Superconducting nanowire single-photon detectors (SNSPDs) offer high-quantum-efficiency and low-dark-count-rate single-photon detection. In a growing number of cases, large magnetic fields are being incorporated into quantum microscopes, nanophotonic devices, and sensors for nuclear and high-energy physics that rely on SNSPDs, but superconducting devices generally perform poorly in large magnetic fields. Here, we demonstrate robust performance of amorphous SNSPDs in magnetic fields of up to $\pm 6$ T with a negligible dark-count rate and unchanged quantum efficiency at typical bias currents. Critically, we also show that the SNSPD can be used as a magnetometer with a sensitivity of better than $100\mu \mathrm{T}/\sqrt{\mathrm{Hz}}$ and as a thermometer with a sensitivity of $20\mu \mathrm{K}/\sqrt{\mathrm{Hz}}$ at 1 K. Thus, a single-photon detector integrated into a quantum device can be used as a multifunctional quantum sensor capable of describing the temperature and magnetic field on chip simply by varying the bias current to change the operating modality from single-photon detection to thermometry or magnetometry.

Currently, thermally excited magnons are being intensively investigated, owing to their potential in computing devices and thermoelectric conversion technologies. We report the detection of a thermal magnon current propagating in a magnetic insulator yttrium iron garnet under a temperature gradient using a quantum sensor: electron spins associated with nitrogen-vacancy (N-V) centers in diamond. A thermal magnon current is observed as modified Rabi-oscillation frequencies of N-V spins hosted in a beam-shaped bulk diamond that is resonantly coupled with coherent magnon propagating over a long distance. Additionally, using a nanodiamond, alteration in N-V spin-relaxation rates, depending on the applied temperature gradient, are observed under nonresonant N-V excitation conditions. The demonstration of probing a thermal magnon current mediated by coherent magnons via N-V spin states serves as a basis for creating a device platform that hybridizes spin caloritronics and spin qubits.

In this research, we propose and design an acoustic metamuffler (AMM) by coupling a microperforated plate and a composite waveguide formed by a main waveguide and a Helmholtz resonator. The proposed mechanism and deliberately designed structure are conducive to generating multimode resonances that help to improve the coupling absorption effect and lead to broadband (4 octaves) sound insulation. We develop an effective circuit model to analytically predict the insulation bandwidth and put forward numerical and experimental measurements that demonstrate the effectiveness of the proposed concept. The designed AMM produces sound insulation with an average of 20 dB of sound-transmission loss at a low-frequency range, extending from 100 to 1600 Hz, while having an ultrathin thickness of 6.2 cm (1/55λ for the lowest working frequency). Our findings could have pragmatic applications for acoustic insulators or absorbers.

High-precision frequency estimation is an ubiquitous issue in fundamental physics and a critical task in spectroscopy. Here, we propose a quantum Ramsey interferometry to realize high-precision frequency estimation in a spin-$1$ Bose-Einstein condensate via driving the system through quantum phase transitions (QPTs). In our scheme, we combine adiabatically driving the system through QPTs with $\pi /2$ pulse to realize the initialization and recombination. Through adjusting the laser frequency under fixed evolution time, one can extract the transition frequency via the lock-in point. The lock-in point can be determined from the pattern of the population measurement. In particular, we find the measurement precision can approach the Heisenberg-limited scaling. Moreover, the scheme is robust against detection noise and nonadiabatic effect. Our proposed scheme does not require single-particle resolved detection and is within the reach of current experiment techniques, which may point out an alternative way for high-precision frequency estimation.

We report on a detailed study of low-temperature photoluminescence (PL) and magnetophotoluminescence under perpendicular magnetic fields (up to 30 T) and circularly polarized excitation on high-quality monolayer (ML) ${\mathrm{WS}}_2$ on a thick layer (120 nm) of talc dielectric. Remarkably, we obtained high-quality samples without any evidence of localized exciton emission at low temperature and a PL linewidth comparable to that of ${\mathrm{WS}}_2/h$-$\mathrm{BN}$ samples. As a consequence, we observe well-resolved emission peaks at low temperature due to the formation of excitonic complexes, including a dark-trion (DT) state and phonon replicas of the DT without the application of an in-plane magnetic field. The nature of the emission peaks, the magnetic field dependence of the degree of polarization, and g factors are discussed in detail and compared with the corresponding results obtained for $h$-$\mathrm{BN}$ encapsulated transition-metal dichalcogenide (TMD) samples. We observe that under ${\sigma }^{+}$-polarized excitation the sign of the circular polarization of biexcitons is reversed under higher magnetic fields. In addition, the dark-trion polarization increases considerably with increasing perpendicular magnetic field, demonstrating different behavior compared with previous studies of dark trions on monolayer ${\mathrm{WSe}}_2$. Our results suggest that talc is indeed a promising layered material for the surface protection of ML TMDs and to explore fundamental physics in view of applications in optoelectronic devices.

We evaluate the influence of Coulomb drag of electrons and holes in gated $n$ and $p$ regions caused by ballistic electrons and holes generated in a depleted $i$ region due to interband tunneling on the current-voltage characteristics and impedance of $p^{+}$-$p$-$i$-$n$-$n^{+}$ graphene tunneling transistor structures (GTTSs). This drag leads to current amplification in the gated $n$ and $p$ regions and positive feedback between the amplified dragged current and the injected tunneling current. A sufficiently strong drag can result in a negative real part of the GTTS impedance, enabling a plasma instability and self-excitation of plasma oscillations in the terahertz (THz) frequency range. This effect might be used for the generation of THz radiation.

Based on the self-consistent calculations of the eight-band $\mathbf{k}\cdot \mathbf{p}$ model and Poisson equation, we theoretically demonstrate that the intersubband optical absorption of $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ type-II quantum wells (QWs) is effectively tuned by the electric-field-driven band-inversion transition. By utilizing this property, $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ QWs can be made into highly efficient electroabsorption modulators (EAMs) for TM-polarized mid-infrared radiation (MIR) (approximately $4.2$–$12\mu \mathrm{m}$) and TE-polarized far-infrared radiation (FIR) (approximately $12$–$100\mu \mathrm{m}$). At low temperature, an example $p$-$i$-$n$ waveguide EAM composed of six-period $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ QWs achieves a drive voltage of $<0.5$ V, a 3-dB cutoff bandwidth of approximately $24.6$ GHz, and a dynamic power consumption of $\le 0.06$ mW/GHz for a 20-dB on:off ratio modulation of $4.46$-$\mu \mathrm{m}$ TM-polarized radiation. Although the modulation efficiencies drop by 70%–80% when the temperature increases, the performance of $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ EAMs is (at least) comparable with conventional $1.55$-$\mu \mathrm{m}$ EAMs at 300 K. These results suggest that $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ EAMs are promising devices for high-speed and low-power-consumption MIR (or FIR) free-space optical communications.

Phonon drag due to momentum exchange between electrons and phonons can lead to a substantially increased Seebeck coefficient desirable for thermoelectric energy conversion. However, this effect is only usually observable at low temperatures when the phonon mean free path is long, and it is thought to become negligible above room temperature or in heavily doped materials due to strong phonon scattering. Here, we present first-principles calculations of all thermoelectric transport properties of silicon-germanium alloys from 150 to 1100 K. Results show that phonon drag is dominant at low temperatures. At 1100 K, phonon drag still contributes to 10%–20% of the thermoelectric figure of merit, zT, and its relative contribution increases with higher carrier concentration. The favorable comparison between our calculations and reported experiments brings us closer to predicting the thermoelectric transport properties of alloys using first-principles simulations. The surprising insights in phonon drag could stimulate the search for better thermoelectric materials.

Quantum key distribution (QKD), which enables the secure distribution of symmetric keys between two legitimate parties, is highly useful in future network security. An access network that connects multiple end users with one network backbone can be combined with QKD to build security for end users in a scalable and cost-effective way. In this paper, we show that a downstream-access network can be implemented using continuous-variable (CV) QKD. The security analysis is conducted independently with other network nodes, which greatly relax the trustfulness requirement on the network nodes. The security is then obtained against other network parties even when the quantum signals are broadcast throughout the network. The passive beam splitter is sufficient to deterministically distribute the quantum signals, and standard CV QKD systems can be directly fitted in the downstream-access network, which makes it more applicable for practical implementations. Numerous simulation results are provided to demonstrate the performance of the CV QKD downstream-access network, where up to 64 end users are shown to be feasible to access the network. We provide the security-analysis framework for realizing CV QKD in the downstream-access network, which will boost the diversity for constructing practical CV QKD networks.

The nitrogen-vacancy ($\mathrm{N}$-$V$) center in diamond has a wide range of potential applications in quantum metrology, communications, and computation. The key to its use lies in how large the optical spin contrast is and the associated fidelity of the spin-state readout. In this paper, we propose a mechanism for improving contrast with a spin-to-charge protocol that relies on the use of an external electrode and cryogenic temperatures to discretize the diamond conduction band for spin-selective resonant photoionization. We use effective-mass theory to calculate the discrete eigenenergies in this system and use them to formulate a spin-to-charge protocol that involves resonant photoionization out of the $\mathrm{N}$-$V$ ground state into the diamond conduction band. The major sources of broadening, which guide the design of the experiment, are also addressed. With this mechanism, we calculate an optical spin contrast that and an associated spin-readout fidelity of 85%. This significant improvement can be applied to a number of cryogenic quantum technologies.

Single-photon detectors operating on satellites for use in a quantum communications network can incur large dark-count-rate increases from the natural radiation environment of space. Displacement damage to the material lattice of a detector from the ionizing radiation can result in a permanent dark-count increase in the detector. In this work, we analyze the radiation-induced dark-count rate of a silicon single-photon avalanche diode onboard a satellite at different orbiting altitudes, as well as, the additional radiation from a nuclear-disturbed environment caused by a high-altitude nuclear explosion. For detectors on low Earth orbit satellites, protons are the biggest source of radiation damage and are best mitigated by choosing an orbit that minimizes exposure when passing through the South Atlantic Anomaly and Polar Cusps. Detectors on medium Earth orbit and geostationary orbit satellites, if shielded by more than 10 mm of aluminum, provide the best platform in terms of the least amount of radiation damage to the detectors. In the event of a high-altitude nuclear explosion, the artificial radiation belts produced by the explosion will cause too much damage to silicon single-photon detectors on low Earth orbit satellites and render them unfit for quantum communications in less than a day. Higher orbit satellites will only suffer minor dark-count-rate increases from the artificial radiation belts.

We present a simple approach to produce multiple correlated photon pairs through spontaneous parametric down-conversion. We develop Gaussian masks to subdivide the pump beam before passing it through a nonlinear medium. In this way, we are able to observe simultaneous separate down-converted emission cones with spatial overlap. The technique we employ could be used to greatly increase the dimensionality of entangled photonic systems generated from spontaneous parametric down-conversion, affording greater scalability to optical quantum computing than previously explored.

We show scalable and complete suppression of the recently reported terahertz-pulse-induced switching between different resistance states of antiferromagnetic $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$ thin films by ultrafast gating. The gating functionality is achieved by an optically generated transiently conductive parallel channel in the semiconducting substrate underneath the metallic layer. The photocarrier lifetime determines the timescale of the suppression. As we do not observe a direct impact of the optical pulse on the state of $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$, all observed effects are primarily mediated by the substrate. The sample region of suppressed resistance switching is given by the optical spot size, thereby making our scheme potentially applicable for transient low-power masking of structured areas with feature sizes of about 200 nm and even smaller.

Conventional polarized light interference is realized by using the natural crystals sandwiched between two linear polarizers, which has applications in optical signal modulation and photomechanics. However, due to a shortage of suitable birefringent materials, a similar effect has not been reported in the low-frequency range. In this paper, a compact and tunable polarization interference system working in the microwave band has been proposed, employing artificially structured planar materials, i.e., two metal screens perforated with subwavelength apertures. One screen splits the incident linear polarization into two orthogonal electric components and the other combines the parallel electric components (in a certain direction) to induce an interference. We find that the near-field coupling between the two metal screens plays a crucial role and the polarization interference effect is strongly dependent on the lateral dislocation of the system. The theory and experiment demonstrated that, by laterally displacing one metal screen (at the millimeter scale), a nearly linear modulation of transmission intensity (93.1%–1.4%) can be achieved. The proposed effect can be extended to the THz range and may be valuable for constructing the microwave or THz modulators.

The coherent transduction between microwave and optical frequencies is critical to interconnect superconducting quantum processors over long distances. However, it is challenging to establish such a quantum interface with high efficiency and small added noise based on the standard direct conversion scheme. Here, we propose an electro-optic transduction system based on continuous-variable quantum teleportation. Reliable quantum-information transmission can be realized with an arbitrarily small cooperativity, in contrast to the direct conversion scheme, which requires a large minimum cooperativity. We show that the teleportation-based scheme maintains a significant rate advantage robustly for practical thermal noise and all values of cooperativity. We further investigate the performance in the transduction of complex quantum states such as cat states and Gottesman-Kitaev-Preskill (GKP) states and show that a higher fidelity can be achieved with the teleportation-based scheme. Our scheme significantly reduces the device requirement, and makes quantum transduction between microwave and optical frequencies feasible in the near future.

Self-injection locking is a dynamic phenomenon, which provides passive stabilization of a laser emission frequency via resonant optical feedback. The stabilization coefficient depends on the level of the feedback and quality factor of the external resonant structure creating the feedback. A conventional self-injection locked laser (SIL) based on a dielectric cavity involves barely tunable resonant Rayleigh scattering (RRS). In this work we study theoretically a scheme of a SIL via a high-$Q$ microresonator with a drop-port coupled mirror, in which the optical feedback level is optimally adjusted by tuning the drop-port mirror coupling. We show that the additional reflector can improve the laser stabilization and power handling efficiency if compared with the classic RRS-based scheme.

Establishing and approaching the fundamental limit of orbital-angular-momentum (OAM) multiplexing are necessary and increasingly urgent for current multiple-input multiple-output research. In this work, we elaborate the fundamental limit in terms of independent scattering channels (or the degrees of freedom of scattered fields) through angular-spectral analysis, in conjunction with a rigorous Green’s function method. The scattering-channel limit is universal for arbitrary spatial-mode multiplexing, which is launched by a planar electromagnetic device, such as antenna, metasurface, etc., with a predefined physical size. As a proof of concept, we demonstrate both theoretically and experimentally the limit by a phase-only metasurface hologram that transforms orthogonal OAM modes to plane-wave modes scattered at critically separated angular-spectral regions. Particularly, a minimax optimization algorithm is applied to suppress angular-spectrum aliasing, achieving good performances in both full-wave simulation and experimental measurement at microwave frequencies. This work offers a theoretical upper bound and corresponding approach route for engineering designs of OAM multiplexing.

We investigate the ultrastrong tunable coupler for coupling of superconducting resonators. The obtained coupling constant exceeds 1 GHz, and the wide-range tunability is achieved for both antiferromagnetics and ferromagnetics from $-1086$ to 604 MHz. The ultrastrong coupler is composed of a rf superconducting quantum interference device (SQUID) and dc SQUID as tunable junctions, which connect to resonators via shared aluminum thin-film meander lines enabling such a huge coupling constant. The spectrum of the coupler obviously shows the breaking of the rotating-wave approximation, and our circuit model treating the Josephson junction as a tunable inductance reproduces the experimental results well. The ultrastrong coupler is expected to be utilized in quantum annealing circuits and/or noisy intermediate-scale quantum devices with dense connections between qubits.

We propose a one-step scheme to implement the time-optimal nonadiabatic holonomic three-qubit controlled gates in Rydberg atoms by modulating Rabi frequencies of driving fields on the target atom. We perform the time-optimal operations following the brachistochrone curve, after which the scheme manifests completely different characteristics in contrast to the conventional nonadiabatic holonomic quantum computation (NHQC) that all operations require exactly the same evolution time, even for small angular rotations. Since the present three-qubit controlled-gate scheme significantly reduces the evolution time, the influence of decoherence can be greatly reduced. On the other hand, the robustness on the systematic error can also be guaranteed by the feature of geometric phase. The simulation through master equation verifies the high fidelity and robustness of our scheme. Although we use the Rydberg-atom system to verify the feasibility of our scheme, any quantum platforms capable of achieving the same form of effective Hamiltonian is feasible, which means the proposed three-qubit time-optimal-control nonadiabatic holonomic gates is independent of the particular physical platform and may have the potential for wider application.