Scattering wave systems that are periodically modulated in time offer many new degrees of freedom to control waves in both the spatial and frequency domains. Such systems, albeit linear, do not conserve frequency and require the adaptation of the usual theories and methods. In this paper, we provide a general extension of transmission-line or telegraph equations to periodically time-modulated systems. As a byproduct of the theory, we obtain a general approach to compute and measure the complete scattering matrix of such systems. Finally, the proposed theory and methods are applied and validated on a concrete practical example in the realm of airborne acoustics: a time-modulated actively controlled loudspeaker membrane terminating a monomode waveguide. Different modulation functions and parameters are tested. The experimental results are compared to both numerical simulation and an analytical model based on a two-time-scale method.

We present a scheme to enhance the atom number in magneto-optical traps of strontium atoms operating on the 461-nm transition. This scheme consists of resonantly driving the ${}^1{\mathrm{S}}_0\to {}^3{\mathrm{P}}_1$ intercombination line at 689 nm, which continuously populates a short-lived reservoir state and, as expected from a theoretical model, partially shields the atomic cloud from losses arising in the 461-nm cooling cycle. We show a factor of 2 enhancement in the atom number for the bosonic isotopes ${}^{88}\mathrm{Sr}$ and ${}^{84}\mathrm{Sr}$, and the fermionic isotope ${}^{87}\mathrm{Sr}$, in good agreement with our model. Our scheme can be applied in the majority of strontium experiments without increasing the experimental complexity of the apparatus, since the employed 689-nm transition is commonly used for further cooling. Our method should thus be beneficial to a broad range of quantum science and technology applications exploiting cold strontium atoms, and could be extended to other atomic species.

Spiking neural networks aim to emulate the brain’s properties to achieve similar parallelism and high processing power. A caveat of these neural networks is the high computational cost for emulation, while current proposals for analogue implementations are energy inefficient and not scalable. We propose a device based on a single magnetic tunnel junction to perform neuron firing for spiking neural networks without the need for any resetting procedure. We leverage two areas of physics, magnetism and thermal effects, to obtain biorealistic spiking behavior analogous to the Hodgkin-Huxley model of the neuron. The device is also able to emulate the simpler leaky-integrate-and-fire model. Numerical simulations using experimental-based parameters demonstrate firing frequency in the megahertz to gigahertz range under constant input at room temperature. The compactness, scalability, low cost, CMOS compatibility, and power efficiency of magnetic tunnel junctions advocates for their broad use in hardware implementations of spiking neural networks.

High conversion efficiency is vital for the acquisition of UV light through nonlinear processes. Here, we investigate an alternative method to realize highly efficient second-harmonic generation (SHG) based on the interface of $\beta$ barium metaborate ($\beta \mathrm{BBO}$) and magnesium-doped lithium niobate ($\mathrm{MgO}$:LN). Theoretical simulations indicate that the SHG efficiency of our method is significantly higher than other phase-matching conditions. In the experiment, we demonstrate that higher conversion efficiency can be realized compared with only employing a single BBO crystal. Besides, the shortest output wavelength of SHG can reach 205 nm in our method. The method proposed here can open another way for the integrated ultraviolet laser and may promote the development of UV micromachining in the future.

The recently synthesized two-dimensional van der Waals (vdW) ferromagnet 1T-${\mathrm{MnSe}}_2$ has attracted great attention due to its room-temperature ferromagnetism. By using ab initio quantum transport simulations with noncollinear spins, we demonstrate a monotonic increasing (decreasing) tendency of the angle-resolved tunneling magnetoresistance (spin-transfer torque) with the increasing graphene layer number n in the ${\mathrm{MnSe}}_2$/n-layer graphene/${\mathrm{MnSe}}_2$ (n = 1–4) vdW magnetic tunnel junctions (MTJs). A surprising tunneling magnetoresistance of around ${10}^6$% is obtained when $\theta ={180}^{\circ }$ and n = 4 owing to the nearly perfect spin polarization; this is 3 orders of magnitude larger than that of the commonly used $\mathrm{MgO}$-based MTJ (1000% at 5 K). The maximal linear-response spin-transfer torque of the ${\mathrm{MnSe}}_2$/graphene/${\mathrm{MnSe}}_2$ MTJ (5260 µeV/V) is 2 orders of magnitude larger than that of the $\mathrm{Fe}/\mathrm{MgO}/\mathrm{Fe}$ MTJ. The decay rate of the spin-transfer torkance at zero bias is revealed to be nonmonotonically n dependent. Meanwhile, the total spin-transfer torque in the device changes from linear dependence to nonlinear dependence as the bias voltage increases. Our calculation suggests that the 1T-${\mathrm{MnSe}}_2$-based vdW MTJs are promising in next-generation room-temperature nonvolatile memories.

Coupling of space-separated resonators is interesting for quantum and communication technologies. In this work, we show that antiferromagnetic resonance in separated parallel-plane slabs of hematite ($\alpha$-${\mathrm{Fe}}_2{\mathrm{O}}_3$) couple cooperatively to terahertz electromagnetic cavity modes formed by the slabs themselves. We demonstrate control of these hybridized magnon-polariton modes either by tuning the distance between the slabs in the range of up to a few millimters or by scanning their temperatures in a range above room temperature. Analysis of measured spectra with three distinct theoretical models shows that the best approximations can be obtained with a model based on classical electromagnetism. Cavity-mediated coupling allows the engineering of resonators at millimeter-range frequencies (millielectronvolts).

Balanced homodyne interferometry is a well-known detection technique that allows for sensitive characterization of light fields. Conventionally a homodyne interferometer is operated by locking the relative phase of a reference beam to the signal beam by means of an active feedback loop. A less often used method is to perform a slow continuous modulation of the reference beam arm length that corresponds to averaging all relative phases during the measurement. Here we show theoretically and experimentally that this quadrature averaging can be advantageous in estimating the parameters of a resonant optical cavity. We demonstrate that the averaging turns the transduction function, from cavity frequency fluctuations into the interferometer signal, into a simple function of the laser detuning that, notably, does not depend on the parameters of possible nonresonant channels present in the system. The method needs no active feedback and gives results that are easy to interpret. Moreover, the phase-averaged measurement allows characterization of the absolute magnitude of a cavity frequency modulation.

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.