In this work, we study magnetization dynamics in superconductor-ferromagnet-superconductor thin-film structures. Results of the broad-band ferromagnetic resonance spectroscopy are reported for a large set of samples with varied thickness of both superconducting and ferromagnetic layers in a wide frequency, field, and temperature ranges. Experimentally the one-dimensional anisotropic action of superconducting torque on magnetization dynamics is established; its dependence on thickness of layers is revealed. It is demonstrated that experimental findings support the recently proposed mechanism of the superconducting torque formation via the interplay between the superconducting imaginary conductance and magnetization precession at superconductor-ferromagnet interfaces. Microwave spectroscopy studies in this work are supplemented by investigations of the crystal structure and the microstructure of studied multilayers.

Nonlinear superconducting devices, typically based on Josephson-junction (JJ) nonlinearities, are the basis for superconducting quantum electronics, enabling, e.g., the formation of isolated two-level superconducting qubits and amplifiers. While emerging spin, hybrid spin-superconducting (including Majorana), and nanomagneto-optical quantum systems could benefit tremendously from superconducting nonlinearities, the presence of strong magnetic fields in these systems is incompatible with conventional JJ devices, which are highly sensitive to applied magnetic fields. One potential solution is the use of kinetic inductance (KI) nonlinearity. To date, only linear KI devices have been shown to operate in high magnetic fields, while nonlinear KI device operation in high magnetic fields has received virtually no attention. Here, we study the nonlinearity of superconducting nanowire (NW) KI resonators and their performance as parametric amplifiers for in-plane magnetic fields. We study the Kerr coefficients of NW KI resonators made from 10-nm-thick $\mathrm{Nb}\text{−}\mathrm{Ti}\text{−}\mathrm{N}$ films with characteristic impedance up to 3 $\mathrm{k}\mathrm{\Omega }$, and demonstrate both nondegenerate and degenerate parametric amplification, at magnetic fields up to 2 T. We find that narrow KI resonators of width $0.1\text{μ}\mathrm{m}$ are robust, in terms of gain, dynamic range and noise, to magnetic fields up to approximately $2\mathrm{T}$, for up to 12 dB (16 dB) of phase-insensitive (phase-sensitive) gain. In comparison, wider KI resonators of width $1\text{μ}\mathrm{m}$ suffer significant suppression in the gain at fields well below $2$ T. Around 8-dB deamplification is also observed for coherent states for a 0.1-$\text{μ}\mathrm{m}$ NW KI resonator, implying the capability of noise squeezing. These results open a pathway to developing nonlinear quantum devices that operate in or generate high magnetic fields such as spin, hybrid spin-superconducting, and magneto-optomechanical devices.

Neutral silicon-vacancy ($\text{Si-}{V}^0$) centers in diamond are promising candidates for quantum network applications because of their exceptional optical properties and spin coherence. However, the stabilization of $\text{Si-}{V}^0$ centers requires careful Fermi-level engineering of the diamond host material, making further technological development challenging. Here, we show that $\text{Si-}{V}^0$ centers can be efficiently stabilized by photoactivated itinerant carriers. Even in this nonequilibrium configuration, the resulting $\text{Si-}{V}^0$ centers are stable enough to allow for resonant optical excitation and optically detected magnetic resonance. Our results pave the way for on-demand generation of $\text{Si-}{V}^0$ centers as well as other emerging quantum defects in diamond.

Voltage-tunable superconductor-semiconductor devices offer a unique platform to realize dynamic tunability in superconducting quantum circuits. By galvanically connecting a gated $\mathrm{In}\mathrm{As}$-$\mathrm{Al}$ Josephson junction to a coplanar waveguide resonator, we demonstrate the use of a superconducting element with wideband gate tunability. We show that the resonant frequency is controlled via a gate-tunable Josephson inductance and that the nonlinearity of the $\mathrm{In}\mathrm{As}$-$\mathrm{Al}$ junction is nondissipative as is the case with conventional ${\mathrm{Al}\mathrm{O}}_{\mathrm{x}}\text{-Al}$ junctions. As the gate voltage is decreased, the inductive participation of the junction increases up to $44\mathrm{\%}$, resulting in the resonant frequency being tuned by over $2\mathrm{GHz}$. Utilizing the wide tunability of the device, we demonstrate that two resonant modes can be adjusted such that they strongly hybridize, exhibiting an avoided-level crossing with a coupling strength of $51\mathrm{MHz}$. Implementing such voltage-tunable resonators is the first step toward realizing wafer-scale continuous voltage control in superconducting circuits for qubit-qubit coupling, quantum limited amplifiers, and quantum memory platforms.

The spin Hall nano-oscillator (SHNO) is a promising spintronic device to produce high-frequency and low-linewidth microwave signals. We study a type of SHNO based on the $L{1}_0\text{−}\mathrm{Fe}\mathrm{Pt}/\mathrm{Ni}\mathrm{Fe}/\mathrm{Pt}$ exchange-spring system with an ultrahigh perpendicular magnetic anisotropy by micromagnetic simulation. The hard-soft magnetic exchange-spring system has a very high interlayer-exchange-interaction field, resulting in a more than 30-GHz emission frequency and 38.6-GHz/T field tunability. Apart from conventional localized and nonlocalized auto-oscillation modes, localized or nonlocalized modes with central antisymmetry also appear in this SHNO. Two oscillation centers possess nonequal amplitudes and opposite phases in these antisymmetrical modes. Different modes are determined by the competition between three factors: the exchange-interaction field, the perpendicular anisotropic field, and spin-orbit torque. The spin dynamics in the SHNO exhibit complex field and current dependence. Our results provide a possible direction for the design of SHNOs, which may be beneficial for the research and development of spintronic nano-oscillators.

Biphoton frequency combs (BFCs) are promising quantum sources for large-scale and high-dimensional quantum information and networking systems. In this context, the spectral purity of individual frequency bins will be critical for realizing quantum networking protocols like teleportation and entanglement swapping. Measurement of the temporal autocorrelation function of the unheralded signal or idler photons comprising the BFC is a key tool for characterizing their spectral purity and in turn verifying the utility of the biphoton state for networking protocols. Yet the experimentally obtainable precision for measuring BFC correlation functions is often severely limited by detector jitter. The fine temporal features in the correlation function—not only of practical value in quantum information, but also of fundamental interest in the study of quantum optics—are lost as a result. We propose a scheme to circumvent this challenge through electro-optic phase modulation, experimentally demonstrating time-resolved Hanbury Brown–Twiss characterization of BFCs generated from an integrated 40.5-GHz ${\mathrm{Si}}_3{\mathrm{N}}_4$ microring, up to a $3\times 3$-dimensional two-qutrit Hilbert space. Through slight detuning of the electro-optic drive frequency from the comb’s free spectral range, our approach leverages Vernier principles to magnify temporal features, which would otherwise be averaged out by detector jitter. We demonstrate our approach under both continuous-wave and pulsed-pumping regimes, finding excellent agreement with theory. Our method reveals not only the collective statistics of the contributing frequency bins but also their temporal shapes—features lost in standard fully integrated autocorrelation measurements.

Unitary transformations formulate the time evolution of quantum states. How to learn a unitary transformation efficiently is a fundamental problem in quantum machine learning. The most natural and leading strategy is to train a quantum machine learning model based on a quantum dataset. Although the presence of more training data results in better models, using too much data reduces the efficiency of training. In this work, we solve the problem on the minimum size of sufficient quantum datasets for learning a unitary transformation exactly, which reveals the power and limitation of quantum data. First, we prove that the minimum size of a dataset with pure states is $2^n$ for learning an $n$-qubit unitary transformation. To fully explore the capability of quantum data, we introduce a practical quantum dataset consisting of $n+1$ elementary tensor product states that are sufficient for exact training. The main idea is to simplify the structure utilizing decoupling, which leads to an exponential improvement in the size of the datasets with pure states. Furthermore, we show that the size of the quantum dataset with mixed states can be reduced to a constant, which yields an optimal quantum dataset for learning a unitary. We showcase the applications of our results in oracle compiling and Hamiltonian simulation. Notably, to accurately simulate a three-qubit one-dimensional nearest-neighbor Heisenberg model, our circuit only uses $96$ elementary quantum gates, which is significantly less than 4080 gates in the circuit constructed by the Trotter-Suzuki product formula.

This work presents the possibility of considering ${\mathrm{Mo}\mathrm{O}}_2$ nanorods synthesized by the carbothermal reduction of ${\mathrm{Mo}\mathrm{O}}_3$ as an intercalation- or insertion-type anode for lithium-ion capacitor (LIC) assembly. The mechanism for $\mathrm{Li}$ intercalation into ${\mathrm{Mo}\mathrm{O}}_2$ carbon composite (${\mathrm{Mo}\mathrm{O}}_2$-$\mathrm{C}$) is studied in a half-cell assembly within the potential window of 0.8–3 V vs ${\mathrm{Li}}^{+}/\mathrm{Li}$. The material can deliver an initial discharge capacity of about 225–250 mAh ${\mathrm{g}}^{-1}$ with excellent cyclic stability over 500 galvanostatic charge-discharge cycles at a current density of 100 mA ${\mathrm{g}}^{-1}$. Furthermore, the performance of ${\mathrm{Mo}\mathrm{O}}_2$-$\mathrm{C}$ as a battery-type (intercalation) anode in LIC assembly is studied by pairing it with a commercial activated carbon (AC) cathode. The role of prelithiation and operating-voltage window on the electrochemical performance of AC$//{\mathrm{Mo}\mathrm{O}}_2$-$\mathrm{C}$ LIC is analyzed. Such a LIC assembly without prelithiation, when tested within the voltage window of 1.7–3.2 V, can provide an energy density of about 75.27 Wh ${\mathrm{kg}}^{-1}$ and maintain about 70% of its initial capacity after 4500 cycles. Moreover, such a LIC prototype’s low- and high-temperature performance is also evaluated. This study clearly shows the possibility of considering ${\mathrm{Mo}\mathrm{O}}_2$-based LIC systems as a strong competitor for ${\mathrm{Li}}_4{\mathrm{Ti}}_5{\mathrm{O}}_{12}\text{−}$based LIC assembly, providing much safer features than graphite-based LIC configurations.

Two-mode squeezed vacuum states are a crucial component of quantum technologies. In the microwave domain, they can be produced by the Josephson ring modulator that acts as a three-wave mixing nondegenerate parametric amplifier. Here, we numerically solve the full master equation of three bosonic modes describing the Josephson ring modulator to compute squeezing of output fields and gain at low signal power. We show that the third-order interaction from the three-wave mixing process intrinsically limits squeezing and reduces gain. Since our results are related to other general cavity-based three-wave mixing processes, these imply that any nondegenerate parametric amplifier will have an intrinsic squeezing limit in the output fields.

Electrides are a special family of materials with exotic physical properties and various crystal structures in different dimensions. Among them, alkaline-earth metal nitrides (${\mathrm{AE}}_2\mathrm{N}$) have attracted enormous attention, since the synthesis of ${\mathrm{Ca}}_2\mathrm{N}$ as a two-dimensional (2D) electride. However, apart from ${\mathrm{Ca}}_2\mathrm{N}$, the physicochemical properties of other ${\mathrm{AE}}_2\mathrm{N}$ are not well understood, and in some cases, their crystal structures remain unknown. To overcome these issues, here, we conduct systematic investigations into the crystal structures, electronic structures, and optical properties of two typical ${\mathrm{AE}}_2\mathrm{N}$, i.e., ${\mathrm{Mg}}_2\mathrm{N}$ and ${\mathrm{Be}}_2\mathrm{N}$, based on first-principles calculations. By symmetry analysis, we derive a crystal structure with $R\overline{3}m$ symmetry for ${\mathrm{Be}}_2\mathrm{N}$, which is energetically more stable than all the other structures proposed previously. Our calculations show that $R\overline{3}m$ ${\mathrm{Be}}_2\mathrm{N}$ is a 2D electride with the anionic electrons located in the interstitial space, forming a 2D Kagome lattice. Moreover, distinct from traditional 2D electrides, it exhibits anomalous physicochemical properties, such as having a small interstitial space, a high work function, and a large cleavage energy, mainly due to the relatively strong bonding effect between anionic electrons and cationic framework. For ${\mathrm{Mg}}_2\mathrm{N}$, on the other hand, we find that the most stable structure has R3m symmetry, which shows a total energy of 0.005 eV per formula unit lower than the previously predicted Cmcm structure. Moreover, different from $R\overline{3}m$ ${\mathrm{Be}}_2\mathrm{N}$, R3m ${\mathrm{Mg}}_2\mathrm{N}$ is identified as a zero-dimensional (0D) electride with a semiconducting band structure, since its anionic electrons are confined in separated 0D cavities and cannot interact with each other. These studies thus provide a deeper understanding of the crystal structures and physicochemical properties of ${\mathrm{Be}}_2\mathrm{N}$ and ${\mathrm{Mg}}_2\mathrm{N}$ as potential electride materials.

We introduce an analytical framework for near-field radiative heat transfer in bulk plasmonic and polar media. Considering material dispersion, we derive a closed-form expression for the radiative thermal conductance, which disentangles the role of optical loss from other material dispersion characteristics, such as the spectral width of the Reststrahlen band in polar dielectrics, as well as from the temperature. We provide a universal condition for maximizing heat transfer that defines the optimal interplay between a material’s optical loss and polariton resonance frequency, based on which we introduce tight bounds to near-field heat transfer. With this formalism, one can quantitatively evaluate all polaritonic materials in terms of their performance as near-field thermal emitters.

Topological insulators demonstrate high charge-spin conversion efficiency due to their spin-momentum locking at the Dirac surface states. However, the surface states are sensitive to disruption caused by exchange coupling when interfaced with a ferromagnet. Here, we demonstrate the use of various nonmagnetic insertion layer materials, $\mathrm{Ti},\mathrm{Cu},$ and $\mathrm{Pt}$, at the $\mathrm{Co}/\mathrm{Bi}$-$\mathrm{Sb}(012)$ interface to preserve the topological surface state and promote spin-orbit-torque efficiency through the crystallinity control of $\mathrm{Bi}$-$\mathrm{Sb}(012)$. For 20-nm-thick $\mathrm{Bi}$-$\mathrm{Sb}$, a spin Hall angle of up to 8.93 is observed with the use of a $\mathrm{Pt}$ insertion layer, while it is otherwise negligible for $\mathrm{Co}/\mathrm{Bi}$-$\mathrm{Sb}(012)$ interfaces. We further explore the enhancement of $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystallinity with increasing $\mathrm{Bi}$-$\mathrm{Sb}$ thickness, revealing a rapidly increasing spin-orbit-torque efficiency that gradually saturates above 30 nm. A clear correlation between spin-orbit-torque efficiency and $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystalline size is identified using x-ray diffractometry, establishing the origin of the high spin-orbit efficiency to be the $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystalline orientation. Our work demonstrates the spin-orbit-torque origin in $\mathrm{Bi}$-$\mathrm{Sb}$ experimentally and paves the way for the adaptation of topological insulators as a class of low-energy spin source material for spintronics applications.

We experimentally demonstrate a microwave quantum storage for two spectral modes of microwave radiation in an on-chip system of eight coplanar superconducting resonators. Single-mode storage shows a power efficiency of up to $60\pm 3\mathrm{\%}$ at single photon energy and more than $73\pm 3\mathrm{\%}$ at higher intensity. The noiseless character of the storage is confirmed by coherent-state quantum process tomography. The demonstrated efficiency is an order of magnitude higher than the previously reported data for multimode microwave quantum memory. The proposed on-chip quantum memory architecture can be easily integrated into the state-of-the-art superconducting quantum circuit technology without any coherence compromises. The obtained results are in good agreement with the proposed theory, which pave the way for building a practical multimode microwave memory for superconducting quantum circuits.

A collective outcome or decision from a crowd often prevails over that of a single expert. Here we study this phenomenon through the lens of quantum machine learning. We compare the performance of an expert, a highly trained quantum network, to a crowd, several poorly trained ones, for quantum information processing tasks such as quantum tomography and entanglement recognition. We also show quantum-enhanced performance from a temporal crowd by using time multiplexing on a single poorly trained quantum network. Given the same resources for training, we show that the crowd outperforms the expert by a definitive margin.

This paper develops a systematic approach to realizing linear detectors with an optimized sensitivity, allowing for the detection of extremely weak signals. First, general constraints are derived on a specific class of input-output transfer functions of a linear detector. Then a physical realization of transfer functions in that class is found using the quantum network synthesis technique, which allows for the inference of the physical setup directly from the input-output transfer function. By exploring a minimal realization which has the minimum number of internal modes, it is shown that the optimal such detectors are internal squeezing schemes. Then, investigating nonminimal realizations, which is motivated by parity-time symmetric systems, a quantum nondemolition measurement is systematically recovered.

We demonstrate a bright and polarized fiber in-line single-photon source based on plasmon-enhanced emission of colloidal single quantum dots into an optical nanofiber. We show that emission properties of single quantum dots can be strongly enhanced in the presence of single gold nanorods leading to a bright and strongly polarized single-photon emission. The single photons are efficiently coupled to guided modes of the nanofiber and eventually to a single-mode optical fiber. The brightness (fiber-coupled photon count rate) of the single-photon source is estimated to be $12.2\pm 0.6$ MHz, with high single-photon purity [$g^2(0)=0.20\pm 0.04$] and degree of polarization as high as 94–97 %. The polarized and fiber-coupled single photons can be implemented for potential applications in quantum photonics.