Due to its tunable infrared band gap and its anisotropic conduction properties, black phosphorus represents a very unique two-dimensional (2D) material, the potential of which in the engineering of new devices still needs to be fully explored. Here, we investigate the nonlinear terahertz (THz) electrodynamics of black phosphorus along the more conducting armchair direction. Similarly to the case of other 2D systems such as graphene and topological insulators, the THz saturable-absorption properties of black phosphorus can be understood within a thermodynamic model by assuming a fast thermalization of the electron bath. While black phosphorus does not display the presence of massless fermions at ambient pressure and temperature, our analysis shows that its anomalous THz nonlinear properties can be accounted for by a relativistic massive Dirac dispersion, provided that the Fermi temperature is low enough. An optimal tuning of the Fermi level therefore represents a strategy to engineer a strong THz nonlinear response in other massive Dirac materials, such as transition-metal dichalchogenides or high-temperature superconductors.

We investigate a time-efficient and robust preparation of spin-squeezed states—a class of states of interest for quantum enhanced metrology—in internal bosonic Josephson junctions with a time-dependent nonlinear coupling strength between atoms in two different hyperfine states. We treat this state-preparation problem, which had previously been addressed using shortcuts to adiabaticity (STA), using the recently proposed analytical modification of this class of quantum control protocols that has become known as the enhanced STA (eSTA) method. We characterize the state-preparation process by evaluating the time dependence of the coherent spin-squeezing and number-squeezing parameters and the target-state fidelity. We show that the state-preparation times obtained using the eSTA method compare favorably to those found in previously proposed approaches. We also demonstrate that the increased robustness of the eSTA approach—compared to its STA counterpart—leads to additional advantages for potential experimental realizations of strongly spin-squeezed states in internal bosonic Josephson junctions.

Continuous-variable (CV) quantum communication is an important branch of quantum information technology and promises information-theoretical secure communication. High-performance and lightweight CV quantum devices are one of the keys to realizing quantum networks. Among them, the Gaussian-modulation module still has problems such as cumbersome photoelectric calibration and inconvenient use in the experimental stage, which introduces modulation noise and brings additional excess noise to the system, reducing the signal-to-noise ratio of the system and thus reducing system performance. To this end, by designing a field-programmable gate array and integrating all optical components (except the laser source), we achieved a miniaturized and low-cost Gaussian-modulation unit compatible with the existing fiber-optic communication infrastructure, realizing quantum signal modulation under adaptive photoelectric calibration. Taking CV quantum key distribution as the verification object, we analyzed the signal characteristics and the stability of the unit experimentally. The experimental results show that the proposed scheme exhibits quality Gaussian properties and correlations. Moreover, the temporal stability of the device is relatively high (the mean fluctuation is on the order of ${10}^{-31}$ and the variance fluctuation is 0.0363), which provides more possibilities for the subsequent popularization of quantum communication and the development of quantum information networks.

We theoretically and numerically study the dual-pumped optical parametric oscillation in Kerr nonlinear microresonators with and without the thermo-optic effect. Parametric gain at degeneracy points, which exceeds the intrinsic loss of the waveguide, can be achieved with both normal and anomalous dispersion by optimizing the pump power, detuning, and frequency offset. Our theoretical modeling includes the various forms of dynamic behavior of the dual-pumped cavity, such as build-up, bistability, and chaotic behavior. Besides, the conditions for spectral phase transition, including the power threshold and critical point, have also been numerically investigated to achieve degenerate parametric oscillation. Notably, we analyze the phase-sensitive amplification regimes leading to phase bifurcation, which enable various applications in quantum random number generation, all-optical coherent Ising machines, and on-chip photon-pair generation. Our study will hopefully provide guidelines for Kerr-based microresonator designs, which aim for degenerate optical parametric oscillation.

Gate-defined quantum dots in silicon-germanium heterostructures have become a compelling platform for quantum computation and simulation. Thus far, developments have been limited to quantum dots defined in a single plane. Here, we propose to advance beyond planar systems by exploiting heterostructures with multiple quantum wells. We demonstrate the operation of a gate-defined double quantum dot in a strained germanium double quantum well, where both quantum dots are tunnel coupled to both reservoirs and parallel transport occurs. We analyze the capacitive coupling to nearby gates and find both quantum dots to be accumulated under the central plunger gate. We extract their position and size, from which we conclude that the double quantum dots are vertically stacked in the two quantum wells. We discuss the challenges and opportunities of multilayer devices and outline some potential applications in quantum computing and quantum simulation.

Dispersing organic fluorophores into a host matrix for doped organic light-emitting diodes (OLEDs) is an efficient method to improve the device optoelectronic performance. However, the effects of host-molecule polarity (i.e., the preference of charge-carrier transporting ability) on the carrier dynamic processes and efficiency roll-off of OLEDs are still vague. Herein, we find that the transient electroluminescence (TEL) decay behaviors reflecting the carrier dynamics depend remarkably on the host-molecule polarity. That is, the TEL curves of devices with the bipolar transport host decay slowly after turning off the voltage pulse, while those from devices with the unipolar one decay rapidly within several microseconds. In addition, through the comprehensive measurements and analyses of TEL, magnetoconductance responses, capacitance curves, and electroluminescence efficiency measured from the control devices with unipolar, bipolar, and mixed unipolar host materials, we conclude that all of these experimental methods, especially the TEL measurement, can serve as effective tools to obtain insights into the complex charge-carrier dynamic processes in devices possessing different charge-balance states. We propose an alternative experimental approach that reducing the emission-layer (EML) thickness appropriately in OLEDs with unipolar hosts can greatly suppress their efficiency roll-off. Specifically, the critical current density of $J_0$ (i.e., the current density at which the external quantum efficiency drops to half of its maximum value) obtains a significant increase from 37.14 mA/${\mathrm{cm}}^2$ to 149.76 mA/${\mathrm{cm}}^2$ in the unipolar host-based device by decreasing the EML thickness from 60 to 20 nm. Thus, this work not only presents the key role of host-molecule polarity in charge dynamic processes but also provides a good methodology for visualizing these intricate processes and alleviating the efficiency roll-off of OLEDs.

The cross-resonance gate (cr gate) architecture that exploits fixed-frequency transmon qubits and fixed couplings is a leading candidate for quantum computing. Nonetheless, without the tunability of qubit parameters such as qubit frequencies and couplings, gate operations can be limited by the presence of quantum crosstalk arising from the always-on couplings. When increasing system sizes, this can become even more serious, considering frequency collisions caused by fabrication uncertainties. Here, we introduce a cr-gate-based transmon architecture with passive mitigation of both quantum crosstalk and frequency collisions. Assuming typical parameters, we show that $ZZ$ crosstalk can be suppressed while maintaining $XY$ couplings to support fast, high-fidelity cr gates. The architecture also allows one to go beyond the existing literature by extending the operating regions in which fast, high-fidelity cr gates are possible, thus alleviating the frequency-collision issue. To examine the practicality, we analyze the cr-gate performance in multiqubit lattices and provide an intuitive model for identifying and mitigating the dominant source of error. For the state-of-the-art precision in setting frequencies, we further investigate its impact on the gates. We find that $ZZ$ crosstalk and frequency collisions can be largely mitigated for neighboring qubits, while interactions beyond near-neighbor qubits can introduce new frequency collisions. As the strength is typically at the submegahertz level, adding weak off-resonant drives to selectively shift qubits can mitigate the collisions. This work could be useful for suppressing quantum crosstalk and improving gate fidelities in large-scale quantum processors based on fixed-frequency qubits and fixed couplings.

We have explored different aspects of charge-carrier dynamics and relaxation in lead-free ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ double perovskite using dielectric spectroscopy and assessed its electrochemical response. The cubic phase $(Fm\overline{3}m)$ with a lattice constant of 11.644 Å is confirmed for synthesized perovskite. The phonon dispersion illustrated by density-functional theory indicates the existence of soft optical modes triggered by anharmonic rattling of $\mathrm{Cs}$ atoms and dynamical rotation of ${\mathrm{Sn}\mathrm{I}}_6$ octahedra. Complex impedance spectra have provided details of the contributions of grain boundaries, grains, and anharmonic rattling to charge-carrier dynamics. The ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ exhibits electrical conductivity of $3.77\times {10}^{-5}\mathrm{S}{\mathrm{cm}}^{-1}$ at ambient conditions. The values of the power-law exponent for all temperatures suggest superlinear power-law (SPL) behavior of the ac conductivity. The relaxation time and the stretched exponent in the Kohlrausch-Williams-Watts (KWW) function of the electric modulus are caused by charge-carrier short-range mobility and the hopping of rattling-assisted polarons. The supercapacitor fabricated with ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ as the electrode has delivered a specific capacitance of 3830 F ${\mathrm{g}}^{-1}$ at a current density of 2 A ${\mathrm{g}}^{-1}$. A quasi-solid-state asymmetric supercapacitor device was also fabricated, which delivered an energy density of 51 Wh ${\mathrm{kg}}^{-1}$ and a power density as high as 852 W ${\mathrm{kg}}^{-1}$ at a current density of 1 A ${\mathrm{g}}^{-1}$. We believe this work will open up the avenue to another generation of lead-free, perovskite-based, sustainable energy-storage systems.

Beam arrays with structured polarization states offer distinct advantages over single-structured beams in various applications. In this work, we propose an alternative way to synthesize the beam arrays carrying the controllable spatially inhomogeneous polarization states on the higher-order Poincaré spheres. The protocol is based on the vectorial spatial-coherence engineering of a partially coherent beam source, wherein a higher-order polarization array is first encoded into the spatial-coherence matrix of the source. We demonstrate that the encoded higher-order polarization array can then be reconstructed well in the received plane even when the partially coherent source is significantly blocked by an opaque obstacle or experiences strong phase perturbation. This highlights the high robustness of the synthesized higher-order polarization array. Additionally, we discuss the influence of the spatial-coherence area of the source on the robustness of the beam arrays. The findings of this work may have practical implications for multiparticle manipulation and multichannel optical communication in complex environments.

We investigated the heralded generation of two-qubit dual-rail-encoded states by programmable linear optics. Two types of scheme generating the states from four single photons, which is the minimum possible to accomplish the task, were considered. The schemes have different detection patterns heralding successful generation events; namely, one-mode heralding, in which the two auxiliary photons are detected in one mode, and two-mode heralding, in which single photons are detected in each of the two modes simultaneously. We show that the dependencies of the schemes’ success probabilities on the target state’s degree of entanglement are essentially different. In particular, one-mode heralding yields greater efficiency for highly entangled states if the programmable interferometers can explore the full space of the unitary transfer matrices. The reverse is the case for weakly entangled states, where two-mode heralding is better. We found a minimal decomposition of the scheme with two-mode heralding that is programmed by one variable phase shift. We infer that the linear optical schemes designed specifically for the generation of two-qubit states are more efficient than schemes implementing known linear optical gate–based circuits. Our results reveal a substantial reduction of physical resources needed to generate some types of nonmaximally entangled multiqubit states used in quantum computing.

Space-time modulation-based metamaterials have recently spurred considerable interest, owing to the fundamental addition of the time dimension to the medium parameters and resulting novel properties and potential applications. However, the implementation of most related structures—e.g., involving step, slab, or gradient discontinuities—has been hindered by the impossible requirement of infinitely or prohibitively large device sizes. Here, we provide a solution to this issue, consisting in a space-time transposition of the conventional Fresnel prism, whereby a copy of the target modulation is periodically reinjected at the input of a Fresnel-reduced finite structure, so as to provide the same anharmonic and nonreciprocal frequency conversion as the target space-time interface in the case of a modulation step. This concept, which may readily extend to slab or gradient modulations, as well as accelerated profiles for space-time chirping operations, may pave the way for the practical development of a wide range of novel microwave and optical space-time systems.

In this work, we develop a homogenization model to determine the effective response of a metallic nanowire array embedded in an electric gyrotropic material. We study the interaction of electromagnetic waves with the metamaterial and demonstrate that the nanowire array can greatly enhance the nonreciprocal response of the gyrotropic substrate. In particular, the metamaterial can either absorb the incoming energy almost entirely or reflect it with little loss, depending on the sign of the incidence angle. We explore the implications of our theory in the context of Kirchhoff's law of thermal radiation. Our results demonstrate that the wire array can boost the difference between the emissivity and absorptivity over a broad spectrum of frequencies and incidence angles compared to an unstructured gyrotropic substrate. These findings suggest potential applications for the nonreciprocal wire medium in thermal management, radiative cooling, and others.

High-dimensional entangled states in an orbital-angular-momentum (OAM) basis offer several unique advantages for quantum information applications. However, for the optimal performance of a given application, one requires a generation technique for OAM entangled states that is completely postselection-free and fully controllable. Nonetheless, despite several efforts in the past, no such technique currently exists. In this paper, we propose just such a technique and experimentally demonstrate postselection-free generation of up to about 150-dimensional OAM entangled states. We report the generation accuracy, which is a measure of the control, to be more than $98\mathrm{\%}$ for states with Gaussian and triangular OAM Schmidt spectra, and up to $90\mathrm{\%}$ for the maximally entangled OAM states, which have rectangular spectra.

Thin-film lithium niobate (LN) has become an enabling platform in the development of quantum integrated circuits, for applications including microwave-to-optical converters and electromechanical transducers. Incorporation of high-quality planar superconducting resonators will further elevate the performance of these devices. However, alongside the desired interactions originated from LN, its piezoelectric nature also introduces parasitic coupling between microwave and phononic modes, resulting in excessive dielectric loss. In this work, we investigate such coupling by studying niobium nitride resonators directly deposited on patterned LN thin films. By varying device configuration, we pinpoint that overlapping electric field with LN can inadvertently excite bulk acoustic modes within the substrate, inducing microwave loss independent of the acoustic dissipation rate. The experimental results are supported by simulation and theoretical modeling. Our approach can be applied to different piezoelectric systems and provide insights into the design of microwave components for quantum transduction devices.

Achieving desirable multifunctional optoelectronic devices requires consideration of indexes such as photoelectrical response, polarization sensitivity, energy consumption, power conversion efficiency (PCE), and direction dependence. To satisfy these requirements, we propose an anisotropic ${\mathrm{Hf}\mathrm{Ge}\mathrm{Te}}_4$-based optoelectronic device driven by the anisotropic photogalvanic effect (PGE) for high-performance photodetection and solar harvesting. We find a robust anisotropic PGE photocurrent $J_{\mathrm{ph}}$ ($J_x$ = 5.12, and $J_y$ = 0.07) is generated under the illumination of linearly polarized light due to the noncentrosymmetric nature of pristine ${\mathrm{Hf}\mathrm{Ge}\mathrm{Te}}_4$. Through appropriate mechanical bending and heterostructure assembled with a black phosphorus monolayer, the photocurrent and anisotropic ratio can be substantially enhanced by 8 times and 22.98%, respectively. Moreover, it exhibits a very high PCE of 20.19% and a large optical conductivity of about 13 × ${10}^3{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$. Our results show a fascinating functional coupling architecture that simultaneously implements high polarization-resolved photodetection and solar-energy harvesting in self-powered low-dimensional devices, suggesting an efficient avenue to achieve multifunctional integrated optoelectronic devices.

We detect correlations in qubit-energy fluctuations of non-neighboring qubits defined in isotopically purified Si/Si-Ge quantum dots. At low frequencies (where the noise is strongest), the correlation coefficient reaches 10% for a next-nearest-neighbor qubit-pair separated by 200 nm. Correlations with the charge-sensor signal reach up to 70%, proving that the observed noise is of electrical origin. A simple theoretical model quantitatively reproduces the measurements and predicts a polynomial decay of correlations with interqubit distance. Our results quantify long-range correlations of noise in quantum-dot spin-qubit arrays, essential for their scalability and fault tolerance.

We present and experimentally verify a matrix approach for determining how to optimally sculpt an input wavefront in both space and time for any desired wave-control functionality, irrespective of the complexity of wave scattering. We leverage a singular-value decomposition of the transport matrix that fully captures how both the spatial and temporal degrees of freedom available to shape the input wavefront impact the output wavefront’s spatial and temporal form. In our experiments in the microwave domain, we use our formalism to successfully tackle three iconic wave-control tasks in a disordered cavity: (i) reflectionless transient excitation (“virtual perfect absorption”), (ii) optimal energy deposition, and (iii) scattering-invariant time-varying states.

We present a blueprint for building a fault-tolerant quantum computer using the spin states of electrons on the surface of liquid helium. We propose to use ferromagnetic micropillars to trap single electrons on top of them and to generate a local magnetic field gradient. Introducing a local magnetic field gradient hybridizes charge and spin degrees of freedom, which allows us to benefit from both the long coherence time of the spin state and the long-range Coulomb interaction that affects the charge state. We present concrete schemes to realize single- and two-qubit gates and quantum nondemolition readout. In our framework, the hybridization of charge and spin degrees of freedom is large enough to perform fast qubit gates and small enough not to degrade the coherence time of the spin state significantly, which leads to the realization of high-fidelity qubit gates.

The atomically precise placement of dopants in semiconductors using scanning tunneling microscopes has been used to create planar dopant-based devices, enabling the exploration of novel classical or quantum computing concepts, which often require precise control over tunneling rates in their operation. While the geometry of the dopants can be defined to subnanometer precision, imperfections can still play a significant role in determining the tunneling rates. Here we investigate the influence of different imperfections in phosphorus $\delta$-layer tunnel junctions in silicon: variations of $\delta$-layer thickness and tunnel gap width, interface roughness, and charged impurities. It is found that while most of the imperfections moderately affect the tunneling rate, a single charged impurity in the tunnel gap can alter the tunneling rate by more than an order of magnitude, even for relatively large tunnel gaps. Moreover, it is also revealed that the tunneling rate strongly depends on the electrical charge sign of the impurity.