inherit the spirit of adiabatic evolution, are novel approaches to tackle combinatorial problems with potential runtime speedups. However, hurdled by the limited quantum resources nowadays, QAOAs are infeasible to manipulate large-scale problems. To address this issue, here we revisit the MaxCut problem via the divide-and-conquer heuristic: seek the solutions of subgraphs in parallel and then merge these solutions to obtain the global solution. Due to the $\mathbb{Z}_2$ symmetry in MaxCut, we prove that the merging process can be further cast into a new MaxCut problem and thus be addressed by QAOAs or other MaxCut solvers. With this regard, we propose QAOA-in-QAOA ($\text{QAOA}^2$) to solve arbitrary large-scale MaxCut problems using small quantum machines. We also prove that the approximation ratio of $\text{QAOA}^2$ is lower bounded by 1/2. Experiment results illustrate that under different graph settings, $\text{QAOA}^2$ attains a competitive or even better performance over the best known classical algorithms when the node count is around 2000. Our method can be seamlessly embedded into other advanced strategies to enhance the capability of QAOAs in large-scale combinatorial optimization problems.

Hidden Inverses: Coherent Error Cancellation at the Circuit Level

Abstract: Coherent gate errors are a concern in many proposed quantum-computing architectures. Here, we show that certain coherent errors can be reduced by a local optimization that chooses between two forms of the same Hermitian and unitary quantum gate. We refer to this method as hidden inverses, and it relies on constructing the same gate from either one sequence of physical operations or the inverted sequence of inverted operations. We use parity-controlled $Z$ rotations as our model circuit and numerically show the utility of hidden inverses as a function of circuit width $n$. We experimentally demonstrate the effectiveness for $n=2$ and $n=4$ qubits in a trapped-ion quantum computer. We numerically compare the method to other gate-level compilations for reducing coherent errors.

SENSEI: Characterization of Single-Electron Events Using a Skipper Charge-Coupled Device

Abstract: We use a science-grade skipper charge-coupled device (skipper CCD) operating in a low-radiation background environment to develop a semiempirical model that characterizes the origin of single-electron events in CCDs. We identify, separate, and quantify three independent contributions to the single-electron events, which were previously bundled together and classified as “dark counts”: dark current, amplifier light, and spurious charge. We measure a dark current, which depends on exposure, of (5.89±0.77)×10−4e−/pix/day, and an unprecedentedly low spurious charge contribution of (1.52±0.07)×10−4e−/pix, which is exposure independent. In addition, we provide a technique to study events produced by light emitted from the amplifier, which allows the detector’s operation to be optimized to minimize this effect to a level below the dark-current contribution. Our accurate characterization of the single-electron events allows one to greatly extend the sensitivity of experiments searching for dark matter or coherent neutrino scattering. Moreover, an accurate understanding of the origin of single-electron events is critical to further progress in ongoing research and development efforts of skipper and conventional CCDs.Received 2 July 2021Revised 22 September 2021Accepted 23 December 2021DOI:https://doi.org/10.1103/PhysRevApplied.17.014022© 2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasOptoelectronicsParticle dark matterTechniquesCalorimetersDark matter detectorsMulti-purpose particle detectorsNeutrino detectorsPrecision measurementsSolid-state detectorsParticles & FieldsInterdisciplinary PhysicsGeneral PhysicsGravitation, Cosmology & Astrophysics

Accurate Self-Configuration of Rectangular Multiport Interferometers

Abstract: Multiport interferometers based on integrated beamsplitter meshes are widely used in photonic technologies. While the rectangular mesh is favored for its compactness and uniformity, its geometry resists conventional self-configuration approaches, which are essential to programming large meshes in the presence of fabrication error. Here, we present a configuration algorithm, related to the $2\ifmmode\times\else\texttimes\fi{}2$ block decomposition of a unitary matrix, that overcomes this limitation. Our proposed algorithm is robust to errors, requires no prior knowledge of the process variations, and relies only on external sources and detectors. We show that self-configuration using this technique reduces the effect of fabrication errors by the same quadratic factor observed in triangular meshes. This relaxes a significant limit to the size of multiport interferometers, removing a major roadblock to the scaling of optical quantum and machine-learning hardware.

Electronic Transport Properties and Nanodevice Designs for Monolayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mrow><mml:mi>Mo</mml:mi><mml:mi>Si</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mrow><mml:mi ma

Abstract: A family of $M{A}_{2}{Z}_{4}$ materials has recently inspired great interest due to its exotic geometry and intriguing electronic properties. Here we investigate the electronic transport and photoelectric properties of ${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{P}}_{4}$ monolayer (MSP ML) that has a small direct gap using first-principles calculations. We design several model nanodevices based on MSP ML, including p-n junction diodes, p-i-n junction field-effect transistors, and photoelectric transistors. We demonstrate that these MSP-ML-based nanodevices yield superb transport properties, including significant rectifying effect, high electrical anisotropy, pronounced field-effect behavior, strong photoelectric response, and large photovoltaic power. These findings reveal the multifunctional nature of ${\mathrm{Mo}\mathrm{Si}}_{2}{\mathrm{P}}_{4}$ monolayer, promising its application as a designer material in next-generation ultrathin flexible semiconductor nanodevices.

Temperature-Gradient-Driven Magnetic Skyrmion Motion

Abstract: The static and dynamic properties of skyrmions have recently received increased attention due to the potential application of skyrmions as information carriers and for unconventional computing. While the current-driven dynamics has been explored deeply, both theoretically and experimentally, the theory of temperature gradient-induced dynamics - Skyrmion-Caloritronics - is still at its early stages of development. Here, we move the topic forward by identifying the role of entropic torques due to the temperature dependence of magnetic parameters. Our results show that, skyrmions move towards higher temperatures in single-layer ferromagnets with interfacial Dzyaloshinski-Moriya interactions, whereas, in multilayers, they move to lower temperatures. We analytically and numerically demonstrate that the opposite behaviors are due to different scaling relations of the material parameters as well as a non-negligible magnetostatic field gradient in multilayers. We also find a spatially dependent skyrmion Hall angle in multilayers hosting hybrid skyrmions due to variations of the thickness dependent chirality as

Dipole-Engineering Strategy for Regulating the Electronic Contact of a Two-Dimensional <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Sb</mml:mi></mml:math> <i>X</i> /Graphene ( <i>X</i> = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML

Abstract: A Schottky barrier, formed in the contact of a two-dimensional (2D) semiconductor and metal electrode, seriously degrades device performance. Herein, we propose a dipole-engineering strategy to regulate the electronic contact properties of a 2D polar $\mathrm{Sb}$X (X = $\mathrm{P}$, $\mathrm{As}$, $\mathrm{Bi}$) and graphene ($\mathrm{Gr}$) van der Waals interface. Owing to the mirror asymmetry of $\mathrm{Sb}$X, we construct seven vertical heterostructures in the form of X$\mathrm{Sb}$-$\mathrm{Gr}$ and $\mathrm{Sb}$X$\text{\ensuremath{-}}\mathrm{Gr}$. Tunable Schottky barrier height and contact type can be obtained by using different atomic terminals to contact with $\mathrm{Gr}$. Based on the first-principles calculations, the dipole and its associated potential step are found to be responsible for the regulating effect. Moreover, owing to the remarkable properties of the $\mathrm{Sb}\mathrm{Bi}$-$\mathrm{Gr}$ heterostructure, such as Ohmic contact and low tunneling barrier, we design an optoelectronic field-effect transistor, which exhibits considerable responsivity (0.089 ${\mathrm{AW}}^{\ensuremath{-}1}$) and external quantum efficiency (28.57%). Our findings further confirm that regulating the electronic contact properties by the dipole in the heterostructure is a feasible strategy, which provides meaningful guidance for designing high-performance electronic and optoelectronic devices.

Dependence of Tunneling Mechanism on Two-Dimensional Material Parameters: A High-Throughput Study

Abstract: A band-to-band tunneling FET (TFET) with an atomical two-dimensional (2D) channel is a potential candidate for the next-generation electronic device in view of its steep subthreshold swing and low power consumption. However, how to establish a precise physical model between the band property of 2D channel materials and the device performance of 2D TFETs is the key to accelerate their practical applications. Herein, through high-throughput first-principles calculations, we study the tunneling transport properties of 44 representative 2D materials with four kinds of crystal system. Particularly, we propose a well-defined and striking exponential scaling law $(\mathrm{SS}=A{\mathrm{e}}^{B{m}_{r}}+C)$ between the subthreshold swings (SSs) and reduced effective masses $({m}_{r})$ of 10-nm TFETs. According to the exponential model, 2D orthorhombic and trigonal crystal TFETs with the steep SS hold the reduced effective masses of more than 0.1 and $0.2\phantom{\rule{0.25em}{0ex}}{m}_{0}$, which could inhibit tunneling leakage current. These insights provide guidance for material screening in the construction of post-Moore 2D low-power transistors.

Generalizable Framework of Unpaired Domain Transfer and Deep Learning for the Processing of Real-Time Synchrotron-Based X-Ray Microcomputed Tomography Images of Complex Structures

Abstract: Mitigating greenhouse gas emissions by underground carbon dioxide storage or by coupling intermittent renewable energy with underground hydrogen storage are solutions essential to the future of energy. Of particular importance to the success of underground storage is the fundamental understanding of geochemical reactions with mineralogical phases and flow behavior at the length scale at which interfaces are well resolved. Fast synchrotron-based three-dimensional x-ray microcomputed tomography (\textmu{}CT) of rocks is a widely used technique that provides real-time visualization of fluid flow and transport mechanisms. However, fast imaging results in significant noise and artifacts that complicate the extraction of quantitative data beyond the basic identification of solid and void regions. To address this issue, an image-processing workflow is introduced that begins with unpaired domain transfer by cycle-consistent adversarial network, which is used to transfer synchrotron-based \textmu{}CT images containing fast-imaging-associated noise to long-scan high-quality \textmu{}CT images that have paired ground truth labels for all phases. The second part of the workflow is multimineral segmentation of images using convolutional neural networks (CNNs). Four CNNs are trained using the transferred dynamic-style \textmu{}CT images. A quantitative assessment of physically meaningful parameters and material properties is carried out. In terms of physical accuracy, the results show a high variance for each network output, which indicates that the segmentation performance cannot be fully revealed by pixel-wise accuracy alone. Overall, the integration of unpaired domain transfer with CNN-based multimineral segmentation provides a generalizable digital material framework to study the physics of porous materials for energy-related applications, such as underground ${\mathrm{CO}}_{2}$ and ${\mathrm{H}}_{2}$ storage.

qopt: An Experiment-Oriented Software Package for Qubit Simulation and Quantum Optimal Control

Abstract: Realistic modeling of qubit systems including noise and constraints imposed by control hardware is required for performance prediction and control optimization of quantum processors. We introduce qopt, a software framework for simulating qubit dynamics and robust quantum optimal control considering common experimental situations. To this end, we model open and closed qubit systems with a focus on the simulation of realistic noise characteristics and experimental constraints. Specifically, the influence of noise can be calculated using Monte Carlo methods, effective master equations or with the efficient filter function formalism, which enables the investigation and mitigation of auto-correlated noise. In addition, limitations of control electronics including finite bandwidth effects as well as nonlinear transfer functions and drive-dependent noise can be considered. The calculation of gradients based on analytic results is implemented to facilitate the efficient optimization of control pulses. The software easily interfaces with QuTip, is published under an open source license, well-tested and features a detailed documentation.