We perform a study of the transport properties of cubic boron nitride at low to high electric fields based on calculations performed by density-functional theory and full-band ensemble Monte Carlo. The full band structure and the carrier-phonon scattering rates are computed from first principles using density-functional theory with a hybrid functional and density-functional perturbation theory, respectively. The results of these calculations are used in an ensemble Monte Carlo simulator to calculate the velocities, energies, and impact ionization coefficients in different crystallographic directions. In conjunction, we have also developed the form-factor parameters to recreate the full band structure based on the empirical pseudopotential method, which is less computationally expensive and more accessible. The results of the transport calculations performed using the two differently computed band structures are compared.

The cryogenic hardware required to build a superconducting-qubit-based quantum computer demands a variety of microwave components. These elements include microwave couplers, filters, amplifiers, and circulators and/or isolators. Traditionally implemented as discrete components, integration of this peripheral hardware, in an effort to reduce overall footprint, thermal load, and added noise, is a key challenge to scaling modern quantum processors with qubit counts climbing over the one hundred plus mark. Ferrite-based microwave isolators, generally employed in the readout chain to decouple qubits and resonators from readout electronics, persist as one of the volumetrically largest devices still utilized as discrete components. Here, we present an alternative two-port isolating integrated circuit derived from the dc superconducting quantum interference device (dc SQUID). Nonreciprocal transmission is achieved using the three-wave microwave-mixing properties of a flux-modulated dc SQUID. We show that when multiple dc SQUIDs are embedded in a multipole admittance-inverting filter structure, the three-wave mixing derived from the flux pumping of the dc SQUIDs can provide directional microwave power flow. For a three-pole filter device, we experimentally demonstrate a directionality greater than 15 dB over a 600-MHz bandwidth.

The proton-boron (pB) reaction has attracted extensive interest for confinement fusion since it produces energetic $\alpha$ particles without neutrons. Based on the pitcher-catcher geometry, a scheme to achieve a high-yielding pB nuclear reaction is proposed, where a meshed catcher target is used. The hot electrons generated by the interaction between laser and pitcher target induce a sheath field in the meshes that accelerates protons in the catcher target to drive a nuclear reaction. Not only is the number of protons increased because the action area of the sheath field becomes larger, but also the energy of protons is well controlled to meet the optimal value by adjusting the size of the mesh based on laser intensity. Two-dimensional particle-in-cell simulations show that our scheme has a 6-times improvement in the yield of nuclear reaction compared with the traditional solid catcher target. With a further increase of laser intensity, the gain of nuclear reaction yield also increases. Our approach provides an idea for the research of laser-driven pB nuclear reaction studies and compact $\alpha$ sources.

Surface acoustic waves (SAWs) provide an efficient dynamical coupling between strain and magnetization in micro- and nanometric systems. Using a hybrid device composed of a piezoelectric, GaAs, and a ferromagnetic Heusler alloy thin film, ${\mathrm{Fe}}_3\mathrm{Si}$, we are able to quantify the amplitude of magnetoacoustic waves generated with SAWs via magnetic imaging in an x-ray photoelectron microscope. The cubic anisotropy of the sample, together with a low damping coefficient, allows for the observation of resonant and nonresonant magnetoelastic coupling. Additionally, via micromagnetic simulation, we verify the experimental behavior and quantify the magnetoelastic shear strain component in ${\mathrm{Fe}}_3\mathrm{Si}$, which appears to be large ($b_2=10\pm 4{\mathrm{MJm}}^{-3}$).

Understanding the nature of active sites and the catalytic properties of oxide-supported bimetallic clusters under reaction conditions remains challenging. In this study, we combine first-principles calculations with genetic algorithm and grand canonical Monte Carlo methods to reveal the structures and compositions of ${\mathrm{Ce}\mathrm{O}}_2$-supported $\mathrm{Cu}$-based bimetallic clusters in an oxygen-rich environment. Oxidized ${\mathrm{Cu}}_4{X}_4$ (X = $\mathrm{Pd}$, $\mathrm{Pt}$, and $\mathrm{Rh}$) bimetallic clusters on ${\mathrm{Ce}\mathrm{O}}_2(111$) are stable and exhibit different catalytic properties during $\mathrm{CO}$ oxidation compared with the pristine bimetallic clusters. Microkinetic simulations predict that ${\mathrm{Ce}\mathrm{O}}_2(111)$-supported ${\mathrm{Cu}}_4{\mathrm{Pd}}_4{\mathrm{O}}_{10}$, ${\mathrm{Cu}}_4{\mathrm{Pt}}_4{\mathrm{O}}_{11}$, and ${\mathrm{Cu}}_4{\mathrm{Rh}}_4{\mathrm{O}}_{14}$ clusters have much higher $\mathrm{CO}$ oxidation activity than the supported ${\mathrm{Cu}}_4{\mathrm{Pd}}_4$, ${\mathrm{Cu}}_4{\mathrm{Pt}}_4$, and ${\mathrm{Cu}}_4{\mathrm{Rh}}_4$ clusters; this is ascribed to the moderate $\mathrm{CO}$ adsorption strength and active oxygen on oxidized alloy clusters. A mechanistic study suggests that $\mathrm{CO}$ oxidation occurs via the ${\mathrm{O}}_2$ associative reaction mechanism on the ${\mathrm{Cu}}_4{\mathrm{Pd}}_4{\mathrm{O}}_{10}$ and ${\mathrm{Cu}}_4{\mathrm{Pt}}_4{\mathrm{O}}_{11}$ clusters, while it proceeds through the ${\mathrm{O}}_2$ dissociative reaction mechanism on the ${\mathrm{Cu}}_4{\mathrm{Rh}}_4{\mathrm{O}}_{14}$ cluster. Our calculations further predict that $\mathrm{CO}$ oxidation on the ${\mathrm{Cu}}_4{\mathrm{Rh}}_4{\mathrm{O}}_{14}$ cluster exhibits a low apparent activation energy, indicating that the oxidized cluster possesses excellent $\mathrm{CO}$ oxidation activity. This work demonstrates that the catalytic activity and reaction mechanism vary with the composition and oxidation state of the alloy nanocluster under the reaction conditions and emphasizes the influence of the reaction atmosphere on the reaction mechanisms and catalytic activity of oxide-supported alloy catalysts.

We study relaxation processes in dipolar-coupled negatively charged nitrogen-vacancy ($\mathrm{N}$-$V^{-}$) centers close to zero field. Specifically, we uncover regimes where flip-flop and double-flip processes, as well as mixing induced by local electric fields, play a significant role in $\mathrm{N}$-$V$–$\mathrm{N}$-$V$ cross relaxation. Our results are relevant for understanding decoherence in many-body spin systems, as well as for high-sensitivity magneto- and electrometry with long-lived interacting solid-state spins. As a proof of principle, we present an orientation and microwave-free magnetometer based on cross relaxation.

Accurate characterization of the single-photon detection efficiency (SPDE) of single-photon detectors (SPDs) is an essential but nontrivial task for various applications. Conventional methods require detailed detector models to calculate the estimated SPDE, which are not always available. To overcome this limitation, we propose a generalized method based on decoy state for accurate characterization of SPDE with experimental demonstration and verification in this work. The decoy-state scheme uses only three weak laser intensities to probe the response of an SPD and calculate rigorous bounds on the background noise and SPDE without prior knowledge of the detector model. To show the versatility, the decoy-state scheme is applied to the single-photon avalanche diode and the homodyne detector. The effects of time-dependent noise and multiphoton response to our scheme are analyzed. The experimental results show that the effects of time-dependent noise can be eliminated, and an accuracy bound of efficiency can be achieved with a bound range of 2.6%. The relaxed assumptions on the detector model, straightforward measurement process, rigorous calculation, and high accuracy, could open possibilities for our generalized toolbox as a device calibration standard, fundamental physics research, quantum information, lidar, and biomedical applications.

This paper solves a long-standing problem in resonant ultrasound spectroscopy (RUS) for cubic solids through the use of two-dimensional deep learning (DL). By means of inverse methods, conventional RUS can determine all independent elastic constants of a crystalline solid specimen by comparing measured and calculated free-vibration resonance frequencies. However, to avoid invalid local minima in the inverse process, good initial guesses of the elastic constants must be available. Here, we propose a DL scheme to remedy this problem, which utilizes an input elasticity image composed of three layers obtained from resonance frequency data. After network training, this scheme is executed in two steps: DL processing by a neural network to output elastic constants in a Blackman diagram classification, followed by the use of a regression network around the classified point in this diagram for more accurate determination. By means of simulations, we demonstrate that this DL scheme yields the principal elastic constants within an error of approximately $5\mathrm{\%}$ without any inverse processing even in the unfavorable case of five missing modes for 111 existing cubic crystals.

In recent years, optical control of mechanical oscillators has emerged as a critical tool for everything from information processing to laser cooling. While traditional forms of optomechanical cooling utilize systems comprising discrete optical and mechanical modes, it has recently been shown that cooling can be achieved in a chip-based system that possesses a continuum of modes. Through Brillouin-mediated phonon-photon interactions, cooling of a band of traveling acoustic waves can occur when anti-Stokes scattered photons exit the system more rapidly than the relaxation rate of the mechanical waves, to a degree determined by the acousto-optic coupling. Here, we demonstrate that a continuum of traveling-wave phonons can be cooled within an optical fiber, extending this physics to macroscopic length scales. Leveraging the large acousto-optic coupling permitted within a liquid-core optical fiber, heterodyne spectroscopy reveals power-dependent changes in spontaneous-Brillouin-scattering spectra that indicate a reduction of the thermal phonon population by 21 K using 120 mW of injected laser power. These results provide alternative ways to manipulate phonon populations that could enable acousto-optic applications with reduced noise or provide ways to control traveling-wave phonons at the quantum level.

Understanding the mechanical behavior of rocks is crucial in subsurface activities, including storage of carbon dioxide and hydrogen gases, which both rely on shale caprocks as potential sealing barriers. Several current large-scale initiatives focus on potential carbon storage in North Sea aquifers. The Draupne Formation contains a series of shale layers interbedded with sandstone layers, the overall thickness of which varies in the range from tens to hundreds of meters. Injection of carbon dioxide into the underlying sandstone reservoirs leads to changes in the surrounding stress field, which can result in fault reactivation or the creation of microfractures, and thus, alter the performance of the shale caprock. Time-resolved microcomputed tomography (4D $\text{μ}$CT) has, in recent years, become a powerful technique for studying the mechanical properties of rocks under stress conditions similar to those prevailing in geological reservoirs. Here, we present results from experiments performed on Draupne shale using a triaxial rig combined with 4D $\text{μ}$CT based on synchrotron radiation. Detailed mechanical analysis of the tomography datasets by digital volume correlation reveals the three-dimensional pattern of the temporally evolving deformation field. Intermittent bursts of deformation at different locations within the specimen are observed, which eventually evolve into a major fracture plane extending laterally across the whole sample. This study suggests that the pseudolinear-elastic-appearing behavior in the macroscopic stress-strain relationship previously reported for Draupne shale samples could consist of a series of irreversible processes occurring at various weak points within the sample. The combination of 4D $\text{μ}$CT imaging with strain analysis enables in situ investigations of deformation processes via quantification of shear and volumetric strains within the sample, thus providing an improved understanding of the fracture dynamics of shales.

Nanowire lasers can be monolithically and site-selectively integrated onto silicon photonic circuits. To assess their full potential for ultrafast optoelectronic devices, a detailed understanding of their lasing dynamics is crucial. However, the roles played by their resonator geometry and the microscopic processes that mediate energy exchange between the photonic, electronic, and phononic subsystems are largely unexplored. Here, we study the dynamics of $\mathrm{Ga}\mathrm{As}$-$\mathrm{Al}\mathrm{Ga}\mathrm{As}$ core-shell nanowire lasers at cryogenic temperatures using a combined experimental and theoretical approach. Our results indicate that these NW lasers exhibit sustained intensity oscillations with frequencies ranging from $160\text{GHz}$ to $260\text{GHz}$. As the underlying physical mechanism, we have identified self-induced electron-hole plasma temperature oscillations resulting from a dynamic competition between photoinduced carrier heating and cooling via phonon scattering. These dynamics are intimately linked to the strong interaction between the lasing mode and the gain material, which arises from the wavelength-scale dimensions of these lasers. We anticipate that our results could lead to optimised approaches for ultrafast intensity and phase modulation of chip-integrated semiconductor lasers at the nanoscale.

Variational algorithms have received significant attention in recent years due to their potential to solve practical problems using noisy intermediate-scale quantum (NISQ) devices. A fundamental step of these algorithms is the evaluation of the expected value of Hamiltonians, and hence efficient schemes to perform this task are required. The standard approach employs local measurements of Pauli operators and requires a large number of circuits. An alternative is to use entangled measurements, which might introduce additional gates between physically disconnected qubits that harm the performance. To solve this problem, we propose hardware-efficient entangled measurements (HEEMs), that is, measurements that permit only entanglement between physically connected qubits. We show that this strategy enhances the evaluation of molecular Hamiltonians in NISQ devices by reducing the required circuits without increasing their depth. We provide quantitative metrics of how this approach offers better results than local measurements and arbitrarily entangled measurements. We estimate the ground-state energy of the ${\mathrm{H}}_2\mathrm{O}$ molecule with classical simulators and quantum hardware using the variational quantum eigensolver with HEEM.

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.