We develop a theory for nondegenerate parametric resonance in a tunable superconducting cavity. We focus on nonlinear effects that are caused by nonlinear Josephson elements connected to the cavity. We analyze parametric amplification in a strong nonlinear regime at the parametric-instability threshold, and we calculate maximum gain values. Above the threshold, in the parametric-oscillator regime, the cavity linear response diverges at the oscillator frequency at all pump strengths. We show that this divergence is related to the continuous degeneracy of the free oscillator state with respect to the phase. Applying on-resonance input lifts the degeneracy and removes the divergence. We also investigate quantum noise squeezing. It is shown that in the strong amplification regime, the noise undergoes four-mode squeezing, and that, in this regime, the output signal-to-noise ratio can significantly exceed the input value. We also analyze the intermode frequency conversion and identify the parameters at which full conversion is achieved.

We study the key domain-wall properties in segmented nanowire loop-based structures used in domain-wall-based sensors. The two reasons for device failure, namely, distribution of the domain-wall propagation field (depinning) and the nucleation field are determined with magneto-optical Kerr effect and giant-magnetoresistance (GMR) measurements for thousands of elements to obtain significant statistics. Single layers of ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, a complete GMR stack with ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ as a free layer, and a single layer of ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}$ are deposited and industrially patterned to determine the influence of the shape anisotropy, the magnetocrystalline anisotropy, and the fabrication processes. We show that the propagation field is influenced only slightly by the geometry but significantly by material parameters. Simulations for a realistic wire shape yield a curling-mode type of magnetization configuration close to the nucleation field. Nonetheless, we find that the domain-wall nucleation fields can be described by a typical Stoner-Wohlfarth model related to the measured geometrical parameters of the wires and fitted by considering the process parameters. The GMR effect is subsequently measured in a substantial number of devices (3000) in order to accurately gauge the variation between devices. This measurement scheme reveals a corrected upper limit to the nucleation fields of the sensors that can be exploited for fast characterization of the working elements.

In order to enhance the performance of advanced granular recording media and understand the physics behind the mechanism of the reversal process, an atomistic spin-dynamics simulation is used to investigate theoretically the magnetic properties and the magnetization-reversal behavior for a composite media design. This model allows us to investigate the effect of the magnetostatic interaction and inter- and intralayer exchange coupling for a realistic system. The composite granular medium investigated consists of hard and soft composite layers in which the grains are well segregated with a continuous capping layer deposited to provide uniform exchange coupling. We present a detailed calculation aimed to reveal the reversal mechanism. In particular, the angular dependence of the critical field is investigated to understand the switching process. The calculations show a complex reversal mechanism driven by the magnetostatic interaction. It is also demonstrated, at high sweep rates consistent with the recording process, that thermal effects lead to a significant and irreducible contribution to the switching field distribution.

Quantum-transport measurements in advanced silicon-germanium heterojunction bipolar transistors (SiGe HBTs) are presented and analyzed, including tunneling spectroscopy of discrete impurity levels localized within the transistor and the dependence on an applied magnetic field. The collector current at millikelvin temperatures is well accounted for by ideal electron tunneling throughout the entire base. The amplification principle at millikelvin temperatures is fundamentally quantum mechanical in nature: an increase in base voltage, requiring a moderate base current, creates an equal and opposite decrease in the tunneling barrier seen by the electrons in the emitter, thereby increasing the collector current significantly more than the base current, producing current gain. Highly scaled SiGe HBTs operate predictably at millikelvin temperatures, thus opening the possibility of viable SiGe millikelvin circuitry.

The effects of polytypism on thermoelectric properties of Si and Ge nanowires (NWs) are systematically investigated by using a combination of density functional theory and Boltzmann transport calculations. The calculations for NWs with various polytypes demonstrate that the thermoelectric figure of merit $ZT$ of Si and Ge NWs can be tuned by changing different stacking sequences along the growth direction. In particular, the $ZT$ values of $n$-type Si and Ge NWs with hexagonal ($2H$) structure at 300 K are significantly larger than $ZT$ of Si and Ge NWs with a cubic ($3C$) structure. The underlying mechanism for the enhanced $ZT$ value in Si NWs with a $2H$ structure compared with a $3C$ structure is attributed by the different Seebeck coefficient depending on the structure, while that in Ge NWs is due to the difference in both the Seebeck coefficient and electrical conductivity. The calculated results offer a way to design high-performance thermoelectric materials by controlling the polytype of NWs.

We report on measurements of a subwavelength focusing of an ultrasound beam by a polymer ball-shaped lens (Rexolite) immersed in water at room temperature. The beam arises in the near field of the ball-lens shadow side. Considering a $8.2\lambda$-diameter ball lens, with $\lambda$ being the wavelength, we find a superfocused beam with 14.4-dB-intensity gain, a full width and a full length at half maximum of $\lambda /2$ and $2.2\lambda$, respectively, and sidelobes under $-10\mathrm{dB}$. The observed phenomenon is in excellent agreement with the theoretical and computational predictions based on the partial-wave expansion and finite-element methods. Our results may foster the design of simple lens elements for super-resolution acoustic microscopy and ultrasound imaging.

Josephson parametric amplifiers (JPAs) have become key devices in quantum science and technology with superconducting circuits. In particular, they can be utilized as quantum-limited amplifiers or as a source of squeezed microwave fields. Here, we report on the detailed measurements of five flux-driven JPAs exhibiting a hysteretic dependence of the resonant frequency on the applied magnetic flux. We model the measured characteristics by numerical simulations based on the two-dimensional potential landscape of the dc superconducting quantum interference devices, which provide the JPA nonlinearity for a nonzero screening parameter ${\beta }_L>0$ and demonstrate excellent agreement between the numerical results and the experimental data. Furthermore, we study the nondegenerate response of different JPAs and accurately describe the experimental results with our theory.

Grid-based binary holography (GBH) is an attractive method for patterning with light or matter waves. It is an approximate technique in which different holographic masks can be used to produce similar patterns. Here we present an optimal design method for GBH masks that allows for freely selecting the fraction of open holes in the mask from below 10% to above 90%. Open fraction is an important design parameter when making masks for use in lithography systems. The method also includes a rescaling feature that potentially enables a better contrast of the generated patterns. Through simulations, we investigate the contrast and robustness of the patterns formed by masks generated by the proposed optimal design method. It is demonstrated that high contrast patterns are achievable for a wide range of open fractions. We conclude that reaching a desired open fraction is a trade-off with the contrast of the pattern generated by the mask.

We present evidence of axial banding in rotated, binary granular beds comprising particles of equal size but differing material density. It is demonstrated that the presence of differing particle densities alone may produce limited, localized axial segregation arising due to end-wall effects, but that true axial banding, i.e., axial segregation patterns pervading the full extent of the system may be induced through the use of a rotating tumbler whose internal geometry comprises alternating convex and concave segments. The segregation patterns formed are observed to be stable and reproducible, unlike the unpredictable, metastable banding typically observed in systems containing particles differing in size. Moreover, we demonstrate that, by varying the axial extent and positioning of the individual convex and concave segments, the system geometry may be deliberately tuned in order to directly control both the positions and the widths of the axial bands produced—a finding with significant potential benefits for a range of industrial processes.

We show experimentally how quantum interference can be produced using an integrated quantum system comprising an arch-shaped short quantum wire (or quantum point contact, QPC) of 1D electrons and a reflector forming an electronic cavity. On tuning the coupling between the QPC and the electronic cavity, fine oscillations are observed when the arch QPC is operated in the quasi-1D regime. These oscillations correspond to interference between the 1D states and a state which is similar to the Fabry-Perot state and suppressed by a small transverse magnetic field of $\pm 60\mathrm{mT}$. Tuning the reflector, we find a peak in resistance which follows the behavior expected for a Fano resonance. We suggest that this is an interesting example of a Fano resonance in an open system which corresponds to interference at or near the Ohmic contacts due to a directly propagating, reflected discrete path and the continuum states of the cavity corresponding to multiple scattering. Remarkably, the Fano factor shows an oscillatory behavior taking peaks for each fine oscillation, thus, confirming coupling between the discrete and continuum states. The results indicate that such a simple quantum device can be used as building blocks to create more complex integrated quantum circuits for possible applications ranging from quantum-information processing to realizing the fundamentals of complex quantum systems.

The influence of geometric parameters on the magnetic fine structure of domain walls in bent nanowires is investigated. The domain pattern in the soft-magnetic ${\mathrm{Co}}_{39}{\mathrm{Fe}}_{54}{\mathrm{Si}}_7$ alloy is studied via scanning electron microscopy with polarization analysis and modeled via micromagnetic simulations. It is demonstrated that the bending angle affects details of the microstructure as well as the preponderant domain-wall type. A phenomenological model is developed that provides the global energy minimum of individual types of domain walls as a function of the geometric parameters of the wire. The results can be directly transferred to permalloy wires, as permalloy and ${\mathrm{Co}}_{39}{\mathrm{Fe}}_{54}{\mathrm{Si}}_7$ alloy have a comparable magnetostatic exchange length.

The development of quantum relays for long-haul and attack-proof quantum communication networks operating with weak coherent laser pulses requires entangled photon sources at telecommunication wavelengths with intrinsic single-photon emission for most practical implementations. Using a semiconductor quantum dot emitting entangled photon pairs in the telecommunication O band, we demonstrate a quantum relay fulfilling both of these conditions. The system achieves a maximum fidelity of 94.5% for implementation of a standard four-state protocol with input states generated by a laser. We further investigate robustness against frequency detuning of the narrow-band input and perform process tomography of the teleporter, revealing operation for arbitrary pure input states, with an average gate fidelity of 83.6%. The results highlight the potential of semiconductor light sources for compact and robust quantum-relay technology that is compatible with existing communication infrastructures.

We theoretically investigate a novel and powerful double-pole nanoscale relay based on a carbon nanotube, which is one of the nanoelectromechanical switches being able to work under the strong nuclear radiation, and analyze the physical mechanism of the operating stages in the operation, including “pull in,” “connection,” and “pull back,” as well as the key factors influencing the efficiency of the devices. We explicitly provide the analytical expression of the two important operation voltages, $V_{\text{pull in}}$ and $V_{\text{pull back}}$, therefore clearly showing the dependence of the material properties and geometry of the present devices by the analytical method from basic physics, avoiding complex numerical calculations. Our method is easy to use in preparing the design guide for fabricating the present device and other nanoelectromechanical devices.

The GaN-based $p\text{−}n$ junction rectifications are important in the development of high-power electronics. Here, we demonstrate that $p\text{−}n$ junction rectifying characteristics can be realized with pure $n$-type structures by inserting an (In,Ga)N quantum well into the $\mathrm{GaN}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}/\mathrm{GaN}$ double heterostructures. Unlike the usual barriers, the insertion of an (In,Ga)N quantum well, which has an opposite polarization field to that of the (Al,Ga)N barrier, tailors significantly the energy bands of the system. The lifted energy level of the GaN spacer and the formation of the $(\mathrm{In},\mathrm{Ga})\mathrm{N}/\mathrm{GaN}$ interface barrier can improve the reverse threshold voltage and reduce the forward threshold voltage simultaneously, forming the $p\text{−}n$ junction rectifying characteristics.

We study the effects of electrostatic gating on the magnetization auto-oscillations induced by the local injection of electric current into a ferromagnet–heavy-metal bilayer. We find that the characteristic currents required for the excitation, the intensity, and the spectral characteristics of the generated dynamical states can be tuned by the voltage applied to the metallic gate separated from the bilayer by a thin insulating layer. We show that the effect of electrostatic gating becomes enhanced in the strongly nonlinear oscillation regime at sufficiently large driving currents. Analysis shows that the observed effects are caused by a combination of electric-field-dependent surface anisotropy and electric-field-dependent contribution to the current-induced spin-orbit torques. The demonstrated ability to control the microwave emission and spectral characteristics provides an efficient approach to the development of electrically tunable microwave nano-oscillators.

Superconducting qubits with in situ tunable properties are important for constructing a quantum computer. Qubit tunability, however, often comes at the expense of increased noise sensitivity. Here, we propose a flux-tunable superconducting qubit that minimizes the dephasing due to magnetic flux noise by engineering controllable flux “sweet spots” at frequencies of interest. This is realized by using a SQUID with asymmetric Josephson junctions shunted by a superinductor formed from an array of junctions. Taking into account correlated global and local noises, it is possible to improve dephasing time by several orders of magnitude. The proposed qubit can be used to realize fast, high-fidelity two-qubit gates in large-scale quantum processors, a key ingredient for implementing fault-tolerant quantum computers.

We study a superconducting transmission line (TL) formed by distributed $LC$ oscillators and excited by external magnetic fluxes which are aroused from random magnetization ($A$) placed in substrate or ($B$) distributed at interfaces of a two-wire TL. The low-frequency dynamics of a random magnetic field is described based on the diffusion Langevin equation with a short-range source caused by ($a$) a random amplitude or ($b$) the gradient of magnetization. For a TL modeled as a two-port network with open and shorted ends, the effective magnetic flux at the open end has nonlocal dependency on noise distribution along the TL. The flux-flux correlation function is evaluated and analyzed for the regimes ($Aa$), ($Ab$), ($Ba$), and ($Bb$). Essential frequency dispersion takes place around the inverse diffusion time of random flux along the TL. Typically, noise effect increases with size faster than the area of the TL. The flux-flux correlator can be verified both via the population relaxation rate of the qubit, which is formed by the Josephson junction shunted by the TL with flux noises, and via random voltage at the open end of the TL.

Two-color spatial carrier wave interferometry employing pulsed 532- and 671-nm lasers is used to measure the electron-density and heavy-particle-density profiles in the stagnation point of a stable, axially blown arc in argon for currents of 50 to 200 A and stagnation point pressures of 0.2 to 16 bar. This technique takes advantage of the fact that the free-electron contribution to the refractive index depends strongly on the wavelength, while that of the heavy particles does not. The high spatial resolution achieved allows the hot core of the arc to be readily distinguished from the surrounding boundary layer. A custom-built test device is used to ensure flow conditions that lead to a stable, axisymmetric arc; this permits the reconstruction of the density and temperature profiles using a single projection (interferometric image) of the refractive-index distribution through the arc (at two wavelengths). The arc radius determined from the heavy-particle density decreases with increasing stagnation pressure and increases with the current. These measurements are in good agreement with a simple axially blown arc model taking into account Ohmic heating, radiation losses, and enthalpy flow for core temperatures of approximately 16 500 K. The measured electron density at the center of the arc agrees well with a prediction based on local thermodynamic equilibrium.

While surface acoustic wave (SAW) sensors have been used to measure temperature, pressure, strains, and low magnetic fields, the capability to measure bipolar fields and high fields is lacking. In this paper, we report magnetic surface acoustic wave sensors that consist of interdigital transducers made of a single magnetostrictive material, either Ni or ${\mathrm{TbFe}}_2$, or based on exchange-biased ($\mathrm{Co}/\mathrm{IrMn}$) multilayers. By controlling the ferromagnet magnetic properties, high-field sensors can be obtained with unipolar or bipolar responses. The issue of hysteretic response of the ferromagnetic material is especially addressed, and the control of the magnetic properties ensures the reversible behavior in the SAW response.

We present low-temperature measurements of low-loss superconducting nanowire-embedded resonators in the low-power limit relevant for quantum circuits. The superconducting resonators are embedded with superconducting nanowires with widths down to 20 nm using a neon focused ion beam. In the low-power limit, we demonstrate an internal quality factor up to $3.9\times {10}^5$ at 300 mK [implying a two-level-system-limited quality factor up to $2\times {10}^5$ at 10 mK], not only significantly higher than in similar devices but also matching the state of the art of conventional Josephson-junction-embedded resonators. We also show a high sensitivity of the nanowire to stray infrared photons, which is controllable by suitable precautions to minimize stray photons in the sample environment. Our results suggest that there are excellent prospects for superconducting-nanowire-based quantum circuits.