We present the results of experiments on the generation of periodical trains of subnanosecond $K_a$-band pulses in the electron generator, which is based on passive mode locking. This technique is broadly used in laser physics for the generation of ultrashort pulses (USPs) but has not been previously employed in high-power microwave electronics. The microwave USP generator comprises a helical-waveguide gyrotron traveling wave tube and a saturable absorber in the feedback loop. Saturable absorption is implemented in an auxiliary section using the cyclotron resonance interaction with an initially rectilinear electron beam, for which nonlinear saturation is caused by the effect of the relativistic dependence of the gyrofrequency on the particle’s energy. In agreement with theoretical predictions, periodical trains of 0.4-ns pulses with a peak power of 100 kW and repetition of 2.5 ns are measured. Phase coherence of radiated pulses is demonstrated based on analysis of the autocorrelation function. The experiments performed open a wide range of possibilities for the generation of coherent broadband radiation, which is in high demand for numerous applications.

We can control the concentration of chemicals in microdroplets by using vertical contact control of droplets. When a microdroplet contacts an opposite microdroplet, chemicals diffuse between the droplets. Then, the coalescent droplet is separated into two microdroplets. Contact control is attractive for use in biochemical applications because it realizes three-dimensional microfluidic systems. However, in vertical contact control, gravity causes a volume difference of the microdroplets, ΔV, after separation. To solve the problem of gravity, we analyze ΔV quantitatively. In experiments, we measure the relationship between ΔV and the size of microdroplets, R. As a result, we find that ΔV increases proportionally to $R^5$, which agrees with a simple theoretical model based on the balance between gravity and surface tension. Our model suggests that ΔV of the microdroplets after vertical contact control can be predicted quantitatively with the material parameters of the density and surface tension of the liquid.

Highly efficient separation of tightly bound excitons (photoinduced electron-hole pairs) at an interface is crucial for high-performance two-dimensional (2D) heterostructure solar cells. Here we propose and demonstrate a strategy—dipole engineering—to effectively separate excitons at a 2D interface, which could evidently enhance power-conversion efficiency (PCE) via Heyd-Scuseria-Ernzerhof density functional calculations. The proposed dipole engineering is based on the synergetic effect of the intrinsic dipole in 2D Janus materials and interface dipole caused by charge transfer at the interface of 2D van der Waals heterostructures, in stark contrast to those strategies previously used, such as applying a strain or electric field. Taking the Janus ${\mathrm{Ga}}_2\mathrm{Se}\mathrm{Te}/\mathrm{In}\mathrm{S}$ heterostructure as an example, we demonstrate that the PCE can be promoted to a peak value of 23.2% when the intrinsic and interface dipoles are in the same direction, when accompanied by an appropriate applied compression strain. In addition, the Janus heterostructure possesses suitable band-edge positions of redox reaction potentials for full water splitting. Our work may herald an intrinsic yet effective way for enhancing the performance of optoelectronic devices, in contrast to the conventional external stimuli strategy.

Fixed-frequency qubits can suffer from always-on interactions that inhibit independent control. Here, we address this issue by experimentally demonstrating a superconducting architecture using qubits that comprise of two capacitively shunted Josephson junctions connected in series. Historically known as tunable coupling qubits (TCQs), such two-junction qubits support two modes with distinct frequencies and spatial symmetries. By selectively coupling only one type of mode and using the other as our computational basis, we greatly suppress crosstalk between the data modes while permitting all-microwave two-qubit gates.

We report on the impact of structural defects on mid-infrared intersubband (ISB) properties of $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ heterostructures grown by ammonia molecular beam epitaxy (${\mathrm{NH}}_3$ MBE). Twenty-period $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ multi-quantum-well (MQW) heterostructures are grown on co-loaded a-plane freestanding $\mathrm{Ga}\mathrm{N}$ substrates and heteroepitaxial a-plane $\mathrm{Ga}\mathrm{N}$ on r-plane sapphire templates (a-$\mathrm{Ga}\mathrm{N}$/r-sap) for three different quantum-well (QW) widths (3.0, 3.3, and 3.7 nm). Co-loaded structures grown on freestanding a-plane with no basal-plane stacking faults (BSFs), prismatic stacking faults (PSFs), and partial dislocations (PDs), with low threading dislocation (TD) densities of about ${10}^5{\mathrm{cm}}^{-2}$ are compared with those grown on a-$\mathrm{Ga}\mathrm{N}$ templates on $(10\overline{1}2)$ r-sapphire with BSF, PSF, PD, and TD densities of about 4 × ${10}^5$ to ${10}^6{\mathrm{cm}}^{-2}$, 5 × ${10}^3$ to 2 × ${10}^4{\mathrm{cm}}^{-2}$, about 9 × ${10}^{10}$ to 2 × ${10}^{11}{\mathrm{cm}}^{-2}$, and about ${10}^{10}{\mathrm{cm}}^{-2}$, respectively. Fourier-transform infrared absorption spectroscopy indicates ISB transition energies in the range of about 250–300 meV (wavelength range 4.1–4.8 µm) for MQWs with different QW widths. The ISB absorption spectra indicate about 5% smaller transition energies and only about 10%–20% larger spectral linewidths for structures grown on a-$\mathrm{Ga}\mathrm{N}/$r-sapphire templates compared with those on freestanding $\mathrm{Ga}\mathrm{N}$ substrates. The strong defect tolerance in the nonpolar a-plane ISB structures could be due to the nature of defects and their energy levels with respect to the conduction-band minima, which do not affect the ISB properties. Our results pave the way toward the production of low-cost scalable nonpolar III-nitride MQW heterostructures for a variety of passive and active optical materials and devices based on intersubband transitions.

Classical simulations of time-dependent quantum systems are widely used in quantum control research. In particular, these simulations are commonly used to host iterative optimal control algorithms. This is convenient for algorithms that are too onerous to run in the loop with current-day quantum hardware, as well as for researchers without consistent access to hardware. However, if the model used to represent the system is not selected carefully, an optimized control protocol may be rendered futile when applied to hardware. We present a series of models, ordered in a hierarchy of progressive approximation, which appear in quantum control literature. The validity of each model is characterized experimentally by designing and benchmarking control protocols for an IBMQ cloud quantum device. This result demonstrates error amplification induced by the application of a first-order perturbative approximation. Furthermore, the emergence of errors that cannot be corrected by simple amplitude scaling of control pulses is demonstrated in simulation, due to an underlying mistreatment of noncomputational dynamics. Finally, an evaluation of simulated control dynamics reveals that despite the substantial variance in numerical predictions across the proposed models, the complexity of discovering local optimal control protocols appears invariant in the simple control scheme setting.

Recently, space-time plasma-steering source has been proposed as an innovative microwave plasma source to meet the challenge of controlling plasmas in overmoded cavities. This concept has been successfully demonstrated experimentally, allowing the space-time control of nanosecond microwave plasmas on initiators. This paper gives insights into the path that shall be taken to reach full space-time control of plasmas in overmoded cavities. To that end, a key criterion, namely the “plasma-steering criterion,” is introduced and verified with a numerical model. This criterion must be respected for an accurate space-time control of plasmas in overmoded cavities. The usefulness of the plasma density (depending on its value with respect to the critical plasma density) on the plasma-control capabilities is also highlighted.

Scanning tunneling microscope lithography can be used to create nanoelectronic devices in which dopant atoms are precisely positioned in a $\mathrm{Si}$ lattice within approximately $1$ nm of a target position. This exquisite precision is promising for realizing various quantum technologies. However, a potentially impactful form of disorder is due to incorporation kinetics, in which the number of P atoms that incorporate into a single lithographic window is manifestly uncertain. We present experimental results indicating that the likelihood of incorporating into an ideally written three-dimer single-donor window is $63\pm 10\mathrm{\%}$ for room-temperature dosing, and corroborate these results with a model for the incorporation kinetics. Nevertheless, further analysis of this model suggests conditions that might raise the incorporation rate to near-deterministic levels. We simulate bias spectroscopy on a chain of comparable dimensions to the array in our yield study, indicating that such an experiment may help confirm the inferred incorporation rate.

Continuous-time quantum walks (CTQWs) have shown the capability to perform efficiently the spatial search of a marked site on many kinds of graphs. However, most of such graphs are hard to realize in an experimental setting. Here we study CTQW spatial search on a planar triangular lattice by means of both numerical simulations and experiments. The experiments are performed using two-dimensional waveguide arrays fabricated by femtosecond laser pulses, illuminated by coherent light. We show that the retrieval of the marked site by the quantum walker is accomplished with higher probability than the classical counterpart, in a convenient time window placed early in the evolution.

The preparation of a quantum Gibbs state is an essential part of quantum computation and has wide-ranging applications in various areas, including quantum simulation, quantum optimization, and quantum machine learning. In this paper, we propose variational hybrid quantum-classical algorithms for quantum Gibbs state preparation. We first utilize a truncated Taylor series to evaluate the free energy and choose the truncated free energy as the loss function. Our protocol then trains the parameterized quantum circuits to learn the desired quantum Gibbs state. Notably, this algorithm can be implemented on near-term quantum computers equipped with parameterized quantum circuits. By performing numerical experiments, we show that shallow parameterized circuits with only one additional qubit can be trained to prepare the Ising chain and spin chain Gibbs states with a fidelity higher than 95%. In particular, for the Ising chain model, we find that a simplified circuit ansatz with only one parameter and one additional qubit can be trained to realize a 99% fidelity in Gibbs state preparation at inverse temperatures larger than $2$.

Hole spins in semiconductor quantum dots represent a viable route for the implementation of electrically controlled qubits. In particular, the qubit implementation based on $\mathrm{Si}$ $p$-MOSFETs offers great potentialities in terms of integration with the control electronics and long-term scalability. Moreover, the future down scaling of these devices will possibly improve the performance of both the classical (control) and quantum components of such monolithically integrated circuits. Here, we use a multiscale approach to simulate a hole-spin qubit in a down-scaled $\mathrm{Si}$-channel $p$-MOSFET, the structure of which is based on a commercial 22-nm fully depleted silicon-on-insulator device. Our calculations show the formation of well-defined hole quantum dots within the $\mathrm{Si}$ channel and the possibility of a general electrical control, with Rabi frequencies of the order of $100\mathrm{MHz}$ for realistic field values. A crucial role of the channel aspect ratio is also demonstrated, as well as the presence of a favorable parameter range for the qubit manipulation.

Knowledge of the spin-wave dispersion relation is a prerequisite for the explanation of many magnonic phenomena as well as for the practical design of magnonic devices. Spin-wave dispersion measurement by established optical techniques such as Brillouin light scattering or the magneto-optical Kerr effect at ultralow temperatures is often forbiddingly complicated. By contrast, microwave spectroscopy can be used at all temperatures but it usually lacks spatial and wave-number resolution. Here we develop a variable-gap-propagating-spin-wave-spectroscopy (VGPSWS) method for the deduction of the dispersion relation of spin waves in a wide frequency and wave-number range. The method is based on the phase-resolved analysis of the spin-wave transmission between two antennas with variable spacing, in conjunction with theoretical data treatment. We validate the method for in-plane magnetized $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$ and yttrium iron garnet thin films in $\mathbf{k}\perp \mathbf{B}$ and $\mathbf{k}\parallel \mathbf{B}$ geometries by deducing the full set of material and spin-wave parameters, including spin-wave dispersion, hybridization of the fundamental mode with the higher-order perpendicular standing spin-wave modes, and surface spin pinning. The compatibility of microwaves with low temperatures makes this approach attractive for cryogenic magnonics at the nanoscale.

Dirac plasmons in a two-dimensional (2D) crystal are strongly affected by the dielectric properties of the environment, due to interaction of their electric field lines with the surrounding medium. Using graphene as a 2D reservoir of free carriers, one can engineer a material configuration that provides an anisotropic environment to the plasmons. In this work, we discuss the physical properties of Dirac plasmons in graphene surrounded by an arbitrary anisotropic dielectric and exemplify how $h$-$\mathrm{BN}$–based heterostructures can be designed to bear the required anisotropic characteristics. We calculate how dielectric anisotropy impacts the spatial propagation of the plasmons and find that an anisotropy-induced plasmon mode emerges, together with a damping pathway, that stem from the out-of-plane off-diagonal elements in the dielectric tensor. Furthermore, we find that one can create hyperbolic plasmons by inheriting the dielectric hyperbolicity of the designed material environment. Strong control over plasmon propagation patterns can be realized in a similar manner. Finally, we show that in this way one can also control the polarization of the light-matter excitations that constitute the plasmon. Taken together, our results promote the design of the dielectric environment as an effective path to tailor the plasmonic response of graphene on the nanoscopic level.

We demonstrate the dissipative spatiotemporal patterns that are generated by an active ring resonator based on a magnonic quasicrystal (MQC) with Fibonacci-type structure. The dissipative patterns are formed through the magnetostatic surface spin-wave (MSSW) parametric decay into exchange spin waves (SWs), temporal dispersion of the ring resonator and amplification. In the spatial domain, the MSSW and SW parametric patterns measured with the help of Brillouin light spectroscopy have the quasiperiodic spatial localization in crests and grooves of the MQC, respectively. In contrast to the optical quasiperiodic parametric spatial conservative solitons, the amplitude profiles of the magnonic quasiperiodic parametric spatial dissipative patterns correspond to the profiles of the crests and grooves of the MQC. In the time domain, the packets of the chaotic parametric pulses, which are analogs of temporal solitons, are generated at each point of the spatial patterns. The chaotic nature of such pulses is confirmed by an estimate of a highest Lyapunov exponent from the experimental time series. The pulse packets are formed through the time-filtering technique using the external microwave (MW) pulses to control a ring gain. It is shown that at a certain duty factor of the external MW pulses, the parametric MSSW pulses have the amplitude and phase profiles corresponding to the profiles of a conservative bright envelope soliton.

We investigate carrier transport in gallium-free strained-balanced $\mathrm{In}\mathrm{As}/{\mathrm{In}}_x{\mathrm{As}}_{1-x}\mathrm{Sb}$ type-II superlattices in the presence of positional and compositional disorder. We use a rigorous nonequilibrium Green’s function model based on fully nonlocal scattering self-energies computed in the self-consistent Born approximation and a multiband description of the electronic structure. Layer-thickness fluctuations, nonuniform antimony composition, and segregation throughout the superlattice stack lead to as-grown disordered structures that are quite different from the abrupt interface ideal superlattices. We find that regardless of its nature and cause, disorder significantly affects vertical-carrier-transport properties, by impeding the coherent propagation of carriers in the minibands. In particular, the minority-carrier hole mobility is fundamentally limited by the nonideal properties of the superlattice, namely the layer-thickness fluctuation and the nonuniform antimony distribution. Furthermore, upon reducing the temperature, holes become fully localized and transport occurs by hopping, which explains published measured detector data that demonstrates the quantum efficiency, exhibiting a very strong temperature dependence that degrades as the temperature is reduced. As a result, photodetectors that employ holes as minority carriers will be limited in performance, especially for long-wavelength infrared applications at low temperature. However, we find that minority-carrier electron mobility is largely unaffected by disorder, indicating the $p$-type absorbing layer as the preferred option.