The magnetic anisotropy and magnetic interactions at the interface between $\mathrm{Fe}$ and $\mathrm{Ni}\mathrm{O}$(001) were investigated. Depending on the growth conditions of the $\mathrm{Ni}\mathrm{O}$(001) layers and the postannealing temperature, the preferential magnetization direction of the ultrathin $\mathrm{Fe}$ layer grown on a $\mathrm{Ni}\mathrm{O}$(001) layer changed from the in-plane direction to a direction perpendicular to the film plane. The lattice constant of the $\mathrm{Ni}\mathrm{O}$(001) layers parallel to the growth direction increased with ${\mathrm{O}}_2$ flow rate, while that parallel to the in-plane direction were locked onto the MgO(001) substrate regardless of the growth conditions of the NiO layers. Moreover, perpendicular magnetization was observed only when the NiO layer was grown with ${\mathrm{O}}_2$ flow rates higher than 2.0 sccm corresponding to oxygen-rich NiO. X-ray magnetic circular dichroism measurements revealed an enhancement in anisotropic orbital magnetic moments similar to the origin of perpendicular magnetic anisotropy at the $\mathrm{Fe}$/MgO(001) interface. The interfacial magnetic anisotropy energies were 0.93 and $1.02{\mathrm{mJ}/\mathrm{m}}^2$ at room temperature and at 100 K, respectively, indicating less temperature dependence. In contrast, the coercivity $H_c$ exhibited a significant temperature dependence. Although no signature of exchange bias or unidirectional loop shift was observed, $H_c$ was strongly dependent on the NiO layer thickness, indicating that the exchange interaction at the interface between the ferromagnetic and antiferromagnetic layers was not negligible, despite the NiO(001) being a spin-compensated surface.

We present the compact long-term nearly continuous operation of a room-temperature ${\mathrm{Ca}}^{+}$-optical-clock setup towards a transportable clock, achieving an overall systematic uncertainty of 4.8 × ${10}^{-18}$ and an uptime rate of 97.8% over an 8-day period. The active liquid-cooling scheme is adopted, combined with precise temperature measurement with 13 temperature sensors both inside and outside the vacuum chamber to ensure the accurate evaluation of the thermal environment for the optical clock. The environmental temperature uncertainty is evaluated as 293.31(0.4) K, corresponding to a blackbody-radiation frequency-shift uncertainty of 4.6 × ${10}^{-18}$, which is reduced more than twice compared to our previous work. Through frequency comparison of the room-temperature ${\mathrm{Ca}}^{+}$ optical clock with a cryogenic ${\mathrm{Ca}}^{+}$ optical clock, the overall uncertainty of the clock comparison is 7.5 × ${10}^{-18}$, including a statistical uncertainty of 4.9 × ${10}^{-18}$ and a systematic uncertainty of 5.7 × ${10}^{-18}$. This work provides a set of feasible implementations for high-precision transportable ion optical clocks.

The side-channel-secure (SCS) quantum key distribution (QKD), SCS QKD has attracted much attention due to its higher security compared to the measurement-device-independent protocols. It is not only measurement-device-independent secure, but also secure against source-state side channels provided that Eve has no access to Alice’s or Bob’s setups inside their labs. In the original protocol for SCS QKD, the side channels and the intensities of the coherent sources are allowed to be different in different time windows, but the vacuum sources of Alice and Bob have to be perfect, which is impossible in practice due to the finite extinction ratio of the intensity modulators. In this paper, we present a method that does not need the perfect vacuum source requested by the original protocol and assures the result as secure as the original protocol. With the upper bounds of the amplitudes of the nonvacuum part of the sources, the secure key rate of our SCS QKD protocol here with imperfect vacuum and unstable sources can be calculated. The numerical results show that, the secure distance of the SCS QKD protocol exceeds 150 km, provided that the intensity of the imperfect vacuum source is less than ${10}^{-8}$, which can be achieved in the experiment by a two-stage intensity modulator, where we also show that the active odd-parity pairing method and standard pairing method using two-way classical communication can be applied to the SCS QKD protocol to improve the key rate. Given the side-channel security based on the existing technological level, this work makes it possible to realize side-channel-secure QKD with real devices.

We explore the design and implementation of a passive reconfigurable switch based on nonlinear Fano resonators asymmetrically coupled to two ports. It was recently shown that asymmetric nonlinear resonators could provide passive bias-free nonreciprocal transmission when they were excited only from one port at a time. Here, we show that, by suitably exploiting time-reversal symmetry, this class of devices can also lead to controllable transmission routing, whereby a low-power pulse injected from one port controls the transmission of a larger power signal injected from the opposite port. This effect can be used to realize a power- and/or phase-controlled transmission switch with minimal power consumption. We first analyze the device with a circuit model to gain physical insights into its operation and to establish optimal operation conditions. We then provide a proof-of-principle experimental demonstration, reporting a device whereby the transmission of a 20-dBm signal can be switched from about 10% to 40% by controlling either the power or phase of a signal about 10 times weaker. This work demonstrates an alternative way to develop low-energy power-controlled reconfigurable switches for applications ranging from radar technology to wireless communications.

Banks are required to set aside funds in their income statement, known as a loan-loss provision (LLP), to account for potential loan defaults and expenses. By treating the LLP as a global constraint, we propose a hybrid quantum-classical algorithm to solve a specific case of quadratic constrained binary optimization (QCBO) models for loan-collection optimization. The objective is to find a set of optimal loan-collection actions that maximizes the expected net profit presented to the bank as well as the financial welfare in the financial network of loanees, while keeping the LLP at its minimum. Our algorithm consists of three parts: a classical divide-and-conquer algorithm to enable a large-scale optimization, a quantum alternating operator ansatz (QAOA) algorithm to maximize the objective function, and a classical sampling algorithm to handle the LLP. We apply the algorithm to a real-world data set with 600 loanees and five possible collection actions. The QAOA is performed using up to 35 qubits on a classical computer. We show that incorporating the QAOA can enhance the expected net profit by approximately $70\mathrm{\%}$ in comparison to scenarios where the QAOA is absent from the hybrid algorithm. Although this improvement does not constitute definitive evidence of quantum advantage, our work illustrates the use of near-term quantum devices to tackle real-world optimization problems.

The recently proposed space-time-coding electromagnetic metasurface introduces the temporal dimension into artificial structure design, greatly expanding its digital application in information processing. However, the counterpart in thermal digital metamaterial is still lacking but compelling because the absence of temporal dimension prohibits synergetic modulation of thermal signal in time and space, thus limiting the capacity of information storage and the efficiency of information transmission with heat. Here, we introduce the temporal modulation into existing spatially variable thermal coding structures and propose a space-time thermal binary coding scheme. We design a category of coding units with time-dependent metashells so that they can switch their functionalities between cloaking and concentrating in time sequences. Integrating these identical units on a two-dimensional spatial array, we numerically show that this single metasurface can output various digital signal sequences composed of 0 and 1 in time and space dimensions. We further experimentally realize the temporal modulation with a rotatable concentric multilayered structure driven by a time-stepping motor. These results demonstrate a practical space-time strategy for thermal binary coding, provide a feasible prototype for spatiotemporal regulation of thermal signal, and promisingly open an alternative way for realizing information processing in diffusion systems.

One of the great challenges facing atomically dispersed catalysts, including single-atom catalysts (SACs) and double-atom catalysts (DACs), is their ultralow metal loading (typically less than 5 wt %), which limits their practical catalytic applications, such as in the oxygen-reduction reaction (ORR), which is crucial for hydrogen fuel cells and metal-air batteries. Although some important progress has been achieved in ultrahigh-density (UHD) SACs, reports on UHD DACs with stable uniform dispersion are still lacking. Herein, based on the experimentally synthesized $M_2{\mathrm{N}}_6$ motif ($M=\mathrm{Sc}$-$\mathrm{Zn}$), we theoretically demonstrate the existence of UHD DACs, with a metal loading of >40 wt %, which are confirmed by systematic analyses of dynamic, thermal, mechanical, thermodynamic, and electrochemical stabilities. Furthermore, the ORR activities of the UHD DACs are comparable to or even better than those of their experimentally synthesized low-density counterparts, and the ${\mathrm{Fe}}_2{\mathrm{N}}_6$ and ${\mathrm{Co}}_2{\mathrm{N}}_6$ UHD DACs are located at the peak of the activity volcano with ultralow overpotentials of 0.31 and 0.33 V, respectively. Finally, the spin magnetic moment of the active center is found to be a catalytic descriptor for the ORR on DACs. Our work can stimulate the experimental exploration of ultrahigh-density DACs and provides an insight into the relationship between the ORR activity of DACs and their spin states.

Cavity-optomechanics-based phonon lasers are traditionally described in terms of three-mode resonance, while the energy nonconservation optomechanical terms are conventionally ignored. Here, we present a complete theory of phonon lasers, emphasizing the importance of the previously omitted optomechanical interaction. We show that this indispensable optomechanical interaction is equivalent to a Kerr nonlinear interaction. Analytical results show that the interference between this Kerr-type interaction and three-mode resonance adds a transition channel with no phonon emission or absorption, thus enhancing phonon lasing and realizing an ultralow-threshold phonon laser and even a phonon cooler. The interference effect produces an asymmetric gain profile with a Fano spectral shape in addition to the traditional Lorentzian shape. Our complete theory shows perfect agreement with existing experiments and provides a new degree of freedom to manipulate optomechanical phonon lasers.

Hong-Ou-Mandel (HOM) interferometry is a powerful quantum metrology technique to reconstruct the surface-depth profile of transparent samples with complementary benefits of measurement precision and experimental robustness. It is generally assumed that a trade-off exists between the standard HOM interferometry sensitivity and its ambiguity-free dynamic range. Here we challenge this “well-known” assumption through the implementation of spectrally resolved HOM interferometry with discrete color entanglement, and show that the well-separated frequencies embedded in a quantum state suffices to achieve great advantages in enhancing the temporal resolution but without limiting its ambiguity-free dynamic range. Backed by the Fisher-information analysis, spectrally resolved coincidence detection is an optimal measurement strategy as it enables us to recover the quantum Cramér-Rao bound. These results may significantly facilitate the use of spectrally resolved quantum interference for real-time quantum information processing, such as ultrafast phase transition and spectral-temporal photography.

Measurement-based control, using an active-feedback loop, is a standard tool in technology. Feedback control is also emerging as a useful and fundamental tool in quantum technology and in related fundamental studies, where it can be used to prepare and stabilize pure quantum states in various quantum systems. Feedback cooling of center-of-mass micromechanical oscillators, which typically exhibit high thermal noise far above the quantum regime, has been particularly actively studied and has recently been shown to allow ground-state cooling by means of optical measurements. Here we realize measurement-based feedback operations in an electromechanical system, cooling the mechanical thermal noise down to three quanta, limited by added amplifier noise. We also obtain significant cooling when the system is pumped at the blue optomechanical sideband, where the system is unstable without feedback.

We report on the coupling of spin waves propagating as guided modes of yttrium iron garnet stripes. Three stripes are placed parallel to each other and separated by gaps that are small enough to provide nearest-neighbor coupling. We term this geometry “bilateral stripes.” The origin of the coupling is the long-ranging dynamic, stray (dipole) field of the precessing magnetization vector. We propose controlling characteristics of this coupling through variation of the static magnetization angle with respect to the main axes of the geometry. We verified the functionality of the proposed magnonic coupler with a micromagnetic simulation of spin-wave propagation along the bilateral stripes. The micromagnetic numerical simulation yielded spectra of transmission of spin waves through the device prototype. Analysis of those spectra revealed that the bilateral stripes can be used as a functional unit in planar magnonic networks—they can be employed as a directional coupler, spin-wave multiplexer, or microwave power divider. Using Brillouin light scattering spectroscopy, we experimentally demonstrated spin-wave transport along the bilateral stripes. We were able to control the spin-wave routing between the stripes (“magnetic channels”) by varying the angle of the bias magnetic field.

Sensing microwave (MW) fields through a quantum protocol has attracted increasing interest, and the phase of MW fields is an essential parameter in communication and quantum technologies. This work proposes a quantum-effect-based method for measuring the phase difference of MW magnetic fields under near-field conditions based on the selective transitions of nitrogen-vacancy- ($\mathrm{N}$-V) center spins in diamond. First, the relationship between the evolutions of the $\mathrm{N}$-V-center spin states and the phase difference of the MW magnetic fields, which are resonant and able to drive the transitions of the $\mathrm{N}$-V-center spins between ground-state energy levels, is deduced. Then, using a double-microstrip-line MW antenna, the variations in the Rabi frequency and maximum contrast as a function of the phase difference are experimentally revealed. Finally, the measurement of the phase difference is validated, and the sensitivity of the proof-of-principle method is estimated to be ${0.0915}^{\circ }/\sqrt{\text{Hz}}$. This method paves the way for the comprehensive quantum sensing of MW fields and has potential applications in the communication and quantum information fields.

In the superconducting state, the presence of a finite gap in the excitation spectrum implies that the number of excitations (quasiparticles) is exponentially small at temperatures well below the critical temperature. Conversely, minute perturbations can significantly impact both the distribution in energy and the number of quasiparticles. Typically, the interaction with the electromagnetic environment is the main perturbation source driving quasiparticles out of thermal equilibrium, while a phonon bath is responsible for restoration of equilibrium. Here we derive approximate analytical solutions for the quasiparticle distribution function in superconducting resonators and explore the impact of nonequilibrium on two measurable quantities: the resonator’s quality factor and its resonance frequency. Applying our results to experimental data, we conclude that while at intermediate temperatures there is clear evidence for the nonequilibrium effects due to heating of the quasiparticles by photons, the low-temperature measurements are not explained by this mechanism.

Active manipulation of the terahertz (THz) beam, especially beam deflection, is urgently required for wide THz applications. Although Pancharatnam-Berry (PB) metasurfaces can flexibly control the spatial phase distribution of circularly polarized waves, active power distribution and modulation between two conjugate spin beams are still challenging. Here, we demonstrate an all-dielectric metadevice consisting of an anisotropic metasurface layer and a PB metasurface layer etched on both sides of a silicon wafer. This structure develops a photonic spin-decoupling mechanism for spin-beam manipulation, which actively controls the power distribution between the two deflected spin beams by rotating the incident linearly polarized direction. The power-distribution ratio reaches 14.9:1 in the experiment. Moreover, the deflected beam can also be modulated by active switching via a laser pump, achieving an intensity-modulation depth of over 90%. The experimental modulation depth of the metadevice is 52% higher than that of the bare silicon wafer at the same pump power, which originates from the THz localized field enhancement and the strong dispersion effect of the PB structures. Therefore, this work provides a scheme for actively manipulating THz-beam deflection, realizing controllable wave-division multiplexing, power distribution, polarization control, and modulation.

Terahertz pulses with controlled spectral-temporal waveform have great potential in expanding the scope of state-of-the-art terahertz researches, such as terahertz-driven resonant and nonresonant control over matter and light, ultrafast terahertz spectroscopy, and terahertz-driven acceleration. Here, we propose and demonstrate a method to generate a prescribed terahertz chirped waveform with a well-designed tapered corrugated wakefield structure. We develop a deterministic procedure to derive the parameters of the corrugated structure that generates the desired spectral-temporal waveform with certain shape of chirp, central frequency, bandwidth, and pulse duration. The experimental results from ultrashort electron bunches passing through a well-designed tapered corrugated structure are demonstrated. Time-frequency analysis of the terahertz electric field measured by electro-optical sampling shows clear chirp from 0.38 to 0.6 THz. Calibrated measurements of the radiated energy indicate 10 µJ per pulse for an incident beam charge of 300 pC.

Non-Gaussian modulation can improve the performance of continuous-variable quantum key distribution (CV QKD). For Gaussian-modulated coherent-state CV QKD, photon subtraction can realize non-Gaussian modulation, which can be equivalently implemented by non-Gaussian postselection. However, non-Gaussian reconciliation has not been deeply researched, which is one of the key technologies in CV QKD. In this paper, we propose a non-Gaussian reconciliation method to obtain identical keys from non-Gaussian data. Multidimensional reconciliation and multiedge-type low-density parity-check codes (MET LDPC) are used in a non-Gaussian reconciliation scheme, where the layered belief propagation decoding algorithm of MET LDPC codes is used to reduce the decoding complexity. Furthermore, we compare the error-correction performance of Gaussian data and non-Gaussian data. The results show that the error-correction performance of non-Gaussian data is better than Gaussian data, where the frame error rate can be reduced by 50 % for code rate 0.1 at SNR of 0.1554 and the average iteration number can be reduced by 25 %.

Using the Landau-Ginzburg-Devonshire approach, we study stress-induced transformations of polarization switching in ferrielectric ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanoparticles for three different shapes: a disk, a sphere, and a needle. Semiconducting properties of a nanoparticle are modeled by a surface-charge layer, whose effective screening length can be rather small due to the field effect. We reveal a very strong and unusual influence of hydrostatic pressure on the appearance of polarization switching in ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanoparticles, hysteresis loop shape, magnitude of the remanent polarization, and coercive fields, and explain the effects by the anomalous temperature dependence and “inverted” signs of ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ linear and nonlinear electrostriction coupling coefficients. In particular, by varying the sign of the applied pressure (from tension to compression) and its magnitude (from zero to several hundreds of MPa), quasistatic hysteresisless paraelectric curves can transform into double, triple, pinched, or single hysteresis loops. Unexpectedly, we predict that stressed ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanospheres and nanoneedles reveal sizeable temperature and pressure ranges of triple loop stability, which are very rare in ferroelectrics and antiferroelectrics. A physical origin of triple loops is the coexistence of “small” and “larger” polarizations in the four-well ferrielectric state. Due to the sufficiently wide temperature and pressure ranges of double, triple, and pinched hysteresis loop stability (at least in comparison with many other ferroelectrics), ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanodisks can be of particular interest for applications in energy storage (in the region of double loops), ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanospheres maybe suitable for dynamic random-access multibit memory, and ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanoneedles are promising for nonvolatile multibit memory cells (in the regions of triple and pinched loops). The stress control of the polarization-switching scenario allows the creation of advanced piezosensors based on ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanocomposites.