Terahertz (THz) waves are extremely sensitive to water content and biological composition (proteins, genes, and metabolites), which are the key regulator of cancer development. Hence, the application of THz spectroscopy and imaging is promising in the field of biomedicine, especially in cancer diagnosis. Currently, the diagnosis of cancer patients (including tumor borders, tumor stage, etc.) mainly rely on optical microscope observation of pathological slices. The preparation of the slices is complicated and time consuming, and the microscopic field of view is small, which cannot effectively guide the precise removal of the tumor during the operation in time. To solve this problem, a label-free real-time high-resolution near-field THz spectroscopy imaging technique was developed. In this study, we successfully apply this technique to the oral tissue. Compared with conventional point-by-point scanning terahertz time-domain spectroscopy imaging, this technique can exhibit a real-time imaging rate (as high as 20 frames/s at 512 × 1024 pixels) and a field of view approximately 5 mm in diameter with a spatial resolution up to λ/10 at 1 THz in near-field condition. The experimental results show that the terahertz spectroscopic imaging in this paper can clearly detect and identify not only biological tissues with different components, but also normal and cancerous tissues, as well as tissues with the same composition but different slice directions. This work lays an experimental foundation for the application of THz spectroscopy imaging to rapid label-free intraoperative diagnosis, and it provides a basis for promoting the wide application of THz technology in the biomedical field.

We investigate the transverse trapping force acting on a small particle on or off the central axis of zero-order finite Bessel beams. Combining the simulated fields with the Gorkov force potential, the transverse trapping behaviors for small objects are analyzed, and the reversal of the trapping behaviors is recovered when varying the paraxiality parameter of the Bessel beam. The results prove the possibility of using axisymmetric Bessel beams to trap both dense and stiff particles as well as light and soft ones. The particles can be trapped at the maximum central pressure in the main lobe and also the maximum pressure of the other transverse locations. A lossy acoustic metastructure with the ability to decouple the phase and amplitude modulations is used to generate the desired sound field, which is used to trap a foam ball as an example. Our research opens a promising avenue to the design and development of simplified acoustic tweezers.

Detecting a weak physical signal immersed in overwhelming noises entails separating the two, a task for which machine learning is naturally suited. In principle, such a signal is generated by a nonlinear dynamical system of intrinsically high dimension for which a mathematical model is not available, rendering unsuitable traditional linear or nonlinear state-estimation methods that require an accurate system model (e.g., extended Kalman filters). We exploit the architectures of reservoir computing and feed-forward neural networks (FNNs) with time-delayed inputs to solve the weak-signal-detection problem. As a prototypical example, we apply the machine-learning schemes to Earth’s magnetic anomaly field-based navigation. In particular, the time series are collected from the interior of the cockpit of a flying aircraft during different maneuvering phases, where the overwhelmingly strong noise background is the result of other components of Earth’s magnetic field and the fields generated by the electronic devices in the cockpit. We demonstrate that, when combined with the traditional Tolles-Lawson model for Earth’s magnetic field, the articulated machine-learning schemes are effective for accurately detecting the weak anomaly field from the noisy time series. The schemes can be applied to detecting weak signals in other domains of science and engineering.

Magnetic hysteresis present in tracer materials for magnetic particle imaging (MPI) either by effect of the driving-field frequency or because of interactions among nanoparticles is shown to play a crucial role when such a technique is exploited to get quantitative information about the tissue section being examined. By means of simulations based on a rate-equation model, the nanoparticles are proven to display magnetic hysteresis at the operating frequencies typical of MPI even if they are superparamagnetic in quasistatic conditions. As a consequence, the system function, a quantity crucial for image reconstruction in MPI, becomes markedly different in nature and shape from the curve produced by superparamagnetic nanoparticles. Not taking into account such an effect has detrimental consequences on quantitative MPI, as evidenced by specific examples of image reconstruction involving both monodisperse and polydisperse nanoparticles. The role played in image reconstruction by magnetic dipolar interaction, no longer negligible when nanoparticles accumulate in a small volume of the tissue, is discussed within the rate-equation model.

Quantum key distribution (QKD) protocols utilize single photons for key exchanging. Heralded single-photon sources (HSPSs) with spontaneous parametric down-conversion (SPDC) is a practical and effective way of producing single photons of high fidelity, bringing extra performance especially at long distances. A pulsed laser is usually used as the pump for HSPS QKD, but it has disadvantages in operational stability and practicality compared to a cw one. On the other hand, a precise model describing the performance of HSPS with a cw pump is still missing. It is inaccurate and unsafe to apply the key rate model of the pulse-pumped HSPS QKD directly to the cw one, and it also overlooks the timing jitter and dead time of the single-photon detectors. In this paper, we study the cw case in detail, including practical imperfections of the experimental instruments. Our model shows that a cw-pumped setup can reach a much longer transmission distance (31 % longer) than the fastest pulsed HSPS QKD system at present, together with other advantages such as higher operational stability and better spectral characteristics.

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.