We demonstrate the local control of up to eight two-level systems interacting strongly with a microwave cavity. Following calibration, the frequency of each individual two-level system (qubit) is tunable without influencing the others. Bringing the qubits one by one on resonance with the cavity, we observe the collective coupling strength of the qubit ensemble. The splitting scales up with the square root of the number of the qubits, which is the hallmark of the Tavis-Cummings model. The local control circuitry causes a bypass shunting the resonator, and a Fano interference in the microwave readout, whose contribution can be calibrated away to recover the pure cavity spectrum. The simulator’s attainable size of dressed states with up to five qubits is limited by reduced signal visibility, and—if uncalibrated—by off-resonance shifts of subcomponents. Our work demonstrates control and readout of a quantum coherent mesoscopic multiqubit system of intermediate scale under conditions of noise.

Spin-torque ferromagnetic resonance (ST-FMR) is a common method used to measure spin-orbit torque (SOT) in heavy-metal/ferromagnet bilayer structures. In the course of a measurement, other resonant processes such as spin pumping (SP) and heating can cause spin-current or heat flows between the layers, inducing additional resonant voltage signals via the inverse spin Hall effect (ISHE) and Nernst effects. In the standard ST-FMR geometry, these extra artifacts exhibit a dependence on the angle of an in-plane magnetic field that is identical to the rectification signal from the SOT. We show experimentally that the rectification and artifact voltages can be quantified separately by measuring the ST-FMR signal transverse to the applied current (i.e., in a Hall geometry) in addition to the usual longitudinal geometry. We find that in $\mathrm{Pt}$(6 nm)/${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ samples the contribution from the artifacts is small compared with the SOT rectification signal for ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ layers thinner than 6 nm, but can be significant for thicker magnetic layers. We observe a sign change in the artifact voltage as a function of the ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ thickness that we suggest may be due to competition between a resonant heating effect and the SP-ISHE contribution.

A finite one-dimensional Su-Schrieffer-Heeger (SSH) chain exhibits “zero-energy” boundary-mode solutions that are protected by chiral symmetry. The breaking of chiral symmetry leads to several important consequences, including a shift of the boundary mode energies. Here, we systematically study the coupled acoustic-cavity system (CACS), which is an important acoustic platform for realizing tight-binding models (TBMs). We find that the length and number of coupling waveguides not only affect hopping, but also induce a perturbation to the onsite eigenfrequency, which can be attributed to the breaking of chiral symmetry in the TBM. The acoustic origin of these phenomena is discussed, and the conditions of the exact realization of TBMs are identified. Meanwhile, we build an acoustic second-order topological insulator by extending the SSH model to two dimensions and show that the frequency of the topological corner modes is tunable by the same chiral-symmetry-breaking term. This finding is experimentally validated through the demonstration of in-gap and in-band topological corner modes. Our study provides a detailed and accurate understanding of the CACS and clarifies several important nuances for realizing tight-binding systems in acoustics. These results solidify CACS as a foundation for future studies of topological acoustics and non-Hermitian acoustics.

Sound is usually generated in a medium by an electromechanical vibrating structure. The geometrical size and inertia of the structure set the frequency cutoff in the sound-transduction mechanism and, often, different vibrating structures are necessary to cover the whole range from infrasound to ultrasound. An alternative mechanism without any physical movement of the emitter is the thermoacoustic effect, where sound is produced by Joule heating in a conductive material. Here we show that a single thermoacoustic transducer based on a graphene aerogel can emit ultrabroadband sound from infrasound (1 Hz) to ultrasound (20 MHz), with no harmonic distortion. Since conventional acoustic transducers are frequency band limited due to their transduction mechanism, ultrabroadband graphene aerogels may offer a valid alternative to conventional hi-fi loudspeakers, and infrasound and ultrasound transducers.

The fragility of quantum systems makes them ideally suited for sensing applications at the nanoscale. However, interpreting the output signal of a qubit-based sensor is generally complicated by background clutter due to out-of-band spectral leakage, as well as ambiguity in signal origin when the sensor is operated with noisy hardware. We present a sensing protocol based on optimally band-limited “Slepian functions” that can overcome these challenges, by providing narrowband sensing of ambient dephasing noise, coupling additively to the sensor along the $z$ axis, while permitting isolation of the target noise spectrum from other contributions coupling along a different axis. This is achieved by introducing a finite-difference control modulation, which linearizes the sensor’s response and affords tunable band-limited “windowing” in frequency. Building on these techniques, we experimentally demonstrate two spectral estimation capabilities using a trapped-ion qubit sensor. We first perform efficient experimental reconstruction of a “mixed” dephasing spectrum, composed of a broadband $1/f$-type spectrum with discrete spurs. We then demonstrate the simultaneous reconstruction of overlapping dephasing and control noise spectra from a single set of measurements, in a setting where the two noise sources contribute equally to the sensor’s response. Our approach provides a direct means to augment quantum-sensor performance in the presence of both complex broadband noise environments and imperfect control signals, by optimally complying with realistic time-bandwidth constraints.

Efficient conversion of a spin signal into an electric voltage in mainstream semiconductors is one of the grand challenges of spintronics. This process is commonly achieved via a ferromagnetic tunnel barrier, where nonlinear electric transport occurs. In this work, we demonstrate that nonlinearity may lead to a spin-to-charge conversion efficiency larger than 10 times the spin polarization of the tunnel barrier when the latter is under a bias of a few millivolts. We identify the underlying mechanisms responsible for this remarkably efficient spin detection as the tunnel-barrier deformation and the conduction-band shift resulting from a change of applied voltage. In addition, we derive an approximate analytical expression for the detector spin sensitivity $P_{\text{det}}(V)$. Calculations performed for different barrier shapes show that this enhancement is present in oxide barriers as well as in Schottky-tunnel barriers, even if the dominant mechanisms differ with the barrier type. Moreover, although the spin signal is reduced at high temperatures, it remains superior to the value predicted by the linear model. Our findings shed light onto the interpretation and understanding of electrical spin-detection experiments and open paths to optimizing the performance of spin-transport devices.

Despite the very high efficiency at blue wavelengths, nitride-based light-emitting diodes suffer from low efficiencies in the green and red spectral ranges. Different solutions have been proposed to mitigate the “green gap,” including the addition of an ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ capping layer to the quantum well structure. In this work, we show how the increased polarization field due to such an ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ capping layer has profound effects on carrier location and recombination probabilities. The proper choice of ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ composition leads to an increased electron-hole overlap as well as an enhanced confinement in the quantum well region. Therefore, the combination of band structure and electric field engineering can be a promising approach to mitigate the green gap problem.

We describe a method for the rapid determination of the mass of particles confined in a free-space optical dipole force trap. The technique relies on direct imaging of drop-and-restore experiments without the need for a vacuum environment. In these experiments, the trapping light is rapidly shuttered with an acousto-optic modulator, causing the particle to be released from and subsequently recaptured by the trapping force. The trajectories of both the falls and restorations, imaged using a high-speed CMOS sensor, are combined to determine the particle mass. We corroborate these measurements using an analysis of position autocorrelation functions and the mean-square displacement of the trapped particles. We report a statistical uncertainty of less than 2% for masses on the order of $5\times {10}^{-14}\mathrm{kg}$ using a data acquisition time of approximately 90 s.

As one of the thinnest forms of semiconducting silicon, monolayer (ML) silicane has not only excellent gate electrostatics and carrier transport ability, but also compatibility with well-established silicon-based technology. We explore the device performance limits of sub-5-nm ML silicane metal-oxide-semiconductor field-effect transistors (MOSFETs) by applying ab initio quantum transport simulations. The on-state current, effective delay time, and power-delay product of the optimized n-type and p-type ML silicane MOSFETs can well or nearly meet the high-performance device requirements of the International Technology Roadmap for Semiconductors (ITRS) at a gate length of 5 nm. Those of the optimized n-type ML silicane MOSFETs at a gate length of 3 nm and the p-type ML silicane MOSFETs at a gate length of 5 nm can meet the low-power-device demands of the ITRS. Thus, ML silicane as channel materials can scale the Moore’s law down to 5 nm.

How to achieve the maximal entanglement, for given finite entanglement resources? In this paper, we propose a two-port feedback optical parametric amplifier (TFPA) consisting of a nondegenerate optical parametric amplifier (NOPA) and two linear beam splitters. The former is an entanglement resource, and the latter serve as feedback controllers that feed the two entangled beams generated from the NOPA back into its input ports. By manipulating the reflectivity of the beam splitters and the phase delays introduced by the feedback paths, an entanglement enhancement can be achieved by the TFPA. Furthermore, we demonstrate that such entanglement enhancement can happen at all phase points for the ideal case, and the maximal entanglement can be infinitely large. For the lossy case, our TFPA also has its unique advantages in the robustness of the losses of the system.

Polycrystalline $(\mathrm{Sb},\mathrm{Bi}{)}_2{\mathrm{Se}}_3$ thin-film semiconductors are grown by coevaporation with a subsequent annealing process. It is shown that $\mathrm{Bi}$ can be incorporated into the ${\mathrm{Sb}}_2{\mathrm{Se}}_3$ lattice, substituting up to approximately 60% of the $\mathrm{Sb}$ atoms, while maintaining the orthorhombic crystal structure. Upon $\mathrm{Bi}$ substitution, the lattice expands mainly along the one-dimensional $(\mathrm{Sb},\mathrm{Bi}{)}_4{\mathrm{Se}}_6$ ribbons. In addition, the band gap decreases with a direct (indirect) band gap of 0.891 eV (0.864 eV) for a $({\mathrm{Sb}}_{0.4},{\mathrm{Bi}}_{0.6}{)}_2{\mathrm{Se}}_3$ thin film. A photovoltaic device based on a $(\mathrm{Sb},\mathrm{Bi}{)}_2{\mathrm{Se}}_3$ absorber is fabricated that displays an open-circuit voltage of 133 mV and a short-circuit current density of $18.4\mathrm{mA}/{\mathrm{cm}}^2$, demonstrating the potential of this material for infrared detection or multijunction solar-cell applications.

Integrating quantum key distribution (QKD) on the existing optical fiber network through dense wavelength-division multiplexing (DWDM) can greatly reduce the costs of quantum channels and improve the scalability and flexibility of the quantum communication network. In the scenarios of an access network or local area network with a large number of users, the connected optical fibers are relatively short and the quantum signal can be adjacent to the classical channels. In this case, the four-wave-mixing noise generated by the classical lights will have a severe impact on the performance of the QKD. We establish a theoretical model to characterize the excess noise in continuous-variable QKD induced by the four-wave mixing, and experimentally demonstrate the validity of the theoretical predictions. In addition, the influence of cross-phase-modulation noise between the carrier of the quantum signal and classical data channels on the QKD is investigated. To suppress the effect of the four-wave-mixing noise, an unequal channel spacing technique is employed. We realize coexistence of entangled state-based continuous-variable QKD and five adjacent DWDM classical channels with 10 dBm launch power and nonreturn-to-zero on-off-keying modulation at 2.5 Gb/s (10 Gb/s).

High-$T_c$ superconducting (HTS) dynamos are experimentally proven devices that can produce large (more than a kiloamp) dc currents in superconducting circuits, without the thermal leak associated with copper current leads. However, these dc currents are theoretically controversial, as it is not immediately apparent why a device that is topologically identical to an ac alternator should give a dc output at all. Here, we present a finite-element model and a comparison of it with experiment that fully explain this effect. It is shown that the dc output arises naturally from Maxwell’s laws when time-varying overcritical eddy currents are induced to circulate in a HTS sheet. We first show that our finite-element model replicates all of the experimental electrical behavior reported so far for these devices, including the dc output characteristics and transient electrical waveforms. Direct experimental evidence for the presence of circulating eddy currents is also obtained through measurements of the transient magnetic field profile across the HTS tape, using a linear Hall array. These results are also found to agree closely with predictions from the finite-element model. Following this experimental validation, calculated sheet current densities and the associated local electric fields are examined for a range of frequencies and net transport currents. We find that the electrical output from a HTS dynamo is governed by the competition between transport and eddy currents induced as the magnet transits across the HTS tape. The eddy currents are significantly higher (approximately 1.5 times) than the local critical current density, and hence experience a highly nonlinear local resistivity. This nonlinearity breaks the symmetry observed in a normal ohmic material, which usually requires the net transport current to vary linearly with the average electric field. The interplay between local current densities and nonlinear resistivities (which both vary in time and space) is shown to systematically give rise to the key observed parameters of experimental HTS dynamo devices: the open-circuit voltage, the internal resistance, and the short-circuit current. Finally, we identify that the spatial boundaries formed by each edge of the HTS stator tape play a vital role in determining the total dc output. This offers the potential to develop alternative designs for HTS dynamo devices, in which the internal resistance is greatly reduced and the short-circuit current is substantially increased.

Reverse intersystem crossing (RISC) unexpectedly dominates the interconversion of polaron pairs in the traditional fluorescent material 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB), with a large energy gap (Δ$E_{\mathrm{ST}}$) between singlet and triplet excitons, by employing the magnetoelectroluminescence detection technique. Contrary to RISC among excitons with small Δ$E_{\mathrm{ST}}$ from conventional thermally activated delayed fluorescence, these RISC processes are increased at a high current density and low temperature, which is ascribed to an enhanced direct charge-trapping effect and prolonged lifetime of the triplet-polaron pair, respectively. More intriguingly, the conversion of RISC to ISC is observed by reducing the electron-hole distance of polaron pairs via increasing the concentration of DCJTB dopant. Furthermore, the host must have a high triplet exciton energy to prevent energy back-transfer of the triplet exciton from the guest to the host and ensure efficient occurrence of the RISC process. The RISC-dominated device has a markedly higher external quantum efficiency than that of the ISC-governed one. This work not only deepens the microscopic understanding of the spin-pair states of red-light-emitting DCJTB-doped devices, but also provides a useful method to utilize a conventional fluorescent dopant for designing high-efficiency organic light-emitting diodes.

Quantum cryptographic conferencing (QCC) holds promise for distributing information-theoretic secure keys among multiple users over a long distance. Limited by the fragility of Greenberger-Horne-Zeilinger (GHZ) states, QCC networks based on directly distributing GHZ states over a long distance still face a big challenge. Another two potential approaches are measurement device-independent QCC and conference-key agreement with single-photon interference, which were proposed on the basis of the postselection of GHZ states and the postselection of the W state, respectively. However, implementations of the former protocol are still heavily constrained by the transmission rate $\eta$ of optical channels and the complexity of the setups for postselecting GHZ states. Meanwhile, the latter protocol cannot be cast as a measurement device-independent prepare-and-measure scheme. Combining the idea of postselecting GHZ states and recently proposed twin-field quantum-key-distribution protocols, we report a QCC protocol based on weak coherent-state interferences named “phase-matching quantum cryptographic conferencing,” which is immune to all detector side-channel attacks. The proposed protocol can improve the key-generation rate from $O({\eta }^N)$ to $O({\eta }^{N-1})$ compared with the measurement device-independent QCC protocols. Meanwhile, it can be easily scaled up to multiple parties due to its simple setup.

We fabricate vertical $\mathrm{Nb}/{\mathrm{Ti}\mathrm{O}}_x/\mathrm{Nb}$ Josephson junctions where the ${\mathrm{Ti}\mathrm{O}}_x$ layer is mostly metallic but has a high-resistance interface with the top $\mathrm{Nb}$ electrode. Thus, the junctions are in essence of the superconductor–insulator–normal-metal–superconductor type. The ${\mathrm{Ti}\mathrm{O}}_x$ layer extends beyond the junction area and allows one to send an in-plane current through the normal-metal layer of the junction. We investigate and analyze dc properties (critical current, current-voltage characteristics) as well as the behavior of the junctions in external microwave fields. We find a strong dependence of all properties on the voltage $V_i$ applied across the ${\mathrm{Ti}\mathrm{O}}_x$ lead. The dependence on $V_i$ can be mapped to the dependence of the junction parameters on the bath temperature $T$, when $V_i$ is converted to an effective electronic temperature via $T_e=[T^2+(a{V}_i{)}^2{]}^{1/2}$, with $a=3.14\mathrm{K}/\mathrm{mV}$. This relation was used before in the context of a metallic nanowire contacted by superconducting electrodes. While the geometry of our ${\mathrm{Ti}\mathrm{O}}_x$ layer is far from that of a nanowire, the scaling works perfectly well for all experimental data. Our data also indicate that the electronic temperature is homogeneous throughout the junction area, a result that is unexpected for the large junction sizes we use. The device allows easy and relatively fast in situ manipulation of the junction parameters.