We propose a new phase detection technique based on a flux-switchable superconducting circuit, the Josephson digital phase detector (JDPD), which is capable of discriminating between two phase values of a coherent input tone. When properly excited by an external flux, the JDPD is able to switch from a single-minimum to a double-minima potential and, consequently, relax in one of the two stable configurations depending on the phase sign of the input tone. The result of this operation is digitally encoded in the occupation probability of a phase particle in either of the two JDPD wells. In this work, we demonstrate the working principle of the JDPD up to a frequency of 400 MHz with a remarkable agreement with theoretical expectations. As a future scenario, we discuss the implementation of this technique to superconducting qubit readout. We also examine the JDPD compatibility with the single-flux-quantum architecture, employed to fast-drive and measure the device state.

Processing quantum information using quantum three-level systems or qutrits as the fundamental unit is an alternative to contemporary qubit-based architectures with the potential to provide significant computational advantages. We demonstrate a fully programmable two-qutrit quantum processor by utilizing the third energy eigenstates of two transmons. We develop a parametric coupler to achieve excellent connectivity in the nine-dimensional Hilbert space enabling efficient implementations of two-qutrit gates. We characterize our processor by realizing several algorithms like Deutsch-Jozsa, Bernstein-Vazirani, and Grover’s search. Our hardware-efficient protocols allow us to show that two stages of Grover’s amplification can improve the success rates of an unstructured search with quantum advantage. Our results pave the way for building fully programmable ternary quantum processors using transmons as building blocks for a universal quantum computer.

In this article, we analyze the photovoltaic effect while assuming a fixed ambient temperature and a varying system temperature rather than using the standard fixed system temperature–based approaches. We do so by complementing the photon rate balance equation (detailed balance, circuit model) with the power balance equation of the system. As a result, a simple approach capable of treating any photovoltaic system emerges. Accordingly, we study the potential-dependent current and temperature of solar cells and thermoradiative power generators. We show that the optimal band gap of a solar cell depends on its heat-transfer coefficient and that its efficiency may rise or fall as solar concentration increases, depending on its ability to dissipate heat. We also identify where the cell’s efficiency and temperature turn from a conductive and/or convective-dominated cooling regime to a radiative-dominated one. For the thermoradiative case, we show that its power decreases when heat intake is suppressed and study the degrading effect of nonradiative recombination on this power production scheme. The proposed model converges to the known fixed system temperature–based approaches when an infinite ability to transfer heat is considered.

In precision measurement and quantum metrology, the $2\pi$ periodicity inherent to any interferometric signal sets a fundamental limit in the simultaneous achievement of both high measurement sensitivity and range. Here, based on the extended period of the Lissajous curve, we develop a method to break this sensitivity–maximum-range limit completely. In the measurement of magnetic field using an electronic spin in diamond under ambient conditions, we extend the maximum range by 2 orders of magnitude without sacrificing detection sensitivity and bandwidth for sufficiently long measurement time. Furthermore, we achieve precise detection of a time-varying periodic waveform, where a large measurement range can induce high accuracy and demonstrate a Heisenberg-like scaling within the coherent regime. This Lissajous-curve-based range extension method can be directly applied to any other modulo-limited quantum interferometers, which makes a useful and indispensable stepping stone toward practical interferometric quantum metrology.

We demonstrate the sensitivity enhancement of measurement of microwave electric fields by use of the full information of the complex atomic susceptibility. The transfer characteristics of both the microwave electric fields and the noise relevant to atoms, which are mapped to two quadrature components of the probe light, are systematically measured with use of a Mach-Zehnder interferometer. On the basis of the readout of the full complex atomic susceptibility, a sensitivity enhancement of 12 dB is achieved when the probe light has red detuning of 6 MHz. The results shed light on increasing the sensitivity of atomic sensors of microwave electric fields that is beyond the capability of the traditional antenna.

Emerging applications in the field of finite-frequency mesoscopic physics require accurate modeling tools for the evaluation of carrier-transport dynamics, modulation bandwidth, and frequency conversion effects in nanodevices. With foundations in advanced concepts of many-body and quantum field theories, the nonequilibrium Green’s function technique is widely adopted in the calculation of steady-state carrier-transport properties of nanostrucures, while the evaluation of the frequency response is so far largely unexplored by genuine quantum models. Guided by the connection with drift-diffusion solvers within a local description of carrier-phonon scattering, we propose an accurate, yet computationally efficient nonequilibrium Green’s function model of dissipative carrier transport to study the small-signal properties of semiconductor nanostructures. From the numerical evaluation of steady-state Green’s functions and their functional derivatives, we compute spectrally resolved observables expressed in terms of familiar microscopic quantities germane to the drift-diffusion framework, the most prevalent tool in semiclassical device simulation. Besides drastically improving the convergence properties, the exact Jacobian, complemented with the contribution of the displacement current, gives access to the small-signal admittance. Current-conserving boundary conditions suitable for small-signal analysis provide the correct physical behavior near the contacts. Numerical examples show the accuracy and flexibility of the proposed model.

We experimentally implement a one-directional virtual geometric periodicity in an elastic metamaterial. First, unwanted boundary reflections at the domain ends are cancelled through the iterative injection of the polarity-reversed reflected wave field. The resulting boundless experimental state allows for a much better analysis of the influence of metamaterials on the propagating wave field. Subsequently, the propagating wave field exiting on one end of the structure is reintroduced at the opposite end, creating a virtual geometric periodicity in one propagation direction. We find that the experimentally observed band gap converges to the analytical solution through the introduction of the virtual periodicity. The established work flow introduces an approach to the experimental investigation and validation of metamaterial prototypes in the presence of strongly dispersive wave propagation and internal scattering. The fully data-driven ad hoc treatment of boundary conditions in metamaterial experimentation with arbitrary mechanical properties enables reflection suppression, virtual periodicity, and the introduction of more general fictitious boundaries.

Spintronic nano-oscillators and diodes with reduced nonlinearity benefit from low phase noise and improved device properties. Moreover, they could offer key advantages for realizing neuromorphic applications, such as spike-based neurons and frequency multiplexing in neural networks. Here, we experimentally demonstrate the reduction in nonlinearity of a spin Hall nano-oscillator (SHNO) by compensation of its effective magnetic anisotropy. The study involves optimization of $\mathrm{Co}/\mathrm{Ni}$ multilayer growth to achieve the compensation, followed by spin-diode measurements on patterned microstrips to quantify their anisotropy. The relationship between the second- $(H_{k_2}=0.47\mathrm{mT})$ and first-order $(H_{k_1}^{\mathrm{eff}}=-0.8\mathrm{mT})$ anisotropy fields reveals the existence of an easy cone, thereby validating the presence of compensation. Furthermore, we demonstrate a compensated spin diode that has a fixed frequency when the input power is varied. We then study the current-induced auto-oscillation properties of SHNOs on compensated films by patterning nanoconstrictions of 200 and 100 nm wide. The invariance of the resonance frequency and linewidth of the compensated SHNO with applied dc current indicates the absence of nonlinearity. This independence is maintained irrespective of the applied external fields and their orientations. The compensated SHNO obtained has a linewidth of 1.1 MHz and a peak output power of up to 1 pW/MHz.

Twin-field quantum key distribution (TFQKD) enables two distant parties to establish a shared secret key, by interfering weak coherent pulses (WCPs) in an intermediate measuring station. This allows TFQKD to reach greater distances than traditional QKD schemes and makes it the only scheme capable of beating the repeaterless bound on the bipartite private capacity. Here, we generalize TFQKD to the multipartite scenario. Specifically, we propose a practical conference key agreement protocol that only uses WCPs and linear optics and prove its security with a multiparty decoy-state method. Our protocol allows an arbitrary number of parties to establish a secret conference key by single-photon interference, enabling it to overcome recent bounds on the rate at which conference keys can be established in quantum networks without a repeater.

An efficient mass sensor based on exceptional points (EPs), engineered under a synthetic magnetism requirement, is proposed. The benchmark system consists of an electromechanical (optomechanical) system where an electric (optical) field is driving two mechanical resonators that are mechanically coupled through a phase-dependent phonon hopping. This phase induces a series of EPs once it matches the condition of $\pi /2(2n+1)$. For any perturbation of the system, the phase-matched condition is no longer satisfied, and this lifts the EP degeneracies, leading to a frequency splitting that scales as the square root of the perturbation strength, resulting in a giant sensitivity-factor enhancement. Owing to the set of EPs, our proposal allows a multiple-sensing scheme and performs better than its anti-$PT$-symmetric sensor counterpart. This work sheds light on platforms that can be used for mass-sensing purposes, opening up new opportunities in nanoparticle or pollutant detection and in water treatment.

Preshared entanglement can significantly boost communication rates in the high thermal-noise and low-brightness transmitter regime. In this regime, for a lossy bosonic channel with additive thermal noise, the ratio between the entanglement-assisted capacity and the Holevo capacity—the maximum reliable communication rate permitted by quantum mechanics without any preshared entanglement—scales as $\log (1/{\overline{N}}_S)$, where the mean transmitted photon number per mode, ${\overline{N}}_S\ll 1$. Thus, preshared entanglement, e.g., distributed by the quantum internet or a satellite-assisted quantum link, promises to significantly improve low-power radio-frequency communications. In this paper, we propose a pair of structured quantum transceiver designs that leverage continuous-variable preshared entanglement (generated, e.g., from a down-conversion source), binary phase modulation, and non-Gaussian joint detection over a codeword block, to achieve this scaling law of capacity enhancement. Furthermore, we describe a modification to the aforesaid receiver using a frontend that uses sum-frequency generation sandwiched with dynamically programmable in-line two-mode squeezers, and a receiver backend that takes full advantage of the output of the receiver’s frontend by employing a nondestructive multimode vacuum-or-not measurement to achieve the entanglement-assisted classical communication capacity.

On-chip mode division (de)multiplexing plays a significant role in integrated devices to greatly improve communication capacity. One of the goals of mode division (de)multiplexing is to realize devices with larger bandwidth and better fabrication tolerance. However, integrated broadband mode division (de)multiplexing devices have rarely been reported up to now. In this work, we experimentally realize a broadband mode division demultiplexing device with a high degree of fabrication tolerance based on a lithium niobate-on-insulator waveguide array. By taking advantage of the fact that different modes in the waveguide array have different group velocities, we experimentally confirm that different modes in the waveguide array have different transverse displacements. The implementation of the integrated broadband mode division demultiplexer may have future applications in information processing technology, quantum communication, and quantum computing.

The removal of undesired beam nonlinear energy chirp (time-energy correlation) or linearization of the beam longitudinal phase space (LPS) is crucial for high-brightness linac-based scientific applications, such as x-ray free-electron lasers. In this paper, we propose that a low-density hollow channel plasma can be used as a near-ideal passive linearizer to significantly linearize the beam LPS and, at the same time, preserve the beam emittance. Physically, the passage of the beam through the hollow plasma channel excites a strong quasicosinoidal longitudinal wakefield that acts to mitigate the beam nonlinear energy chirp by superimposing a reverse chirp on the beam. The theoretical analyses and large-scale three-dimensional start-to-end simulations confirm that the beam longitudinal phase space can be almost completely linearized without noticeable beam emittance growth. Application of such a near-ideal linearizer may significantly improve the performance of numerous accelerator-based applications.

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.