We present a way to achieve fully directional, quantum-limited phase-preserving amplification in a four-port, four-mode superconducting Josephson circuit by utilizing interference between six parametric processes that couple all four modes. Full directionality, defined as the reverse isolation surpassing forward gain between the matched input and output ports of the amplifier, ensures its robustness against impedance mismatch that might be present at its output port during applications. Unlike existing directional phase-preserving amplifiers, both the minimal backaction and the quantum-limited added noise of this amplifier remains unaffected by noise incident on its output port. In addition, the matched input and output ports allow direct on-chip integration of these amplifiers with other circuit QED components, facilitating scaling up of superconducting quantum processors.

Spin-photon coupling presents an enticing opportunity for the long-range coupling of spin qubits. The spin-photon coupling rate, $g_s$, is proportional to the charge-photon coupling rate, $g_c$. To move deeper into the strong-coupling regime, $g_c$ can be enhanced by fabricating high-impedance cavities using high-kinetic-inductance films. Here, we report dc transport and microwave response investigations of niobium nitride ($\mathrm{Nb}\mathrm{N}$) films of different thicknesses. The kinetic inductance increases rapidly as the film thickness is reduced below 50 nm and for 15-nm $\mathrm{Nb}\mathrm{N}$ films we measure a sheet kinetic inductance $L_{k,S}=41.2\text{pH}/◻$. As an application of the high-kinetic-inductance films, we fabricate compact $LC$ filters that are commonly used to reduce microwave leakage in circuit quantum electrodynamics (cQED) devices. These filters feature up to 60 dB of attenuation near typical cavity resonance frequencies of $f_c=8$ GHz.

The passive approach to quantum key distribution (QKD) eliminates all optical modulators and random number generators from QKD systems reaching an enhanced simplicity, immunity to modulator side channels, and potentially higher repetition rates. In this work, we provide finite-key security bounds for a fully passive decoy-state Bennett-Brassard 1984 (BB84) protocol, considering a recently presented passive QKD source. With our analysis, the attainable secret-key rate is comparable to that of the perfect parameter-estimation limit, in fact differing from the key rate of the active approach by less than one order of magnitude. This demonstrates the practicality of fully passive QKD solutions.

This paper presents a solution to overcome the inherently limited bandwidth of substrate-integrated waveguide (SIW) slot antennas. It is analytically shown that by decreasing the permittivity of a dielectric loaded slot antenna, the resulting bandwidth increases significantly, where the widest bandwidth can be achieved when the permittivity of the dielectric is less than unity. To demonstrate this concept, a rectangular SIW slot is loaded by an array of thin wires to realize the desired low-index metamaterials (MTMs), which consequently results in a single-layer, compact, and cost-effective structure. We have measured an impedance bandwidth $(|S_{11}|<-10\mathrm{dB})$ of 36.2%, covering the millimeter-wave (mmWave) frequency range of 19.7–28.4 GHz. The radiation efficiency is above 90%, providing at least 7.5 decibels relative to isotropic (dBi) gain through the entire frequency band, making it a potential candidate for industrial, scientific and medical (ISM) and/or automotive radar ($24.125\text{--}24.25$ GHz) and 5G ($24.25\text{--}28.35$ GHz). Measurements show that the proposed antenna not only has a broad impedance bandwidth but also an improved radiation bandwidth.

Spin-orbit torque (SOT) has great potential application for developing next-generation magnetic random-access memory (MRAM). For efficient utilization of the SOT MRAM, most efforts have been focused on reducing power consumption by improving the SOT efficiency. Here, we report that inserting an ultrathin ${\mathrm{Ir}}_x{\mathrm{Mn}}_{1\text{−}x}$ (or ${\mathrm{Pt}}_x{\mathrm{Mn}}_{1\text{−}x}$) layer at the heavy-metal–ferromagnet interface is an effective strategy to increase the SOT efficiency. By performing spin-torque ferromagnetic magnetic resonance and second-harmonic Hall measurements, we found that the absolute values of the charge-to-spin conversion efficiency increase from 0.09 for annealed W-${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ (CFB) sample to 0.15 for annealed $\mathrm{W}\text{−}{\mathrm{Ir}}_x{\mathrm{Mn}}_{1\text{−}x}$-CFB sample. The enhancement of the SOT efficiency can be attributed to the reduction of interfacial spin-memory loss at the annealed $\mathrm{W}\text{−}{\mathrm{Ir}}_x{\mathrm{Mn}}_{1\text{−}x}$ (or ${\mathrm{Pt}}_x{\mathrm{Mn}}_{1\text{−}x})\text{−}\mathrm{CFB}$ samples. Moreover, current-driven magnetization switching with a reduced critical current density has been achieved in the annealed $\mathrm{W}\text{−}{\mathrm{Ir}}_x{\mathrm{Mn}}_{1\text{−}x}\text{−}\mathrm{CFB}$ samples. This study highlights the significant roles of the ${\mathrm{Ir}}_x{\mathrm{Mn}}_{1\text{−}x}$ (or ${\mathrm{Pt}}_x{\mathrm{Mn}}_{1\text{−}x}$) insertion layer on improving the SOT efficiency and provides a strategy to improve the SOT efficiency through nanoengineering of the ${\mathrm{Ir}}_x{\mathrm{Mn}}_{1\text{−}x}$ (or ${\mathrm{Pt}}_x{\mathrm{Mn}}_{1\text{−}x}$) insertion layer for energy-efficient SOT devices.

In the field of quantum computation and communication, there is a compelling need for quantum coherent frequency conversion between microwave electronics and infrared optics. A promising platform for this is an optomechanical crystal resonator that uses simultaneous photonic and phononic crystals to create a colocalized cavity coupling an electromagnetic mode to an acoustic mode, which then via electromechanical interactions can undergo direct transduction to electronics. The majority of the work in this area has been on one-dimensional nanobeam resonators, which provide strong optomechanical couplings but, due to their geometry, suffer from an inability to dissipate heat produced by the laser pumping required for operation. Recently, a quasi-two-dimensional optomechanical crystal cavity has been developed in silicon, exhibiting similarly strong coupling with better thermalization but at a mechanical frequency above optimal qubit operating frequencies. Here, we adapt this design to gallium arsenide, a natural thin-film single-crystal piezoelectric that can incorporate electromechanical interactions, obtaining a mechanical resonant mode at $f_{\mathrm{m}}\approx 4.5\mathrm{GHz}$ that is ideal for superconducting qubits and demonstrating optomechanical coupling of $g_{\mathrm{om}}/(2\pi )\approx 650\mathrm{kHz}$.

One of the most important magnetic spin structures is the topologically stabilized skyrmion quasiparticle. Its interesting physical properties make it a candidate for memory and efficient neuromorphic computation schemes. For device operation, the detection of the position, shape, and size of skyrmions is required and magnetic imaging is typically employed. A frequently used technique is magneto-optical Kerr microscopy, in which, depending on the sample’s material composition, temperature, material growing procedures, etc., the measurements suffer from noise, low contrast, intensity gradients, or other optical artifacts. Conventional image analysis packages require manual treatment, and a more automatic solution is required. We report a convolutional neural network specifically designed for segmentation problems to detect the position and shape of skyrmions in our measurements. The network is tuned using selected techniques to optimize predictions and, in particular, the number of detected classes is found to govern the performance. The results of this study show that a well-trained network is a viable method of automating data preprocessing in magnetic microscopy. The approach is easily extendable to other spin structures and other magnetic imaging methods.

We apply the transfer-matrix method to the equations of wave motion in a magnetoelectroelastic (MEE) solid and compute the dispersion of an MEE slab waveguide. The dispersion curves exhibit periodic regions of resonance, where the group velocity approaches zero and the mechanical coupling is reinforced. Inside these regions, the group velocity of higher-order modes can slow to ${10}^3-{10}^4\mathrm{m}/\mathrm{s}$. Substantial nonreciprocity, e.g., a 14.2% difference in phase velocity between forward and backward waves, can be realized within an MEE waveguide at select directions and frequencies. This suggests MEE waveguides as a potential alternative for magnet-free nonreciprocal devices.

Identifying and calibrating quantitative dynamical models for physical quantum systems is important for a variety of applications. Here we present a closed-loop Bayesian learning algorithm for estimating multiple unknown parameters in a dynamical model, using optimized experimental “probe” controls and measurement. The estimation algorithm is based on a Bayesian particle filter, and is designed to autonomously choose informationally optimized probe experiments with which to compare to model predictions. We demonstrate the performance of the algorithm in both simulated calibration tasks and in an experimental single-qubit ion-trap system. Experimentally, we find that, with $60$ times fewer samples, we exceed the precision of conventional calibration methods, delivering an approximately $93$ times improvement in efficiency (as quantified by the reduction of measurements and resets required to achieve a target residual uncertainty and multiplied by the increase in accuracy). In simulated and experimental demonstrations, we see that successively longer pulses are selected as the posterior uncertainty iteratively decreases, leading to an exponential improvement in the accuracy of model parameters with the number of experimental queries, and a commensurate increase in the per-query time.

Focusing ultrasound through the skull requires the accurate correction of skull-induced aberrations. Acoustic lenses provide an inexpensive and effective measure for correcting these distortions. This study explores a phase unwrapping technique for designing multifrequency acoustic lenses coupled with geometrically focused transducers. Here, we demonstrate numerically and theoretically that, in a homogeneous medium and for a single target steered from the natural focus of the transducer, a wrapped lens can only work at its design frequency, while a phase unwrapped lens is broadband. This concept is further explored in experiments where three human skulls are inserted between a transducer and its natural focus. Unwrapped lenses are used to correct for the skull aberrations at multiple frequencies and are compared with their wrapped counterparts. Such experiments demonstrate that only the unwrapped lenses are capable of correcting for skull aberrations in a broad range of frequencies. At the design frequency of the lenses, the unwrapped lenses perform quantitatively better than their wrapped counterparts since the average intensity behind the three skulls is found to be 49% lower with a phase wrapped lens compared with a phase unwrapped lens, even if both allow the qualitative refocusing of the wave at the desired targets. In addition, such results confirm the necessity of correcting for skull aberrations, since the field may be highly distorted in the absence of aberration correction.

Because of their small size, low loss, and compatibility with magnetic fields, and elevated temperatures, surface acoustic wave (SAW) resonators hold significant potential as future quantum interconnects. Here, we design, fabricate, and characterize gigahertz-frequency SAW resonators with the potential for strong capacitive coupling to nanoscale solid-state quantum systems, including semiconductor quantum dots. Strong capacitive coupling to such systems requires a large characteristic impedance, and the resonators we fabricate have impedance values above $100\mathrm{\Omega }$. We achieve such high impedance values by tightly confining a Gaussian acoustic mode. At the same time, the resonators have low loss, with quality factors of several thousand at millikelvin temperatures. These high-impedance resonators are expected to exhibit large vacuum electric field fluctuations and have the potential for strong coupling to a variety of solid-state quantum systems.

Astronomical kinetic inductance detectors (KIDs), similar to quantum information devices, experience performance-limiting noise from materials. In particular, $1/f$ (frequency) noise arises from two-level system defects (TLSs) in the circuit dielectrics and material interfaces and can be a dominant noise mechanism. Here, we present a dual-resonator KID (DuRKID), which is designed for improved noise-equivalent power relative to standard $1/f$-noise-limited KIDs. In this study we present the DuRKID schematic, a fabricated example, our first measurement results, a theoretical model including $1/f$ noise, and a system-noise model containing additional noise sources. The circuit consists of two superconducting resonators sharing an electrical capacitance bridge of four capacitors, each of which hosts TLSs. The device is intended to operate using hybridization of the modes, which causes TLSs to either couple to one mode or the other, depending upon which capacitor they reside in. In contrast, the signal will affect a resonator inductance and, due to mode hybridization, this causes correlated frequency changes in both modes. Therefore, one can better distinguish the photon signal from the TLS frequency noise. To achieve hybridization, a $\mathrm{Ti}\mathrm{N}$ inductor is current biased to allow tuning of one bare-resonator mode into degeneracy with the other. Measurements show that the resonator modes hybridize as expected. The inter-resonator coupling and unintentional coupling of the two resonators to transmission lines are also characterized in measurements. A quantum information-science model allows device-parameter extraction from experimental data and a $1/f$ noise analysis with uncorrelated noise. A system-noise analysis of the DuRKID, with comparisons to standard KIDs, is performed with generation-recombination noise and amplifier noise. The study reveals that the DuRKID can exhibit a large performance advantage over TLS-limited KID detectors.

We report the formation of arbitrary photoconductive patterns made of tellurium ($\mathrm{Te}$) nanocrystals by exposing a tellurite (${\mathrm{Te}\mathrm{O}}_2$-based) glass to femtosecond laser pulses. During this process, $\mathrm{Te}/{\mathrm{Te}\mathrm{O}}_2$-glass nanocomposite interfaces with photoconductive properties form on the tellurite glass substrate. We show that these laser-written patterns exhibit a photoresponse, from the near ultraviolet (263 nm) to the visible spectrum, stable over a few months. Specifically, high responsivity (16.55 A/W) and detectivity (5.25 × ${10}^{11}$ Jones) of a single laser-written line pattern are measured for an illumination dose of 0.07 $\mathrm{mW}/{\mathrm{cm}}^2$ at 400 nm. This work illustrates a pathway for locally turning a tellurite glass into a functional photoconductor of arbitrary shape, without adding materials and using a single laser process step.

In quantum illumination, various detection schemes have been proposed for harnessing the remaining quantum correlations of the entanglement-based resource state. To date, the only successful implementation in the microwave domain [R. Assouly, R. Dassonneville, T. Peronnin, A. Bienfait, and B. Huard, Nat. Phys. 19, 1418 (2023)] has relied on a specific mixing operation of the respective return and idler modes, followed by single-photon counting in one of the two mixer outputs. We investigate the performance of this scheme for realistic detection parameters in terms of the detection efficiency, dark-count probability, and photon-number resolution. Furthermore, we take the second mixer output into account and investigate the advantage of correlated photon counting (CPC) for a varying thermal background and optimum postprocessing weighting in CPC. We find that the requirements for photon-number resolution in the two mixer outputs are highly asymmetric due to different associated photon-number expectation values.

In high-sensitivity bolometers and calorimeters, the photon absorption often occurs at a finite distance from the temperature sensor to accommodate antennas or avoid the degradation of superconducting circuitry exposed to radiation. As a result, thermal propagation from the input to the temperature readout can critically affect detector performance. In this paper we model the performance of a graphene bolometer, accounting for electronic thermal diffusion and dissipation via electron-phonon coupling at low temperatures in three regimes: clean, supercollision, and resonant scattering. Our results affirm the feasibility of a superconducting readout without Cooper-pair breaking by mid- and near-infrared photons, and provide a recipe for designing graphene absorbers for calorimetric single-photon detectors. We investigate the trade-off between the input-readout distance and detector efficiency, and predict an intrinsic timing jitter of approximately $2.7$ ps. Based on our result, we propose a spatial-mode-resolving photon detector to increase the communication bandwidth.

Next-generation microwave communications systems face several challenges, particularly from congested communications frequencies and complex propagation environments. We present, and experimentally test, a framework based on the coupled-dipole approximation for designing structures composed of a single simple emitter with a passive disordered scattering structure of rods that is optimized to provide a desired radiation pattern. Our numerical method provides an efficient way to model, and then design and test, otherwise inaccessibly large scattering systems.

Digital optical phase conjugation (DOPC) emerges as a promising technique for controllable optical delivery in strongly scattering media. Notably, due to the long-standing challenges in accurate alignment of the wave-front sensor and spatial light modulator (SLM), conventional DOPC systems heavily rely on digital calibration for misalignments between these two devices, which will significantly increase the pixel crosstalk of the SLM and thus degrade the performance of DOPC systems. To circumvent this digital calibration for mitigation of the pixel crosstalk, here we propose and demonstrate a real-space alignment scheme for DOPC systems based on single-pixel imaging, which enables precise alignment of the SLM and wave-front sensor: (i) with off-plane alignment precision of submillimeter and milliradian for axial and angular positions, respectively, and (ii) with in-plane pixel match at subpixel resolution. With additional efforts on fidelity optimization of optical phase-conjugation, deformations on the phase-conjugated wave front can almost be removed and the DOPC system would exhibit ultrahigh performance, with peak-to-background ratio (PBR) approaching the theoretical limit (more than 90%) in the time-reversed optical refocusing test. It can be anticipated that all DOPC-based applications will tremendously benefit from this near-theoretical-limit performance of optical phase conjugation for improved capacity and practical utility.

Vector magnetometry provides more information than scalar measurements for magnetic surveys utilized in space, defense, medical, geological, and industrial applications. These areas would benefit from a mobile vector magnetometer. Here we present a scanning fiber-coupled nitrogen-vacancy (N-$V$) center vector magnetometer. Feedback control of the microwave excitation frequency is used to improve dynamic range and maintain sensitivity during movement of the sensor head. Tracking of the excitation frequency shifts for all four orientations of the N-$V$ center allows us to image the vector magnetic field of a damaged steel plate. We calculate the magnetic tensor gradiometry images in real time, and they allow us to detect smaller damage than is possible with vector or scalar imaging.

To improve the performance of thermoelectric materials, a highly effective and widely implemented approach is to create multiphase composites. These composites are designed to impede phononic heat transport, which accounts for the majority of thermal conductivity in conventional thermoelectric semiconductors. In 1999, Bergman and Fel [J. Appl. Phys. 85(12), 8205–8216 (1999)] reported that also the electronic properties, specifically the power factor $S^2\sigma$, can be significantly enhanced in two-phase composites consisting of a highly conducting, simple metal and a high-performance thermoelectric arranged in an optimal manner, sparking great experimental interest. In this work, we challenge the theoretical results of Bergman and Fel and the conclusions drawn therein by utilizing a simple serial model. We show that, while the improvement of the power factor is indeed extraordinary, the results lead to a misleading interpretation of the overall thermoelectric performance of the material. As a result, we argue that the power factor is not a suitable metric for evaluating multiphase materials and composites, and that the figure of merit $zT$ must be used instead. Nonetheless, we demonstrate that the best thermoelectric composite consists of a highly conductive metal and a high-performance thermoelectric.