The ability to create phase-controlled pulses of light with wavelength tunability has applications spanning quantum and classical communications networks. Traditionally, optical transmitters are able to either produce phase-controlled pulses at a fixed wavelength or require a chain of bulky and expensive external modulators to convert wavelength tunable continuous-wave light into optical pulses. One technology of great interest is quantum key distribution (QKD), a technology for generating perfectly random keys at remote nodes to ensure secure communications. Environments such as data centers, where the user needs change regularly, will require adaptability in the deployment of QKD to integrate into classical optical networks. Here we propose and demonstrate an alternative quantum transmitter design consisting of a multimodal Fabry-Perot laser optically injection locked by a wavelength tunable laser. The transmitter is able to produce phase-controlled optical pulses at GHz speeds with a tunable wavelength range of $>65\mathrm{nm}$ centered at 1550 nm. With this transmitter, we perform proof-of-principle QKD with secure bit rates of order Mb/s.

Rare-earth (RE) atoms in solid-state materials are attractive components for photonic quantum information systems because of their coherence properties even in high-temperature environments. We have experimentally performed the single-site optical spectroscopy and optical addressing of a single RE atom in an amorphous silica optical fiber at room temperature. The single-site optical spectroscopy of the tapered RE-doped fiber shows nonresonant emission lines similar to those seen in the case of an unstructured fiber and the autocorrelation function of photons emitted from the fiber shows the antibunching effect due to the spatial isolation given by the tapered fiber structure. The ability to address single RE atoms at room temperature provides a very stable and cost-effective technical platform for the realization of a solid-state system for a large-scale quantum optical network and other quantum technologies based on a large number of spectral channels from visible to midinfrared wavelengths.

Quantum transduction between microwave and optical photons plays a key role in quantum communication among remote qubits. Although the quantum transduction schemes generating communication photons have been successfully demonstrated by using optomechanical interfaces, the low conversion efficiency remains an obstacle to the implementation of a quantum network consisting of multiple qubits. Here, we present an efficient quantum transduction scheme using a one-dimensional diamond optomechanical crystal cavity tuned at a color-center emission without optomechanical coupling. The optomechanical crystal cavity incorporates a thin aluminum nitride ($\mathrm{Al}\mathrm{N}$) pad piezoelectric coupler near the concentrator cavity region, while retaining ultrasmall mechanical and optical mode volumes of about $1.5\times {10}^{-4}({\mathrm{\Lambda }}_p{)}^3$ and $0.2(\lambda /n{)}^3$, respectively. The energy level of a coherent color-center electron is manipulated by a strong mechanical-mode–color-center electron-coupling rate up to 16.4 MHz. In our system, we theoretically predict that the population-conversion efficiency from a single microwave photon into an optical photon can reach 15% combined with current technologies. The coherent conversion efficiency is over 10% with a reasonably pure decay time of $T_2^{\ast }>10\mathrm{ns}$. Our results imply that an atomic color center strongly coupled to the optomechanical crystal cavity will offer a highly efficient quantum transduction platform.

We report the realization of an acoustic quadratic Weyl semimetal, which hosts a pair of quadratic Weyl points with opposite charges ±2 in the two lowest bands. The quadratic Weyl points, protected by $C_4$ and time-reversal symmetries, have quadratic dispersions along the $k_x$ and $k_y$ directions and linear dispersion along the $k_z$ direction. Nonzero charges of the quadrative Weyl points give rise to the Fermi-arc boundary states. By constructing a domain wall, we further achieve the double negative refraction of the boundary acoustic waves without reflection in experiment. These results evidence the quadratic Weyl semimetal, and have potential applications in innovative acoustic devices.

We present a comprehensive study on the high-frequency magnetization dynamics of homogeneously patterned ultrathin magnetic nanowire arrays. The backward volume magnetostatic spin-wave (BVMSW) and Damon-Eshbach (DE) configurations are studied along with the intermediate transition states to understand the edge mode’s evolution in depth. We find at the sub-$10$-$\text{nm}$ ultrathin regime the dynamics are heavily influenced by geometrical parameters such as magnetic layer thickness ($t_{\mathrm{FM}}$), demagnetization factors, and interfaces. Critical entities such as field separation $\delta H$ between uniform to edge mode increase linearly with $1/t_{\mathrm{FM}}$, while the edge saturation field $H_{\text{sat}}^{\mathrm{edge}}$ increase monotonically with increasing $t_{\mathrm{FM}}$, revealing excellent agreement between findings from experimental and micromagnetic simulations. The dynamics are less sensitive to the width of the nanowire but very sensitive to the adjacent material or the interface of the ferromagnet, especially at the ultrathin limits.

We report on multiphoton statistics of correlated twin beams produced in a periodic poled microring resonator on thin-film lithium niobate. Owing to high-cavity confinement and near-perfect quasi-phase-matching, the photon pairs are produced efficiently in single modes at rates reaching 27 MHz per $\text{μ}\mathrm{W}$ pump power. By using a pump laser whose pulse-width impedance matches with the cavity, those photons are further created in single longitudinal modes with purity reaching 99%, without relying on later-on filtering. With a dual-channel photon-number-resolving detection system, we obtain directly the joint detection probabilities of multiphoton states up to three photons, with high coincidence to accidental contrast for each. Used as a single-photon source, it gives heralded $g_H^{(2)}(0)$ around 0.04 at a single-photon rate of 650 kHz on chip. The findings of our research highlight the potential of this nanophotonic platform as a promising platform for generating nonclassical, few-photon states with ideal indistinguishability, for fundamental quantum optics studies and information applications.

Sophisticated positive-operator-valued measures (POVMs) are fundamental to obtain a quantum advantage in many informational problems but are difficult to implement. We design a universal measurement device composed of cascaded quantum-walk modules able to realize arbitrary entangled POVMs on two qubits. Only parameters in the modules need reprogramming for different POVMs. We apply our device to the task of direction guessing, realizing nine different five-outcome entangled measurements (fidelities above 0.9850) via nine-step photonic quantum walks, recovering optimal direction guessing from nonideal input states. Our work marks an important step toward building a universal versatile quantum measurement device able to implement optimal measurements for various quantum information tasks.

We propose an on-chip fiber-based quantum transducer to convert microwave photons into optical-telecommunication-band photons. The device consists of a coplanar-waveguide microwave resonator and a pair of coupled optical cavities integrated into a rare-earth-doped fiber. The microwave resonator is formed by two aluminum strips deposited onto the cladding of the fiber, whose diameter has been etched down to approximately $8\text{μ}\mathrm{m}$. We study the conversion process using the cavity electro-optic formalism for a system operating at liquid-helium temperature (around 4 K), as well as using a three-wave mixing approach for operation at millikelvin temperatures achievable in a dilution refrigerator. Both methods predict that unity conversion efficiency could be achievable with realistically achievable experimental parameters. The proposed device acts as a quantum transducer for coherent microwave-to-optical conversion, which has great promise for application in all-fiber integrated quantum devices for quantum information processing.

We present a scheme of quantum repeater that uses entangled multimode coherent states that are obtained by electro-optic modulation of symmetric and antisymmetric Schrödinger cat states. Part of the generated entangled frequency modes is sent to a symmetric beam splitter at the central node, while the remaining modes are stored locally in quantum memories. The entangled coherent states between remote quantum memories are conditionally prepared by photon-counting measurements at the output channels of the beam splitter. We study how the effects of decoherence in the quantum channel affect statistics of photocounts and, for the heralding outcomes determined by the parity of photocounts, evaluate the probability of success and the fidelity of the prepared entanglement depending on the symmetry of input cat states. It is demonstrated that the generated entanglement can be employed for teleportation of the phase information from the modulated states that are utilized in quantum key distributions with subcarrier wave encoding.

We develop a computational method, based on Gauss’s law, for calculating the effective permeability tensor of materials with magnetic inclusions. Electromagnetic composites find application in a variety of modern technologies since they have properties that are different from their constituent materials. However, many effective medium calculations for the permittivity or permeability tensor detailed over the past 120 years have issues with a limited range of applicability or with ignoring the coupling between different components of the tensor. Here, a computational approach is developed for calculating the effective tensor permeability of a composite material, which eliminates these issues. We show its validity by comparing to analytic results in the relevant limits, namely composites comprised of thin films and regular inclusions of low filling fraction. Examples of irregularly shaped magnetic inclusions and materials with high magnetic filling fractions are then presented for which analytic effective medium methods cannot make predictions.

We have proposed and experimentally verified a tunable interqubit coupling scheme for large-scale integration of superconducting qubits. The key feature of the scheme is the insertion of connecting pads between the qubit and tunable coupling element. In such a way, the distance between two qubits can be increased considerably to a few millimeters, leaving enough space for arranging control lines, readout resonators, and other necessary structures. The increased interqubit distance provides more wiring space for the flip-chip process and reduces crosstalk between qubits and from control lines to qubits. We use the term tunable coupler with capacitively connecting pad (TCCP) to name the tunable coupling part that consists of a transmon coupler and capacitively connecting pads. With the different placement of connecting pads, different TCCP architectures can be realized. We have designed and fabricated a few multiqubit devices in which TCCP is used for coupling. The measured results show that the performance of the qubits coupled by TCCP, such as $T_1$ and $T_2$, was similar to that of traditional transmon qubits without TCCP. Meanwhile, our TCCP also exhibited a wide tunable range of the effective coupling strength and a low residual $ZZ$ interaction between the qubits by properly tuning parameters on the design. Finally, we successfully implemented an adiabatic controlled-$Z$ gate with TCCP. Furthermore, by introducing TCCP, we also discuss the realization of the flip-chip process and tunable coupling qubits between different chips.

Magnetic exchange springs (ESs) are composed of exchange-coupled hard and soft magnetic layers, i.e., layers with high and low anisotropy, respectively. The moments in the soft layer can be wound up by applying an external field, which has to be smaller than the anisotropy field of the hard layer. Alternatively, an ES can be realized by biasing the soft magnetic layer by two adjacent hard magnetic layers with different magnetic anisotropy directions. We have fabricated an ES layer stack by magnetron sputter deposition. As the hard magnetic bottom layer, we used epitaxial $\mathrm{Fe}\mathrm{Pt}$ L1${}_0$, and as the top layer $\mathrm{Co}$ with both layers having different in-plane easy axes. These hard layers pin the moments of a soft permalloy (${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$) layer sandwiched between them, winding up an ES at remanence. The anisotropy of the polycrystalline top $\mathrm{Co}$ layer was engineered by glancing-angle deposition to have in-plane easy axis anisotropy perpendicular to the easy direction of the bottom layer. Using soft x-ray spectroscopy and magneto-optical measurements, we found the in-plane ES to extend from the soft layer into the top layer of our $\mathrm{Fe}\mathrm{Pt}$/permalloy/$\mathrm{Co}$ trilayer structure.

The properties of spin-wave quanta, referred to as magnons, make them a promising signal carrier for the transmission and processing of information in magnetic materials. In this field, called magnonics, dynamic magnonic crystals attract special attention for applications such as complex real-time spectral transformations, due to on-demand controllability of transmission gaps. Here, we report on the experimental realization of the dynamic control of spin-wave band structures in artificial crystals using the metal-insulator transition in the vanadium dioxide (${\mathrm{VO}}_2$) grating. The obtained results are confirmed by a theoretical approach predicting a dramatic variation of the reflection coefficient from the ${\mathrm{VO}}_2$ stripes upon the phase transition. The proposed structure combines the excellent versatility, fast switching speed, low power consumption, and external control it affords.

The realization of superconducting single-photon detectors operating above the liquid-helium temperature is currently the focus of intense research efforts. Here, we present the fabrication of ultraclean encapsulated nanowires from commercially available thin films of $\mathrm{YBa}$${}_2$$\mathrm{Cu}$${}_3$$\mathrm{O}$${}_{7-\delta }$ by high-energy oxygen-ion irradiation. The structured nanowires, ranging from 0.1 to $5\text{μ}\mathrm{m}$ in width, exhibit sharp resistive transitions to the superconducting state above 85 K. The $I$-$V$ characteristics reveal that the evolution from the superconducting to the normal state shows a large voltage jump in the volt range. This demonstrates that the entire nanowire transits—a prerequisite for the development of a hot spot upon absorption of a single photon. Our results pave the way for the fabrication of $\mathrm{YBa}$${}_2$$\mathrm{Cu}$${}_3$$\mathrm{O}$${}_{7-\delta }$-based scalable superconducting single-photon-detection devices.

The change in the arrangement of magnetic dipole moments in a magnetic metasurface, due to the influence of an external static magnetic field, is discussed. Each meta-atom of the metasurface is composed of three identical disk-shaped resonators (trimer) made of magnetically saturated ferrite. To provide physical insight, full-wave numerical simulations of the near fields and transmission characteristics of the metasurface are complemented by the theoretical description based on the multipole decomposition method. With these methods, the study of eigenmodes and scattering conditions of a single magnetic resonator, trimer, and their array forming the metasurface is performed. It is found that the magnetic dipole-based collective hybrid mode of the trimer can be gradually transformed from the radial (pseudomonopole) to azimuthal (toroidal) order and vice versa by varying the bias magnetic field strength. This is because the magnetic dipole moment of each individual disk constituting the trimer undergoes rotation as the bias magnetic field strength changes. This transition between two orders is accompanied by various patterns of localization of the electric field inside the meta-atoms. Due to the unique field configuration of these modes, the proposed metasurface can be considered for designing magnetic field sensors and nonreciprocal devices.

Recent advances in laser technology demand more-effective measurement of ultrashort optical pulses in fiber optics and nanophotonics devices. Here we propose an in-fiber strategy to characterize ultrashort pulses by incorporating a gallium selenide–assisted microfiber, as a frequency-doubling block, in an all-fiber interferometer. Pulses to be retrieved were first split into the two arms of the interferometer and then overlapped, exciting the interferometric autocorrelated second harmonic in the microfiber due to high second-order nonlinearity of gallium selenide. In the experiment, the autocorrelator shows high accuracy in measuring 7.13-ps, 1239-fs, and 683-fs pulses. In comparison with a commercial frequency-resolved optical gating, the relative errors are only 1.53%, 3.74%, and 6.86%, respectively. Moreover, by attenuation of the incident laser of one arm of the interferometer, the measured peak-to-background ratio was less than the conventional theoretical value. The proposed autocorrelator could be very compatible with current systems for future applications of in-fiber or on-chip ultrafast optical technology.

Resistive switching in memristors is being amply investigated for different applications in nonvolatile memory, neuromorphic computing, and programmable logic devices. Memristors are conducting devices in which the conductance depends on one or more slow internal state variables, and they exhibit strongly nonlinear properties and intense memory effects. Here, we address the characterization of current-controlled memristors by small-perturbation frequency-resolved impedance techniques. We show that the equivalent circuit obtained at different stationary points provides essential information about the dynamic behavior in voltage cycling and transient response to a square perturbation. The general method enables the analysis of stability and hysteresis in current-voltage curves. The current-controlled memristor very naturally produces a negative capacitance effect, and we review several devices reported in the literature, including discharge tubes and metal-oxide memristors, to expose the deep connections between the sign of the capacitance and the type of hysteresis.