While strong system-bath coupling produces rich and interesting phenomena, applications to quantum thermal engines have been so far pointing mainly at detrimental effects. The delicate trade-off between efficiency loss due to strong coupling and power increase due to faster equilibration, while acknowledged, remains largely unexplored owing to the challenge of assessing the equilibration time precisely. Here, we overcome this obstacle by exploiting exact numerical simulations based on the hierarchical equations of motion (HEOM) formalism. We show that a quantum Otto cycle can perform better at strong (but not ultrastrong) coupling in that the product of the efficiency and that output power is maximized in this regime. In particular, we show that strong coupling allows one to obtain engines with larger efficiency than their weakly coupled counterparts, while sharing the same output power. Conversely, one can design strongly coupled engines with larger power than their weakly coupled counterparts, while sharing the same efficiency. Overall, our results provide situations where strong coupling can directly enhance the performance of thermodynamic operations, reinforcing the importance of studying quantum thermal engines beyond standard configurations.

The ability to execute high-fidelity operations is crucial to scaling up quantum devices to large numbers of qubits. However, signal distortions originating from nonlinear components in the control lines can limit the performance of single-qubit gates. In this work, we use a measurement based on error amplification to characterize and correct the small single-qubit rotation errors originating from the nonlinear scaling of the qubit drive rate with the amplitude of the programmed pulse. With our hardware, and for a 15-ns pulse, the rotation angles deviate by up to several degrees from a linear model. Using purity benchmarking, we find that control errors reach $2\times {10}^{-4}$, which accounts for half of the total gate error. Using cross-entropy benchmarking, we demonstrate arbitrary-angle single-qubit gates with coherence-limited errors of $2\times {10}^{-4}$ and leakage below $6\times {10}^{-5}$. While the exact magnitude of these errors is specific to our setup, the presented method is applicable to most sources of nonlinearity. Our work shows that the nonlinearity of qubit drive line components imposes a limit on the fidelity of single-qubit gates, independent of improvements in coherence times, circuit design, or leakage mitigation when not corrected for.

The detection of microelastomers in deep tissue has significant biomedical value. However, elastography of microelastomers is still limited to superficial tissues. Here, we propose photoacoustic multiacoustic spectral elastography based on the photoacoustic oscillation effect. After illumination with a pulsed laser, a light-absorbing elastomer vibrates and emits damped ultrasound waves, i.e., eigenvibro signals, in a short time. The eigenvibro signals contain elastic information. Our proposed method utilizes a causal damped wavelet to enhance and decompose the eigenvibro signals. Each wavelet basis corresponds to a distinct frequency, enabling the wavelet transformation of the photoacoustic signals and their synthesis to generate frequency-specific images. These individual images collectively form multispectral images, effectively displaying elastic features at their respective frequencies. Then, the elastogram of microelastomers in the region of interest is reconstructed and includes the intensity image, peak-frequency image, quality-factor image, and elasticity-to-viscosity ratio image. Both numerical simulations and phantom experiments validate the suitability of our proposed method, even with interference from inelastic optical absorbers. Our study has potential biomedical value for the assessment of diseases resulting from the development of microelastomers.

Excitement about the promise of quantum computers is tempered by the reality that the hardware remains exceptionally fragile and error prone, forming a bottleneck in the development of alternative applications. In this paper, we describe and experimentally test a fully autonomous workflow designed to deterministically suppress errors in quantum algorithms from the gate level through to circuit execution and measurement. We introduce the key elements of this workflow, delivered as a software package called Fire Opal, and survey the underlying physical concepts: error-aware compilation, automated system-wide gate optimization, automated dynamical decoupling embedding for circuit-level error cancellation, and calibration-efficient measurement-error mitigation. We then present a comprehensive suite of performance benchmarks executed on IBM hardware, demonstrating up to $>1000\times$ improvement over the best alternative expert-configured techniques available in the open literature. Benchmarking includes experiments using up to 16 qubit systems executing the following: Bernstein Vazirani, quantum Fourier transform, Grover’s search, quantum approximate optimization algorithm, variational quantum eigensolver, syndrome extraction on a five-qubit quantum error-correction code, and quantum volume. Experiments reveal a strong contribution of Non-Markovian errors to baseline algorithmic performance; in all cases the deterministic error-suppression workflow delivers the highest performance and approaches incoherent error bounds without the need for any additional sampling or randomization overhead, while maintaining compatibility with all additional probabilistic error suppression techniques.

Thermal emissions with both narrow spectral bandwidth and compressed angular distributions suggesting high temporal and spatial coherence are useful for the realization of an alternative family of highly efficient optical devices, and they play an increasingly significant role in numerous applications. Although people have endeavored to realize thermal emitters by harnessing different physics implemented in various artificial metasurfaces mainly made from metals, most of the reported results to date fail to produce emitters possessing the above merits. This work presents a proof-of-principal experimental demonstration on how to make use of the concept of quasiguided modes to realize alternative thermal mid-infrared emitters. Supported by an elaborately designed binary grating structure containing a period-doubling perturbation, these modes possess ultrahigh and perturbation-dependent Q factors. Although an amorphous form of evaporated materials is used, and the measured signal is a superposition of multiangle emission peaks, an emission bandwidth of about 25 nm is still experimentally obtained at the wavelength of 5.8 μm, which is one order of magnitude smaller than conventional thermal emitters based on metallic metamaterials. This bandwidth implies that a similar level of enhancement in the temporal coherence is achieved. Furthermore, our calculations show that the thermal emissions based on the quasiguided modes can achieve a spatial coherence length of 1.7 mm. Both of these two coherence properties are the state of the art to date in the mid-infrared thermal emitters.

It is crucial to realize field-free deterministic current-induced switching in spin-orbit-torque magnetic random-access memory with perpendicular magnetic anisotropy (PMA). A tentative solution has emerged recently, which employs interlayer chiral exchange coupling or the interlayer Dzyaloshinskii-Moriya interaction (IDMI) to achieve symmetry breaking. We hereby investigate the interlayer DMI in a $\mathrm{Pt}/\mathrm{Co}$ multilayer system with orthogonally magnetized layers, using a repeatedly stacked $[\mathrm{Pt}/\mathrm{Co}{]}_n$ structure with PMA and a thick $\mathrm{Co}$ layer with in-plane magnetic anisotropy (IMA). We clarify the origin and direction of such symmetry breaking with relation to the IDMI effective field and show a decreasing trend of the said effective-field magnitude with respect to the stacking number (n). By comparing the current-induced field-free switching behavior of both PMA and IMA layers, we confirm the dominating role of the IDMI in such field-free switching, excluding other possible mechanisms, such as tilted anisotropy and unconventional spin currents that may arise from symmetry breaking.

Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, the London penetration depth - based quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. We study niobium films to provide an example of a comprehensive analytical set that may benefit superconducting circuits such as those used in quantum computers. The films have a superconducting transition temperature of $T_c=9.35$ K and a fairly clean superconducting gap. The estimated superfluid density is enhanced at intermediate temperatures. These observations are consistent with the recent theory of anisotropic strong-coupling superconductivity in $\mathrm{Nb}$ and indicate outstanding quality. However, the response to the magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat dissipation (to a substrate), leading to thermomagnetic instabilities. This may present a challenge for further improvement of transmon quantum coherence. Possible mitigation strategies are discussed.

Entanglement is vulnerable to degradation in a noisy sensing scenario, but surprisingly, the quantum illumination protocol has demonstrated that its advantage can survive. However, designing a measurement system that realizes this advantage is challenging since the information is hidden in the weak correlation embedded in the noise at the receiver side. Recent progress in a correlation-to-displacement conversion module provides a route towards an optimal protocol for practical microwave quantum illumination. In this work, we extend the conversion module to accommodate experimental imperfections that are ubiquitous in microwave systems. To mitigate loss, we propose amplification of the return signals. In the case of ideal amplification, the entire six-decibel error-exponent advantage in target detection error can be maintained. However, in the case of noisy amplification, this advantage is reduced to three decibels. We analyze the quantum advantage under different scenarios with a Kennedy receiver in the final measurement. In the ideal case, the performance still achieves the optimal one over a fairly large range of error probability with only on-off detection. Empowered by photon-number-resolving detectors, the performance is further improved and also analyzed in terms of receiver operating characteristic curves. Our findings pave the way for the development of practical microwave quantum illumination systems.

The measurement-device-independent quantum key distribution (MDI QKD) can resist all attacks against measurement devices. However, its practical performance is still limited due to very low coincidence counting rates in Bell-state measurements. In this paper, we propose a practical memory-assisted MDI QKD protocol by cooperating the heralded single-photon sources and an all-optical quantum memory. By making use of the heralding characteristic of heralded single-photon sources, the earlier photon will be stored in the Sagnac optical quantum memory, and then released till another photon’s arrival. Moreover, by combining with the decoy-state method, the present work exhibits significantly improved key rates and transmission distances compared with all existing MDI QKD protocols without using quantum memories. Therefore, this work can provide valuable references for practical implementation of long-distance quantum communications.

The optical spin Hall effect, as an optical analogue of spin-orbit coupling of fermionic electrons, has attracted increasing interest in fundamental physics and nanophotonic applications. In addition to spin, charge of particles, as the other degree of freedom, plays a significant role in spin-orbit interaction as well. However, the optical counterpart of charge conjugation has been absent due to the charge-free nature of photons. Here, in a one-dimensional photonic crystal, we report reversible optical spin currents from the opposite band edges of the photonic band gap, which can provide an optical analogue of charge conjugation in spin-orbit coupling. The effective magnetic field acting on the photons is controlled by the tunable splitting of the transverse electric and transverse magnetic optical modes, which is confirmed by both experiment and simulation. Based on the tunable effective magnetic field, the optical spin current can be controlled by the tunable cavity length in an open cavity. Our results will facilitate the development of optical spintronic devices and guide the understanding of analogies and generalizations involving quantum and classical wave theories.

We numerically investigate the high-efficiency photovoltaic effect in a lateral $p$-$n$ homojunction based on monolayer black phosphorus (MBP) by using the nonequilibrium Green’s function combined with the density-functional theory. Because of the built-in electric field of the $p$-$n$ junction and the wrinkle structure of MBP, the photocurrent excited by either linearly or elliptically polarized light is significantly enhanced in a wide photon energy range. Moreover, because of the electron-photon interaction, the photocurrent is related to atomic orbitals through the polarizing angle of the polarized light. Therefore, we can read the orbital information of the band structure from the polarizing angular distribution of the photocurrent. These findings suggest the promising application of the MBP-based $p$-$n$ homojunction in high-efficiency photoelectric devices and orbital-resolved photovoltaic detection.

Since the first proof-of-principle experiments 25 years ago, quantum metrology has matured from fundamental concepts to versatile and powerful tools in a large variety of research branches, such as gravitational-wave detection, atomic clocks, plasmonic sensing, and magnetometry. At the same time, two-photon interferometry, which underpins the possibility of entanglement to probe optical materials with unprecedented levels of precision and accuracy, holds the promise to stand at the heart of innovative functional quantum sensing systems. We report a quantum based method for measuring the frequency dependence of the velocity in a transparent medium, i.e., chromatic dispersion (CD). This technique, using energy-time-entangled photons, allows straightforward access to CD value from the visibility of two-photon fringes recorded in a free-evolution regime. In addition, our quantum approach features all the advantages of classical measurement techniques, i.e., flexibility and accuracy, all in a plug-and-play system.

Dispersions around the Dirac points in sonic crystals give rise to various extraordinary transport properties of sound waves. In particular, with tilted Dirac cones, the acoustic Dirac crystals will exhibit obvious anisotropic properties. When the Dirac cone is titled to a certain degree, a unique type-III Dirac point emerges with the extreme anisotropic-wave-transport property, which features the crossing of flat bands and linear dispersions. Recently, some quantum and artificial structures have been proposed to realize different types of Dirac points, where the type-III Dirac point in sonic crystals has rarely been studied so far. Here, we utilize the orbital modes in an acoustic honeycomb lattice to obtain strictly flat bands through eliminating π bonding. The type-III Dirac points emerge with modulated intercell and intracell couplings, which are simulated with specific structural parameters. At type-III Dirac points, the acoustic wave propagation is extremely anisotropic and manifests as plaquette states that exist in the two-dimensional flat-band systems. Inspired by the unique properties at the type-III Dirac points, hybrid acoustic Dirac lattices of regions characterized by different wave-propagation directions are used to conduct the montage operation of plaquette states. The orbital type-III Dirac cones and the montage operation of plaquette states provide opportunities for manipulating energy flow in exotic ways and expanding the potential applications of Dirac materials.

In circuit quantum acoustodynamics (cQAD), superconducting circuits are combined with acoustic resonators to create and control nonclassical states of mechanical motion. Simulating these systems is challenging due to the extreme difference in scale between the microwave and mechanical wavelengths. All existing techniques simulate the electromagnetic and mechanical subsystems separately. However, this approach may not be adequate for all cQAD devices. Here, we demonstrate a single simulation of a superconducting qubit coupled to an acoustic and a microwave resonator and introduce two methods for using this simulation to predict the frequencies, coupling rates, and energy-participation ratios of the electromechanical modes of the hybrid system. We also discuss how these methods can be used to investigate important dissipation channels and quantify the nontrivial effects of mode hybridization in our device. Our methodology is flexible and can be extended to other acoustic resonators and quantum degrees of freedom, providing a valuable tool for designing hybrid quantum systems.

Nanomechanical resonators can serve as high-performance detectors and have the potential to be widely used in industry for a variety of applications. Most nanomechanical-sensing applications rely on detecting changes of resonance frequency. In commonly used frequency-tracking schemes, the resonator is driven at or close to its resonance frequency. Closed-loop systems can continually check whether the resonator is at resonance and adjust the frequency of the driving signal accordingly. In this work, we study three resonance-frequency-tracking schemes, a feedback-free (FF) scheme, a self-sustaining oscillator (SSO) scheme, and a phase-locked loop oscillator (PLLO) scheme. We improve and extend the theoretical models for the FF and the SSO tracking schemes and test the models experimentally with a nanoelectromechanical system (NEMS) resonator. We employ an SSO architecture with a pulsed positive-feedback topology and compare it to the commonly used PLLO and FF schemes. We show that all tracking schemes are theoretically equivalent and that they all are subject to the same speed versus accuracy trade-off characteristics. In order to verify the theoretical models, we present experimental steady-state measurements for all of the tracking schemes. The frequency stability is characterized by computing the Allan deviation. We obtain almost perfect correspondence between the theoretical models and the experimental measurements. These results show that the choice of the tracking scheme is dictated by cost, robustness, and usability in practice as opposed to fundamental theoretical differences in performance.

We use the principles of cavity optomechanics to design a resonant mechanical force sensor for atomic force microscopy. The sensor is based on a type of electromechanical coupling, dual to traditional capacitive coupling, whereby the motion of a cantilever induces surface strain that causes a change in the kinetic inductance of a superconducting nanowire. The cavity is realized by a compact microwave-plasma mode with an equivalent $LC$ circuit involving the kinetic inductance of the nanowire. The device is fully coplanar and we show how to transform the cavity impedance for optimal coupling to the transmission line and the following amplifier. For the device presented here, we estimate the bare kinetic inductive mechanoelectric coupling (KIMEC) rate $g_0/2\pi$ in the range 3–10 Hz. We demonstrate phase-sensitive detection of cantilever motion using a multifrequency pumping and measurement scheme.

The local ordering of atoms in alloys directly has a strong impact on their electronic and optical properties. This is particularly relevant in nonrandom alloys, especially if they are deposited far from the equilibrium processes, as is the case for epitaxial ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ layers. In this work, we investigate the arrangement of $\mathrm{Ge}$ and $\mathrm{Sn}$ atoms in optoelectronic grade ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ epitaxial layers featuring a $\mathrm{Sn}$ content in the 5–14% range by using polarization-dependent Raman spectroscopy and density-functional-theory calculations. The thorough analysis of the polarization-dependent spectra in parallel and perpendicular configuration allowed us to properly tag all the observed vibrational modes, and to shed light on that associated to disorder-assisted Raman transitions. Indeed, with the help of large-scale atomistic simulations, we were able to highlight how the presence of $\mathrm{Sn}$ atoms, that modify the local environments of $\mathrm{Ge}$ atoms, gives rise to two spectral features at different Raman shifts, corresponding to distortions of the atomic bonds. This analysis provides a valuable framework for advancing the understanding of the vibrational properties in ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ alloys, particularly with regard to the impact of local ordering of the different atomic species.