A different type of terahertz emitter is made by coupling a source mesa fabricated from the intrinsic Josephson junctions present in single-crystalline high-$T_c$ superconducting ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ ($\mathrm{Bi}2212$) to a much larger and impedance-matched external resonator with resonance frequencies up to 1.5 THz. The lateral dimensions of the source mesa are only $20\times 20\mu {\mathrm{m}}^2$, about 0.5%–3% of those of conventional $\mathrm{Bi}2212$ mesas, but its thickness of $2\mu \mathrm{m}$ is comparable to them. The observed emission frequencies that lie near to 1.5 THz match very well the resonance frequencies of the external resonator. This result ensures that the functions of resonator and antenna are separated from that of the THz current source in the $\mathrm{Bi}2212$ mesa structure. These facts allow for much greater flexibility in the emitter design to achieve higher power and higher frequency THz wave sources.

This paper presents a low-loss and broadband impedance-interface waveguide and a broadband mode-matching technique to convert transverse electromagnetic (TEM) mode into line-wave (LW) mode, which is formed at the interface between two complementary impedance surfaces. Furthermore, an ultra-wideband Y splitter is designed based on the one-dimensional (1D) impedance-interface waveguides. Broad bandwidth, low loss, tunable mode confinement, and ability for integration are the benefits of 1D impedance-interface waveguides. However, their drawbacks may stem from the lack of and problematic excitation of a pure LW mode at the interface between two planes. Another limitation is the coupling efficiency between 1D impedance-interface waveguides and conventional microwave waveguides. To ratify such limitations, a broadband tapered-slot Vivaldi transition is designed to excite the proposed 1D impedance-interface waveguide. Subsequently, a Y splitter consisting of complementary impedance surfaces is designed, fabricated, and measured. The Y splitter is implemented in the frequency band of 4–16 GHz, having a bandwidth of $120\mathrm{\%}$ and an insertion loss of approximately 1 dB. The aforementioned characteristics of 1D impedance-interface waveguides are fully verified. Consequently, the proposed waveguide, mode-matching technique, and Y splitter could be used for the design of other microwave devices.

The negative-differential-resistance (NDR) response of $\mathrm{Nb}$/${\mathrm{Nb}\mathrm{O}}_x$/$\mathrm{Pt}$ cross-point devices is shown to have a polarity dependence due to the effect of the metal-oxide Schottky barriers on the contact resistance. Three distinct responses are observed under opposite polarity testing: bipolar S-type NDR, bipolar snapback NDR, and combined S-type and snapback NDR, depending on the stoichiometry of the oxide film and device area. In situ thermoreflectance imaging is used to show that these NDR responses are associated with strong current localization, thereby justifying the use of a previously developed two-zone, core-shell thermal model of the device. The observed polarity-dependent NDR responses, and their dependence on stoichiometry and area, are then explained by extending this model to include the effect of the polarity-dependent contact resistance. This study provides an improved understanding of the NDR response of metal-oxide-metal structures and informs the engineering of devices for neuromorphic computing and nonvolatile memory applications.

Single molecules in solids have been considered as an attractive class of solid-state single quantum systems because they can be chemically synthesized to have stable transitions at desired wavelengths. Here, we report and demonstrate single dibenzoterrylene molecules in crystalline anthracene nanosheets as a solid-state platform for delicate photophysics studies and as building blocks of single-photon devices. The nanosheet sample enables studies of single-molecule photophysics at room temperature, including in situ observation of single-molecule insertion-site jump, quantitative measurements of the associated changes of dipole-moment orientation and magnitude, and determination of the excitation-power-dependent intersystem crossing rate and the triplet lifetime. Moreover, we demonstrate the flexible assembly of the nanosheets into a planar Yagi-Uda antenna device to achieve a Gaussian-like angular emission pattern from single dibenzoterrylene molecules. The low thickness (approximately 100 nm), good photostability, and mechanical stability make the dibenzoterrylene-in-anthracene-nanosheet system an excellent candidate for static quantum nodes in integrated photonic circuits.

Inductor-capacitor (LC) passive wireless sensors are implemented by loose coupling between an external read-out coil and an inductor that receives power through this inductive coupling. They allow for continuous real-time detection in harsh or sealed environments where physical access is difficult. Changes in the sensors are wirelessly and remotely detected by the read-out coil. For implanted medical devices or sealed package applications, however, geometrical constraints such as small and noninvasive coils reduce the inductive coupling, turning the remote distance into a fundamental challenge. Here we propose theoretically and demonstrate experimentally that a parity-time (PT) symmetry breaking regime can be utilized to extend the remote distance of the LC passive wireless sensors. In our prototypes, a capacitive humidity sensor is fabricated with graphene oxide (GO) as a sensing material. The variable capacitance in the external read-out coil is cyclic scanned so as to keep the PT-symmetric configuration. Our results show that the read-out distance of the sensors with the PT-symmetric configuration is approximately 4 times as long as that with conventional methods while keeping the same for their sensitivity. The use of PT-symmetry-breaking regimes for the LC passive wireless sensors should enable the interrogation of applications in the implanted or sealed fields.

The engineering of efficient electrical contacts to two-dimensional (2D) layered materials represents one of the major challenges in the development of industrial-grade 2D-material-based electronics and optoelectronics. In this paper, we present a computational study of the contact resistance and current-flow distribution for electrical contacts between 2D materials and three-dimensional (3D) metals and between different 2D materials. We develop models of the electrical contact resistance for 2D/2D and 2D/3D metal/semiconductor contact interfaces based on a self-consistent transmission-line model coupled with a thermionic charge-injection model for 2D materials and first-principles simulation by density-functional theory, which explicitly includes the variation of the electrostatic potential in the contact region. We compare the results of our self-consistent calculations with existing experimental work and obtain excellent agreement. It is found that the presence of contact interface roughness, in the form of fluctuating Schottky barrier heights in the contact region, can significantly reduce the contact resistance of ${\mathrm{Mo}\mathrm{S}}_2$/metal Schottky 2D/3D contacts. Our findings suggest that roughness engineering may offer a possible paradigm for reducing the contact resistance of 2D-material-based electrical contacts.

For thermoelectric applications, thermoelectric $\mathrm{Si}$ thin films with periodic circular pores have been intensively studied because of the low price and earth abundance of $\mathrm{Si}$. In this work, a different periodic nanoporous pattern is investigated for its potential thermoelectric benefit, i.e., a $\mathrm{Si}$ thin film with periodic nanoslots. Inside such structures, the neck between adjacent nanoslots functions as the nanorestriction to suppress the phonon transport, leading to a dramatically reduced lattice thermal conductivity. When the neck width is still longer than the mean free paths of majority charge carriers, bulklike electron transport can be maintained so that the thermoelectric ZT can be enhanced. For the thermal designs of these porous thin films, a simple but accurate analytical model based on the mean-free-path modification with a characteristic length is derived and is used to predict their thermoelectric properties. For heavily doped $\mathrm{Si}$ films with the neck width reduced to 5 nm, the computed ZT can reach 0.58 at 1100 K. The proposed nanoslot pattern can be extended to general thin films and atomic thick materials to tune their transport properties.

Three-dimensional magnetic null points (3D nulls) are sites of dynamic activity and particle acceleration in astrophysical environments. Future laboratory infrastructures to validate theoretical models of observed plasma dynamics about 3D nulls will benefit from an experimental infrastructure that closely reproduces these analytical potential magnetic fields. As such, a conducting-coil assembly is presented that closely matches the theoretical form of a 3D null within an interior volume. Exact magnetic field solutions are attained for a spherical aggregation of circular current-carrying coils with finite cross sections that create a central magnetic null point of vanishing magnetic energy density. This configuration is shown to replicate the fan-plane–spine-axis topology of 3D nulls to a high order within the central volume close to the null. Finally, numerical analyses demonstrate that the investigated architecture is scalable over wide size and energy ranges. Applications of the presented conducting-coil assembly include sophisticated next-generation plasma experiments and particle accelerators.

Deterministic magnetic switching driven by electric fields rather than power-dissipating currents at the nanoscale is fundamentally challenging yet promising for applications to energy-efficient and high-density spintronic devices. Here, we demonstrate an electric-field-controlled deterministic, robust and repeatable $120{}^o$ rotation of “Y”-like magnetic state in a patterned nanoscale multiferroic heterostructure consisting of a concave triangular nanomagnet deposited on $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3$-${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ film. Using phase-field simulations, we find that the rotation of “Y”-like magnetic state is controlled by a co-action of strain-mediated electric-field-induced uniaxial magnetoelastic anisotropy, magnetic in-plane shape anisotropy of the concave-triangle-shaped nanomagnet, and an interfacial exchange-bias field from a juxtaposed antiferromagnetic layer. It is also shown that deterministic magnetic state switching can be accomplished by a pulsed strain, the duration of which can span from ten nanoseconds or longer down to a few nanoseconds, providing great design flexibility. We also discuss the dynamics of electric-field-driven switching of “Y”-like magnetic state as well as the influence of side length, thickness, and shape variation (i.e., concave radius) of the nanomagnet on the critical strain for the switching. These results offer a technologically viable route to designing nanomagnet-based nonvolatile spin memories with high density and low power.

Thin nanomaterials are key constituents of modern quantum technologies and materials research. The identification of specimens of these materials with the properties required for the development of state-of-the-art quantum devices is usually a complex and tedious human task. In this work, we provide a neural-network-driven solution that allows for accurate and efficient scanning, data processing, and sample identification of experimentally relevant two-dimensional materials. We show how to approach the classification of imperfect and imbalanced data sets using an iterative application of multiple noisy neural networks. We embed the trained classifier into a comprehensive solution for end-to-end automatized data processing and sample identification.

The nitrogen-vacancy center in diamond has been explored extensively as a light-matter interface for quantum information applications, however, it is limited by low coherent photon emission and spectral instability. Here, we present a promising interface based on an alternative defect with superior optical properties (the germanium-vacancy) coupled to a finesse-11 000 fiber cavity, resulting in a ${31}_{-15}^{+11}$-fold increase in the spectral density of zero-phonon-line emission. This work sets the stage for cryogenic experiments, where we predict a measurable increase in the spontaneous emission rate.

We propose a scheme to achieve the analogous interface-state laser by dint of the interface between the two intermediate-resonator-coupled non-Hermitian resonator chains. We find that, after introducing the couplings between the two resonator chains and the intermediate resonator at the interface, the photons of the system mainly gather into the three resonators near the intermediate resonator. The phenomenon of the photon gathering towards the certain resonators is expected to construct the photon storage and even the laser generator. We reveal that the phenomenon is induced via the joint effect between the isolated intermediate resonator and two kinds of non-Hermitian skin effects. In particular, we investigate the interface-state laser in a topologically trivial non-Hermitian resonator array in detail. We find that the pulsed interface-state laser can be achieved accompanying with the intermittent proliferation of the photons at the intermediate resonator when an arbitrary resonator is excited. Also, we reveal that the pulsed interface-state laser in the topologically trivial non-Hermitian resonator array is immune to the on-site defects in some cases, whose mechanism is mainly induced by the nonreciprocal couplings instead of the protection of topology. Our scheme provides a promising and excellent platform to investigate interface-state laser in the microresonator array.

There are two main factors limiting the performance of quantum key distribution—channel transmission loss and noise. Previously, a linear bound was believed to put an upper limit on the rate-transmittance performance. Remarkably, the recently proposed twin-field and phase-matching quantum key distribution schemes have been proven to overcome the linear bound. In practice, due to the intractable phase fluctuation of optical signals in transmission, these schemes suffer from large error rates, which renders the experimental realization extremely challenging. Here, we close this gap by proving the security based on a different principle—encoding symmetry. With the symmetry-based security proof technique, we can decouple the privacy from the channel disturbance, and eventually remove the limitation of secure key distribution on bit error rates. As a direct application, we show that the phase-matching scheme can yield positive key rates even with high bit error rates up to 50%. In the simulation, with typical experimental parameters, the key rate is able to break the linear bound with an error rate of 13%. Meanwhile, we provide a simple finite-data size analysis for the phase-matching scheme under this symmetry-based analysis, which can break the bound with a reasonable data size of ${10}^{12}$. Encouraged by high loss and error tolerance, we expect the approach based on symmetry-protected privacy will provide a different insight into the security of quantum key distribution.

Fast, robust two-qubit gate operation with low susceptibility to crosstalk are the key to scalable quantum-information processing. A parametrically driven gate is inherently insensitive to crosstalk, while superadiabatic control can speed up the gate without losing accuracy. We propose and experimentally implement superadiabatic two-qubit gates using parametric modulation on superconducting quantum circuits. Our results demonstrate the preservation of adiabaticity at a gate speed close to the quantum limit, in addition to robustness against control instability. We demonstrate a controlled-$Z$ gate with an error rate of $5.8\mathrm{\%}$, mainly limited by qubit decoherence, promising future improvement and scalable implementation.

We report a spin-rectification effect in a spin-valve structure consisting of ferroelectric croconic acid (${\mathrm{C}}_5{\mathrm{H}}_2{\mathrm{O}}_5$) sandwiched between ferromagnetic electrodes ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ and $\mathrm{Co}$, which can be switched between a high-resistance (off) and a low-resistance (on) state by a poling voltage. In the off state, the magnetoresistance (MR) sign reverses with the measurement voltage with a 0.1-V offset, suggesting a spin-rectification behavior, while in the on state the MR remains negative. These observations can be understood in terms of electrically controlled interfacial energy-band alignment either from the electrostatic effect or from the interfacial redox process. The observed spin-rectification effect suggests the possibility of diodelike devices for spin-polarized current.

A semiclassical model is presented for characterizing the linear response of elementary quantum-optical systems involving cavities, optical fibers, and atoms. Formulating the transmission and reflection spectra using a scattering-wave (transfer-matrix) approach, the calculations become easily scalable. To demonstrate how useful this method is, we consider the example of a simple quantum network, i.e., two cavity-QED systems connected via an optical fiber. Differences between our quasiexact transfer-matrix approach and a single-mode, linearized quantum-optical model are demonstrated for parameters relevant to recent experiments with coupled nanofiber-cavity-QED systems.

In this work, we demonstrate that interlayer exchange-coupled magnetic thin films can be employed to fabricate ultrasensitive and thermally stable magnetic sensors, taking advantage of the linear anomalous Hall effect via tuning the interlayer exchange coupling and magnetic anisotropy. We investigate the magnetotransport and noise properties of these anomalous Hall sensors. The magnetic field detectability reaches $126.1\mathrm{nT/}\sqrt{\mathrm{Hz}}$ at 1 Hz and $4.5\mathrm{nT/}\sqrt{\mathrm{Hz}}$ at 1 kHz, at room temperature. Meanwhile, the achieved dynamic reserve is 103.0 dB. The magnetic field detectability shows a good temperature stability in the 200 to 300 K range. Within the temperature range, the temperature coefficient of sensitivity is determined to be 530 ppm/K. The high temperature stability is attributed to the interlayer exchange coupling, which also improves the stability against in-plane magnetic fields, beneficial for perpendicular magnetic field sensing. The interlayer exchange-coupled magnetic thin-film-based anomalous Hall sensors solve the trade-off between the sensor size, sensing capability, and stability, which are desirable for microimaging and biomedical applications.