Quantum key distribution provides a promising solution for sharing secure keys between two distant parties with unconditional security. Nevertheless, quantum key distribution is still severely threatened by the imperfections of devices. In particular, the classical pulse correlation threatens security when sending decoy states. To address this problem and simplify experimental requirements, we propose a phase-matching quantum key distribution protocol without intensity modulation. Instead of using decoy states, we propose an alternative method to estimate the theoretical upper bound on the phase error rate contributed by even-photon-number components. Simulation results show that the transmission distance of our protocol could reach 305 km in telecommunication fiber. Furthermore, we perform a proof-of-principle experiment to demonstrate the feasibility of our protocol, and the key rate reaches 22.5 bit/s under a 45-dB channel loss. Addressing the security loophole of pulse intensity correlation and replacing continuous random phase with a six- or eight-slice random phase, our protocol provides a promising solution for constructing quantum networks.

The classical Drude model provides an accurate description of the plasma resonance of three-dimensional materials, but only partially explains two-dimensional systems where quantum mechanical effects dominate such as P:$\delta$ layers—atomically thin sheets of phosphorus dopants in silicon that induce electronic properties beyond traditional doping. Previously it was shown that P:$\delta$ layers produce a distinct Drude tail feature in ellipsometry measurements. However, the ellipsometric spectra could not be properly fit by modeling the $\delta$ layer as a discrete layer of classical Drude metal. In particular, even for large broadening corresponding to extremely short relaxation times, a plasma resonance feature was anticipated but not evident in the experimental data. In this work, we develop a physically accurate description of this system, which reveals a general approach to designing thin films with intentionally suppressed plasma resonances. Our model takes into account the strong charge-density confinement and resulting quantum mechanical description of a P:$\delta$ layer. We show that the absence of a plasma resonance feature results from a combination of two factors: (i) the sharply varying charge-density profile due to strong confinement in the direction of growth; and (ii) the effective mass and relaxation time anisotropy due to valley degeneracy. The plasma resonance reappears when the atoms composing the $\delta$ layer are allowed to diffuse out from the plane of the layer, destroying its well-confined two-dimensional character that is critical to its distinctive electronic properties.

Experimentally achieving optimal performance of quantum sensors, such as optically pumped magnetometers, is usually challenging and time consuming due to the complexity of the physical system. For a given experiment scheme, there are usually many experiment parameters that need to be fine tuned for achieving a satisfying performance of the quantum sensor. By employing global optimization algorithms, we propose an automatic and efficient way to optimize the sensor performance. We take the most sensitive magnetic field sensor, i.e., the optically pumped magnetometer, as an example to demonstrate how these algorithms facilitate the automatic optimization of quantum sensors. Both the optimization of zero-field magnetometers in a magnetic shield and the finite-field magnetometers in open Earth’s field are demonstrated. A noise floor of approximately $10\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ is achieved within a short time. This method provides an efficient way for quantum sensor optimization and will be helpful for the quantum sensor design.

Atoms are ideal quantum sensors and quantum light emitters. Interfacing atoms with nanophotonic devices promises nanoscale sensing and quantum optical functionalities. But precise optical control of atomic states in these devices is challenged by the spatially varying light-atom coupling strength, which is generic to nanophotonic devices. We demonstrate numerically that despite the inhomogeneity, composite picosecond optical pulses with optimally tailored phases are able to evanescently control the atomic electric dipole transitions nearly perfectly, with fidelity $f>99\mathrm{\%}$ across volumes large enough for, e.g., controlling cold atoms confined in near-field optical lattices. Our proposal is followed by a proof-of-principle demonstration with a ${}^{85}$$\mathrm{Rb}$-vapor–optical-nanofiber interface, where the excitation by an $N=3$ sequence of guided picosecond $D_1$ control pulses reduces the absorption of a coguided nanosecond $D_2$ probe pulse by up to approximately $70\mathrm{\%}$. The close-to-ideal performance is corroborated by comparing the absorption data across the parameter space with first-principles modeling of the mesoscopic atomic vapor response. Extension of the composite technique to $N\ge 5$ appears highly feasible to support arbitrary local control of atomic dipoles with exquisite precision. This unprecedented ability would allow error-resilient atomic spectroscopy and open up a variety of research opportunities in nanophotonic quantum optics.

Topological interface states and the associated enhancement of sound intensity in periodic systems is of fundamental interest in wave physics and may have wide applications, such as in sound detection and biomedical imaging. Yet their occurrence is hitherto limited to the topological transition where the bands inverse. Here we theoretically design and experimentally demonstrate an “acoustic heterostructure” capable of giving rise to interface states at arbitrary frequency within the topological band gap, which is accomplished by adjusting a single structural parameter. The remarkable sound-intensity enhancement, which reaches ${10}^4$, is not affected by this change but remains near constant within the said frequency range and is tested in both a sonic and an elastic wave setting. We envision our strategy to be useful in the design of alternative acoustic sensors and detectors with high flexibility and sensitivity in diverse scenarios, such as nondestructive detection and medical imaging.

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.