The quantum transduction between microwave and optical photons is essential for realizing scalable quantum computers with superconducting qubits. Due to the large frequency difference between microwave and optical ranges, the transduction needs to be done via intermediate bosonic modes or nonlinear processes. So far, the transduction efficiency $\eta$ via the magneto-optic Faraday effect (i.e., the light-magnon interaction) in the ferromagnet YIG has been demonstrated to be as small as $\eta \sim {10}^{-8}-{10}^{-15}$ due to the weak magneto-optic coupling. Here, we take advantage of the fact that three-dimensional topological insulator thin films exhibit a topological Faraday effect that is independent of the sample thickness in the terahertz regime. This leads to a large Faraday rotation angle and therefore enhanced light-magnon interaction in the thin-film limit. We show theoretically that the transduction efficiency between microwave and terahertz photons can be greatly improved to $\eta \sim {10}^{-4}$ by utilizing the heterostructures consisting of topological insulator thin films, such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ferromagnetic insulator thin films, such as YIG.

Acoustic orbital angular momentum (OAM) beams hold potential for underwater communications, particle manipulation, and biomedical applications. However, adapting air-based OAM technologies to the underwater environment presents unique challenges. In the underwater environment, the medium cannot be considered acoustically soft, and commonly used ultrasound frequencies have shorter wavelengths compared with air, necessitating new design approaches. Conventional underwater ultrasonic lens design methods often rely on simplified models that neglect nonlocal interactions and diffraction within the structure, therefore leading to suboptimal performance. We introduce a novel nonlocal inverse design method by integrating the full-wave models with the nondominated sorting genetic algorithm II (NSGA-II). This approach diverges from conventional phase-based design by optimizing the physical structure of the lens to account for nonlocal interactions within the material. We experimentally demonstrate the performance when generating high-purity OAM beams underwater. Our experimental results show that this method can generate high-purity OAM beams underwater, achieving over 90% purity for the OAM beams of topological charges from m = 1 to m = 4. This is a significant improvement compared with traditional methods, which typically reach 70–89% purity. These findings highlight the practical applicability of our method for nonlocally designing the ultrasonic lens, paving the way for advancements in beam performance for various applications.

Measuring the temperature of a quantum system is an essential task in almost all aspects of quantum technologies. Theoretically, an optimal strategy for thermometry often requires measuring energy, which demands full accessibility over the entire system as well as a complex entangled measurement basis. In this paper, we take a different approach and show that single-qubit sequential measurements in the computational basis not only allow for precise thermometry of a many-body system, but may also achieve precision beyond the thermometry capacity of the probe at equilibrium, given by the Cramér-Rao bound. Thus, using consecutive single-qubit measurements of the probe out of equilibrium is, in most cases, very beneficial, as it achieves lower-temperature uncertainties and avoids demanding energy measurements when compared with probes at thermal equilibrium. To obtain such precision, the time between the two subsequent measurements should be smaller than the thermalization time so that the probe never thermalizes. Therefore, the nonequilibrium dynamics of the system continuously imprint information about temperature in the state of the probe. To demonstrate the generality of our findings, we consider thermometry in both spin chains and the Jaynes-Cummings model.

We report on an experiment in which orbital-angular-momentum (OAM)-entangled photon pairs generated by the spontaneous parametric down-conversion process can be engineered to have particular symmetry properties. Our method is based on the use of a Dove-prism pair in conjunction with Hong-Ou-Mandel (HOM) interferometry resolved in transverse position and OAM. The latter allows us to engineer the postselected two-photon state to exhibit a specific type of symmetry. By selecting particular topological charge values for the pump and for the postselected two-photon state, we can transition from a symmetric two-photon state and a HOM dip to an antisymmetric state and a HOM peak. Spatial resolution allows us to obtain the HOM interferogram both at the single-pixel level and globally by summing over all sensor pixels. Furthermore, through spatially selective OAM projection of the detected photon pairs, we can define multiple transverse regions with different symmetry properties, as verified by our spatially resolved HOM apparatus. Although we used two transverse regions for this proof-of-concept demonstration, this method could in principle be scaled to a larger number of regions, leading to a new technique to be added to the existing toolbox for quantum technologies in the photonic domain.

Many modern-day quantum science experiments rely on high-fidelity measurement of fluorescent signals emitted by the quantum system under study. A pernicious issue encountered when such experiments are conducted near a material interface in vacuum is “laser-induced contamination” (LIC): the gradual accretion of fluorescent contaminants on the surface where a laser is focused. Fluorescence from these contaminants can entirely drown out any signal from, e.g., optically probed color centers in the solid state. Crucially, while LIC appears often in this context, it has not been systematically studied. In this work, we probe the onset and growth rate of LIC for a diamond nitrogen-vacancy center experiment in vacuum, and we correlate the contamination-induced fluorescence intensities to micron-scale physical buildup of contaminant on the diamond surface. Drawing upon similar phenomena previously studied in the space optics community, we use photocatalyzed oxidation of contaminants as a mitigation strategy. We vary the residual oxygen pressure over 9 orders of magnitude and find that LIC growth is inhibited at near-atmospheric oxygen partial pressures, but the growth rate at lower oxygen pressure is nonmonotonic. Finally, we discuss a model for the observed dependence of LIC growth rate on oxygen content and propose methods to extend in situ mitigation of LIC to a wider range of operating pressures.

In most studies utilizing the magneto-optic Kerr effect (MOKE), the detected change of polarized light upon reflection from a magnetized sample is supposed to be proportional to the magnetization $\mathbf{M}$. However, MOKE signatures quadratic in $\mathbf{M}$ have also been identified and utilized, e.g., to sense the structural order in Heusler compounds, to detect spin-orbit torque or to image antiferromagnetic domains. In our study, we observe a strong anisotropic MOKE contribution of third order in $\mathbf{M}$ in $\mathrm{Ni}$(111) thin films, attributed to a cubic magneto-optic tensor $\propto {\mathbf{M}}^3$. We further show that the angular dependence of cubic MOKE (CMOKE) is affected by the amount of structural domain twinning in the sample. Our detailed study on CMOKE will open up opportunities for CMOKE applications with sensitivity to twinning properties of thin films, e.g., CMOKE spectroscopy and microscopy or time-resolved CMOKE. Furthermore, the in-plane magnetization orientation can be detected with CMOKE when perpendicular incidence light is used, which is suitable for MOKE setups that do not provide oblique incidence.

Electron-phonon ($e$-ph) scattering is a key effect in quantum transport and electronic device physics which is, however, often neglected in atomistic device simulation due to its impractical computational burden. Here we investigate e-ph effects in two-dimensional (2D) FETs, where the injecting source is graphene that inject “Dirac” electrons into the FET channel. Such a Dirac-source FET was experimentally known to have excellent transfer characteristics for its lower off-state current due to the electronic structure of the graphene. By using an approximate but computationally efficient technique (the Zacharias-Giustino method) to capture e-ph scattering, we quantitatively analyze to what extent e-ph scattering affects the operation of Dirac source, as a function of temperature. Our nonequilibrium Green’s function density-functional-theory analysis microscopically reveal the e-ph scattering and we make a comprehensive evaluation of it across real, momentum, and energy spaces, covering both the tunneling and thermionic emission regions. The findings suggest that e-ph scattering does not significantly impact the overall performance of the Dirac-source FETs. For the graphene-${\mathrm{WSe}}_2$ device, the e-ph effects amount to somewhat increase the off-state current, which is not significant as to alter the subthreshold property of the transistor. Other factors, such as gate efficiency—determined by the body factor—exhibit a more pronounced influence on device performance for this system.

Alloys based on lanthanum phosphate (${\mathrm{La}\mathrm{PO}}_4$) are often employed as thermal barrier coatings, due to their low thermal conductivity and structural stability over a wide temperature range. To enhance the thermal-insulation performance of these alloys, it is essential to comprehensively understand the fundamental physics governing their heat conduction. Here, we employ the Wigner formulation of thermal transport in conjunction with first-principles calculations to elucidate how the interplay between anharmonicity and compositional disorder determines the thermal properties of ${\mathrm{La}}_{1-x}{\mathrm{Gd}}_x{\mathrm{PO}}_4$ alloys, and discuss the fundamental physics underlying the emergence and coexistence of particlelike and wavelike heat-transport mechanisms. We also show how the Wigner transport equation correctly describes the thermodynamic limit of a compositionally disordered crystal, while the Boltzmann transport equation does not. Our predictions for microscopic vibrational properties (temperature-dependent Raman spectrum) and for macroscopic thermal conductivity are validated against experiments. Finally, we leverage these findings to devise strategies to optimize the performance of thermal barrier coatings.

Nonlinear behavior in the hopping transport of interacting charges enables reconfigurable logic in disordered dopant network devices, where voltages applied at control electrodes tune the relation between voltages applied at input electrodes and the current measured at an output electrode. From kinetic Monte Carlo simulations we analyze the critical nonlinear aspects of variable-range hopping transport for realizing Boolean logic gates in these devices on three levels. First, we quantify the occurrence of individual gates for random choices of control voltages. We find that linearly inseparable gates such as the xor gate are less likely to occur than linearly separable gates such as the and gate, despite the fact that the number of different regions in the multidimensional control voltage space for which and or xor gates occur is comparable. Second, we use principal-component analysis to characterize the distribution of the output current vectors for the (00,10,01,11) logic input combinations in terms of eigenvectors and eigenvalues of the output covariance matrix. This allows a simple and direct comparison of the behavior of different simulated devices and a comparison to experimental devices. Third, we quantify the nonlinearity in the distribution of the output current vectors necessary for realizing Boolean functionality by introducing three nonlinearity indicators. The analysis provides a physical interpretation of the effects of changing the hopping distance and temperature and is used in a comparison with data generated by a deep neural network trained on a physical device.

Zero-noise extrapolation (ZNE), a technique to estimate quantum circuit expectation values through noise scaling and extrapolation, is well studied in the context of quantum computing. We examine the applicability of ZNE to the field of quantum sensing. Focusing on the problem of dc magnetometry using the Ramsey protocol, we show that the sensitivity (in the sense of the minimum detectable signal) does not improve upon using ZNE in the slope detection scheme. On the other hand, signals of sufficiently large magnitude can be estimated more accurately. Our results are robust across various noise models and design choices for the ZNE protocols, including both single-qubit and multiqubit entanglement-based sensing.

A nonadiabatic geometric quantum gate is realized by integrating nonadiabatic geometric phases with global geometric features into the unitary quantum control, thereby removing the limitation of a long evolution time in the adiabatic case. However, systematic errors are inevitable in practical quantum control; these lead to the deviation of the evolution from target conditions to inducing geometric phases, smearing the robustness of the induced geometric quantum gates. Here, we present a general theoretical framework with enhanced robustness for geometric quantum gates by preserving fundamental geometric conditions. We first analytically evaluate the influence of systematic errors on geometric gates and then propose an optimized approach to mitigate this influence. Numerical simulations indicate that, as the geometric conditions are still maintained in the presence of systematic errors in our scheme, the constructed geometric quantum gates exhibit strong robustness, far superior to that of conventional schemes. Furthermore, we propose implementing the scheme in superconducting quantum circuits, where geometric quantum gates can achieve high fidelity with current experimental parameters. Therefore, the enhanced gate performance highlights the promise of our scheme for large-scale quantum computations.

Nanomechanical resonator arrays constitute a promising platform for topological physics and integrated acoustic devices. However, achieving precise control of the couplings between resonators has been a significant challenge for realizing a time-dependent Hamiltonian. In this work, we address this challenge by designing the geometric parameters of the resonators, enabling us to achieve dynamical control over the coupling strength and frequency stability. Our scalable resonator array allows for the dynamic control of coupling strengths between every individual resonator pair, ranging from zero to more than 20 times the dissipation rate. Moreover, we demonstrate Rabi-like oscillations with real-time-varying Rabi frequencies. This dynamically controlled system provides an extended platform for investigating dynamic processes and their applications.

Tunable couplers in superconducting quantum computers have enabled fast and accurate two-qubit gates, with reported high fidelities over 99% in various architectures and gate-implementation schemes. However, there are few tunable couplers whose performance in multiqubit systems is clarified, except for the most widely used one: single-transmon coupler (STC). Achieving similar accuracy to isolated two-qubit systems remains challenging due to various undesirable couplings but is necessary for scalability. In this work, we numerically analyze a system of three fixed-frequency qubits coupled via two double-transmon couplers (DTCs) where nearest-neighbor qubits are highly detuned and also next-nearest-neighbor ones are nearly resonant. The DTC is a recently proposed tunable coupler, which consists of two fixed-frequency transmons coupled through a common loop with an additional Josephson junction. We find that the DTC cannot only reduce undesired residual couplings sufficiently, as well as in isolated two-qubit systems, but also enables implementations of 30-ns $\mathrm{CZ}$ gates and individual and simultaneous 10-ns $\pi /2$ pulses with fidelities over 99.99%. For comparison, we also investigate the system where the DTCs are replaced by the STCs. The results show that the DTC outperforms the STC in terms of both residual coupling suppression and gate accuracy in the above systems. From these results, we expect that the DTC architecture is promising for realizing high-performance, scalable superconducting quantum computers.

Quantum key distribution (QKD) could help to share a secure key between two distant peers. In recent years, twin-field (TF) QKD has been widely investigated because of its long transmission distance. One of the popular variants of TF QKD is sending-or-not-sending (SNS) QKD, which has been experimentally verified to realize 1000-km level fiber key distribution. In this article, we introduce phase postselection into the SNS protocol. With this modification, the probability of selecting “sending” can be substantially improved. The numerical simulation shows that the transmission distance can be improved both with and without the actively odd-parity pairing method. The case of discrete phase randomization is also analyzed, a similar improvement on distance can be seen.

In recent years, notable progress has been made in the study of hole qubits in planar germanium, and circuit quantum electrodynamics (circuit QED) has emerged as a promising approach for achieving long-range coupling and scaling up of qubits. Here, we demonstrate the coupling between holes in a planar germanium double quantum dot (DQD) and photons in a microwave cavity. Specifically, a real-time calibrated virtual gate method is developed to characterize this hybrid system, which in turn allows us to determine the typical parameters sequentially through a single-parameter fitting instead of conventional multiparameter fitting with additional uncertainty, giving a hole-photon coupling rate of $g_0$/2π = 21.7 MHz. This work is a step toward further research on hole-photon interactions and long-range qubit coupling in planar germanium. The experimental method developed in this work contributes to a more accurate and efficient characterization of hybrid cavity-QED systems.

Coupled learning is a contrastive local learning scheme for tuning the properties of individual elements within a network to achieve desired functionality of the system. It takes advantage of physics both to learn using local rules and to “compute” the output response to input data, thus enabling the system to perform decentralized computation without the need for a processor or external memory. We present three proof-of-concept mechanical networks of increasing complexity, and demonstrate how they can learn tasks such as self-symmetrization and node allostery via iterative tuning of individual spring rest lengths. These mechanical networks could feasibly be scaled and automated to solve increasingly complex tasks, hinting at a new class of smart metamaterials.