We propose a scheme to entangle a magnon in an yttrium iron garnet sphere with an atomic ensemble by using an optical cavity as the qubus. The cavity mode couples with the magnon through magnetic dipole interaction, and simultaneously couples with the atomic ensemble through Jaynes-Cummings interaction. Moreover, the magnon couples with the mechanical vibration mode of the yttrium iron garnet sphere via magnetostrictive interaction. In this hybrid system, we find that there exist six kinds of bipartite entanglements, including the entanglement between the magnon and the atomic ensemble, and we use logarithmic negativity $E_N$ for quantification. Our findings demonstrate the phenomena of entanglement sudden death, entanglement sudden revival, and entanglement transfer among the six bipartite entanglements. Then we focus on entanglement between a magnon and an atomic ensemble, and we show that the entanglement can be effectively enhanced by increase of the magnetic dipole coupling or the magnetostrictive coupling. Finally, we discuss the robustness and find that incorporating an atomic ensemble into the system could enhance the steady-state entanglement’s tolerance to the environmental temperature.

We have developed a compact and highly efficient assembly for stimulated laser cooling of thermal rubidium atomic beams traveling upon planar silicon devices. Following precollimation via a silicon microchannel array, we perform beam brightening via a blue-detuned optical molasses [Aspect et al., Phys. Rev. Lett. 57, 1688 (1986)]. Owing to the small size of the chip elements, we require only 8 mW to implement the efficient cooling scheme, making it well suited for future portable atomic sensors that require minimal size, weight, and power consumption. Small mirrors are fabricated and hand-assembled to precisely overlap a strong elliptical standing wave with a sheet-shaped atomic density distribution, with dimensions chosen precisely to match these. We reduce the transverse velocity spread to below 1 m/s within an interaction length of 4.5 mm above the silicon substrate. We use Doppler-sensitive two-photon Raman spectroscopy to characterize the cooling. In contrast to time-of-flight methods utilized previously, this approach requires a much shorter apparatus to achieve similar resolution. This hybrid of passive and active collimation paves the way toward the construction of fully fledged atomic instruments, such as atomic beam clocks and gyroscopes, entirely on-chip through batch-fabricated processes.

Invertible logic is a powerful new unconventional computing paradigm, providing bidirectional operations between inputs and outputs. It has found applications in important critical problems, such as integer factorization and machine learning. Here we propose a network of interconnected nonlinear systems that serve as our probabilistic bits (“$p$-bits”) to implement invertible logic in the presence of a noise floor. In the forward (or directed) mode, the inputs are fixed in our network, yielding outputs in accordance with and, or, nand, and nor logic functions. In the reverse (inverted) mode the output is clamped in the network, and the input nodes fluctuate among all possible logical input values consistent with the different logic functions. So the system acts as a unique invertible logic circuit by exploiting the probabilistic transitions between the dynamical states of the coupled noisy nonlinear systems. Interestingly, both the directed and the inverted mode are most robust and reliable in an optimal band of moderate noise, reminiscent of stochastic resonance. The concept is verified in proof-of-principle electronic circuit experiments, demonstrating the robustness of the architecture and the potential of this idea to be realized in a wide range of physical situations.

Time-resolved photoluminescence (PL) measurements in wide (15 and 25 nm) $(\mathrm{In},\mathrm{Ga})\mathrm{N}/\mathrm{Ga}\mathrm{N}$ quantum well light-emitting diodes (LEDs) reveal that photoexcited charge carriers occupying ground states in the well can be detected up to tens of milliseconds after optical excitation. Moreover, even without photoexcitation, some residual charge carriers are present in the ground states of wide wells. We call them “dark charge,” since they do not recombine radiatively. However, they screen the electric field in the well and lead to the appearance of highly efficient optical transitions through excited states. The intensity of these optical transitions (PL) is sensitive to the population of dark charge. Dark charge can be depleted by short pulses of negative voltage applied to the diode and then it slowly recovers to its equilibrium value. Such long decay-regeneration times on the order of milliseconds are due to negligible overlap of electron and hole ground states in wide wells with a strong electric field. The reverse voltage applied to the LED shortens these lifetimes, since it reduces the electric field in the well. The effect of regeneration of dark charge allows us to estimate the value of electric field in the well. Furthermore, we present a simple model of the carrier dynamics in wide quantum wells.

In quantum information transformation and quantum computation, the most critical issues are security and accuracy. These features, therefore, stimulate research on quantum state characterization. A characterization tool, quantum state tomography, reconstructs the density matrix of an unknown quantum state. Theoretically, reconstructing an unknown state using this method can be arbitrarily accurate. However, this is not so practical owing to the huge burden of measurements and data processing for large numbers of qubits. Even comprising an efficient estimator and a precise algorithm, an optimal tomographic framework can also be overburdened owing to the exponential growth of the measurements. Moreover, the consequential postprocessing of huge amounts of data challenges the capacity of computers. Thus, it is crucial to build an efficient framework that requires fewer measurements but yields an expected accuracy. To this end, we built a tomography schema by which only a few Pauli measurements enable an accurate tomographic reconstruction. Subsequently, this schema was verified as efficient and accurate through numerical simulations on the tomography of multiqubit quantum states. Furthermore, it was proven to be robust through numerical simulations on a noisy superconducting qubit system. Therefore, the tomography schema paves an alternative effective way to reconstruct the density matrix of a quantum state owing to its efficiency and accuracy, which are essential for quantum state tomography.

We theoretically and experimentally study the precision of a quantum clock near zero temperature, explicitly accounting for the effect of continuous measurement. The clock is created by a superconducting transmon qubit dispersively coupled to an open coplanar resonator. The cavity and qubit are driven by coherent fields, and the cavity output is monitored with a quantum-noise-limited amplifier. When the continuous measurement is weak, it induces persistent coherent oscillations (with fluctuating periods) in the conditional moments of the qubit’s energy probability distribution, which are manifest in the output of the resonator. On the other hand, strong continuous measurement leads to an incoherent cycle of quantum jumps. We theoretically find an equality for the precision of the clock in each regime. Independently from the equalities, we derive a kinetic uncertainty relation for the precision, and find that both equalities satisfy this uncertainty relation. Finally, we experimentally verify that our quantum clock obeys the kinetic uncertainty relation for the precision, thus making an explicit link between the (kinetic) thermodynamic behavior of the clock and its precision, and achieving an experimental test of a kinetic uncertainty relation in the quantum domain.

We demonstrate the detection of broadband intense terahertz electromagnetic pulses by Zeeman-torque sampling (ZTS). Our approach is based on magneto-optic probing of the Zeeman torque that the terahertz magnetic field exerts on the magnetization of a ferromagnet. Using an 8-nm-thick iron film as a sensor, we detect pulses from a silicon-based spintronic terahertz emitter with a bandwidth of 0.1–11 THz and a peak field of >0.1 MV/cm. A simple static magneto-optic calibration measurement provides access to absolute transient terahertz field strengths. We show the relevant added value of ZTS compared to electro-optic sampling (EOS): an echo-free transfer function with simple frequency dependence, linearity even at high terahertz-field amplitudes, the straightforward calibration of EOS response functions, and modulation of the polarization-sensitive direction by an external ac magnetic field. Consequently, ZTS has interesting applications even beyond the accurate characterization of broadband high-field terahertz pulses for nonlinear terahertz spectroscopy.

Superconducting circuits are highly controllable platforms to manipulate quantum states, which make them particularly promising for quantum information processing. We show here how the existence of a distance-independent interaction between microwave resonators coupled capacitively through a qubit offers an alternative control parameter toward this goal. This interaction is able to induce an idling point between resonant resonators, and its state-dependent nature allows one to control the flow of information between the resonators. The advantage of this scheme over the previous one is demonstrated through the generation of high-fidelity NOON states between the resonators, with a lower number of operations than previous schemes. Beyond superconducting circuits, our proposal could also apply to atomic lattices with clock transitions in optical cavities, for example.

The photogalvanic effect in ferroelectric (FE) materials attracts considerable interest in solar cells, polarization-sensitive photodetection, and nonvolatile memory applications. However, narrow-band-gap FE materials that cover the full solar spectrum are currently rare, and there is still room to enhance the light-polarization sensitivity and to improve the switching ratio for FE optoelectronic devices. Here, we investigate the photogalvanic effect induced by elliptically polarized light in the narrow-band-gap ($<1.6$ eV) $\mathrm{In}\mathrm{Se}$/${\mathrm{In}}_2{\mathrm{Se}}_3$ FE heterojunction (HJ) stacked with the $\mathrm{In}\mathrm{Se}$ monolayer and $\alpha$-${\mathrm{In}}_2{\mathrm{Se}}_3$ FE monolayer. The dependence of the photocurrent on the polarization angle, $\varphi$, and on the incidence angle, $\alpha$, is formulated within the nonequilibrium Green’s function formalism, with which the photocurrent is then calculated by using first-principles methods. We find that both the linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE) are generated from the infrared to the visible range. The CPGE is only induced at oblique incidence due to $C_{3v}$ symmetry of the HJ and can be ${10}^3$ times larger than the LPGE. The largest photocurrent is achieved when light is irradiated along the zigzag direction, which is dominated by the CPGE. The photocurrent can further be tuned by switching between upward and downward FE polarization, leading to a large switching ratio of 563. The photocurrent also shows an excellent light-polarization sensitivity at normal and oblique incidence, with an impressive extinction ratio of 410. These results shed light on the CPGE of FE materials for applications in low-dimensional FE optoelectronics.

Despite the prospect of next-generation electronic technologies spurring the investigation of the remarkable topological magnetoelectric response, its potential remains largely unexplored in the application of basic electronic devices. In this paper, we undertake this task at the theoretical level by addressing the $\theta$ electrodynamics and examine electromagnetic properties (e.g., tunable inductance, operating frequency range, and power consumption) of three fundamental passive magnetic devices endowed with this effect: the primitive transformer, the bilayer solenoid inductor, and the solenoid actuator. We further exploit the methodology of magnetic circuits to obtain an extended Hopkinson’s law that is valid for both topological and ordinary magnetoelectric responses (provided it is uniform in the bulk). Under low-power conditions, we find out that the functionally passive part of the topological-magnetoelectric transformer, the solenoid inductor as well as the solenoid actuator, is indistinguishable from the conventional situation up to second order in the magnetoelectric susceptibility; we argue that the main benefit of using topological insulators essentially relies on a lower power consumption. Our theoretical framework is also convenient to analyze magnetoelectric inductors endowed with a relatively large magnetoelectric susceptibility, they display a broad inductance tunability of over $200\mathrm{\%}$ up to 100 GHz in the millimeter length scale. Conversely, our treatment predicts that the operating frequency range could be restricted below the ultralow frequency by a significantly strong magnetoelectric response (e.g., retrieved by certain multiferroic heterostructures).

We explore critical field and critical current behavior in inhomogeneous narrow thin-film superconducting strips. Formulations are developed to calculate free energy, critical field, and critical current for strips with inhomogeneous Pearl-length distributions. Our findings show that inhomogeneities, specifically a shorter Pearl length in the middle of the strip, significantly enhance the critical field $B_{c1}$. This has practical implications for achieving complete flux expulsion. While narrow strips have traditionally been considered the most effective approach to improve $B_{c1}$ and eliminate trapped vortices, our results suggest that engineered inhomogeneities offer an alternative method to enhance the critical field and improve flux expulsion without reducing strip width, providing greater design flexibility for superconducting devices. Additionally, we find that for the purpose of increasing the critical current, utilizing an inhomogeneous film with a reduced Pearl length in the middle of the strip is advantageous. The enhancement in critical current arises from the current suppression effect at the edges induced by the inhomogeneous distribution of superfluid density. Furthermore, we demonstrate that an inhomogeneous film with a left-right asymmetric Pearl-length distribution enables control over the nonreciprocity of the critical current, highlighting the potential of engineering inhomogeneous Pearl-length distributions to implement devices exhibiting the superconducting diode effect. Our results provide concrete examples of how manipulating the inhomogeneity of Pearl length can enhance the performance of superconducting devices. Various methods such as doping nonuniform impurities or creating a temperature gradient can be employed to implement an inhomogeneous Pearl-length distribution.

Vertical transistors, with the channel material sandwiched between graphene and metal electrodes, are promising for the development of next-generation electronic devices. However, realizing complementary transport in two-dimensional vertical transistors is challenging due to the significant disorder and severe Fermi-level pinning effects at the metal-semiconductor interface caused by conventional metallization. Here, we report complementary vertical transistors in graphene/${\mathrm{WSe}}_2/$van der Waals metal heterostructures. In this device, the van der Waals metal retains the pristine nature of atomically thin ${\mathrm{WSe}}_2$, minimizing the Fermi-level-pinning effect at the metal-${\mathrm{WSe}}_2$ interface. Thus, the barrier height and the type of majority carrier can be simply controlled by the metal work function. Based on this approach, we achieve n- and p-type ${\mathrm{WSe}}_2$ vertical transistors by laminating $\mathrm{Ag}$ and $\mathrm{Pt}$ as van der Waals contact metals, respectively. Moreover, with highly controllable device polarities, we demonstrate a complementary inverter by integrating two vertical transistors with different polarities. Our work not only enables complementary two-dimensional vertical transistors, but also provides a promising strategy for controlling the polarity of carriers in vertical heterostructures.

Kerr parametric oscillators (KPOs) implemented in the circuit quantum electrodynamic architecture can operate as qubits. Their applications to quantum annealing and universal quantum computation have been studied intensively. For these applications, the readout of the state of KPOs is of practical importance. We develop a scheme of state tomography for KPOs with reflection measurement. Although it is known that the reflection coefficient depends on the state of the KPO, it is unclear whether tomography of a qubit encoded into a KPO can be performed in a practical way mitigating decoherence during the measurement, and how accurate it is. We show that the reflection coefficient has a one-to-one correspondence with a diagonal element of the density matrix of the qubit when a probe frequency is properly chosen and an additional single-photon drive is introduced. Thus, our scheme offers a novel way to read out the qubit along an axis of the Bloch sphere, and therefore the reflection measurement and single-qubit gates can constitute state tomography.

We investigate the magnetic properties of an epitaxial full-Heusler ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ film with an atomically controlled multiferroic interface coupled with a ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ substrate. The real-space observation of the magnetic domain structure using a magneto-optical Kerr microscope reveals perfect domain-pattern transfer from the ferroelectric substrate, indicating strong interface magnetoelectric coupling. The unique anisotropic magnetization reversal process was found to be induced by lateral modulation of the magnetic anisotropy constant through the strain-induced magnetoelectric coupling effect. The strong interface coupling yields efficient electrical manipulation of the magnetic property with the appropriate initial ferroelectric domain pattern. Our finding indicates that the interface multiferroic structure based on an epitaxially grown Heusler thin film is a promising approach for efficient electrical manipulation of magnetization.

Topological phases of matter feature exotic edge states. However, the fractional topological numbers at edges, although predicted long ago by Jackiw and Rebbi, remain elusive in topological photonic systems. Here we report on the observation of fractional topological numbers at the topological edges and corners in one- and two-dimensional photonic crystals. The fractional topological numbers are determined via measurements of the photonic local density of states. In one-dimensional photonic crystals, we witness a rapid change of the fractional topological number at the edges rising from 0 to 1/2 when the photonic band gap experiences a topological transition, confirming the well-known prediction of Jackiw and Rebbi. In two-dimensional systems, we discover that the fractional topological number in the corner region varies from 0 to 1/2 and 1/4 in phases with different photonic band gaps. Our study paves the way toward topological manipulation of fractional quantum numbers in photonics.

Josephson traveling-wave parametric amplifiers (TWPAs) enable the amplification of weak microwave signals close to the quantum limit with large bandwidth, which has a broad range of applications in superconducting quantum computing and in the operation of single-photon detectors. While the large bandwidth allows for their use in frequency-multiplexed detection architectures, an increased number of readout tones per amplifier puts more stringent requirements on the dynamic range to avoid saturation. Here, we characterize the undesired mixing processes between the different frequency-multiplexed tones applied to a Josephson TWPA, a phenomenon also known as intermodulation distortion. The effect becomes particularly significant when the amplifier is operated close to its saturation power. Furthermore, we demonstrate that intermodulation distortion can lead to significant crosstalk and reduction of fidelity for multiplexed readout of superconducting qubits. We suggest using large detunings between the pump and signal frequencies to mitigate crosstalk. Our work provides insights into the limitations of current Josephson TWPAs and highlights the importance of performing further research on these devices.

The effect of laser polarization on the laser wakefield acceleration (LWFA) of electrons has been investigated in the bubble regime, in particular when assisted by ionization injection. By utilizing linear and circular laser polarizations, we discover that circular polarization leads to a dramatic increase in the electron reproducibility rate and also increases the electron charge and beam divergence, while linear polarization yields higher electron peak energy and more stable beam pointing. Our experimental findings are also supported by three-dimensional particle-in-cell simulations. Our study highlights the potential of laser polarization as a simple and effective tool in controlling LWFA and electron-beam properties depending on applications.

Frost damage in porous materials is a weathering mechanism that can cause dangerous rockfalls or damage to built cultural heritage. The volume expansion of 9% when water freezes is usually seen as the cause of frost damage. This does not, however, explain why partially saturated porous stones also show damage despite the fact that ice should have room to grow. By performing experiments both at the scale of a single pore and in a real stone, we propose an explanation for the mechanism of frost damage at low water saturations: the meniscus at an air-water interface confines the water in the pores. Because of this confinement, ice that forms will exert a pressure on the pore walls rather than growing into the pore. The amplitude of stress is found to be larger in small pores and when the meniscus has a larger contact angle with the walls. The contact angle is also observed to increase in the case of multiple freeze-thaw cycles, which increases the likelihood of damage. We find that cracks start first in the ice (being weaker than the confining material), followed by damage in the material itself. Remarkably, when multiple air-water interfaces are induced within limestone samples through a hydrophobic surface treatment, the stones are much more susceptible to frost damage than are uncoated stones, with cracks appearing preferentially at the hydrophilic-hydrophobic interface. This shows that indeed the meniscus confining the water during freezing and consequently the wetting properties are the relevant factors for frost damage in partially saturated porous stones.