Spin-transfer torques in a nanocontact to an extended magnetic film can create spin waves that condense to form dissipative droplet solitons. Here we report an experimental study of the temperature dependence of the current and applied field thresholds for droplet soliton formation, as well as the nanocontact’s electrical characteristics associated with droplet dynamics. Nucleation requires lower current densities at lower temperatures, in contrast to typical spin-transfer-torque-induced switching between static magnetic states. Magnetoresistance and electrical noise measurements (10 MHz-1 GHz) show that droplet solitons become more stable at lower temperature. These results are of fundamental interest in understanding the influence of thermal noise on droplet solitons and have implications for the design of devices using the spin-transfer-torque effects to create and control collective spin excitations.

The mechanical and electrical response of acoustophoretic microfluidic devices attached to an ac-voltage-driven piezoelectric transducer is studied by means of numerical simulations. The governing equations are formulated in a variational framework that, introducing Lagrangian and Hamiltonian densities, is used to derive the weak form for the finite-element discretization of the equations and to characterize the device response in terms of frequency-dependent figures of merit or indicators. The effectiveness of the device in focusing microparticles is quantified by two mechanical indicators: the average direction of the pressure gradient and the amount of acoustic energy localized in the microchannel. Furthermore, we derive the relations between the Lagrangian, the Hamiltonian, and three electrical indicators: the resonance $Q$ value, the impedance, and the electric power. The frequency response of the hard-to-measure mechanical indicators is correlated to that of the easy-to-measure electrical indicators, and, by introducing optimality criteria, it is clarified to which extent the latter suffices to identify optimal driving frequencies as the geometric configuration and the material parameters vary. The latter have been varied by considering both Pyrex and aluminium nitroxide top-lid materials.

We present the “trimon,” a multimode superconducting circuit implementing three qubits with all-to-all longitudinal coupling. This always-on interaction enables simple implementation of generalized controlled-not gates which form a universal set. Furthermore, two of the three qubits are protected against Purcell decay while retaining measurability. We demonstrate high-fidelity state swapping operations between two qubits and characterize the coupling of all three qubits to a neighboring transmon qubit. Our results offer a different paradigm for multiqubit architecture with applications in quantum error correction, quantum simulations, and quantum annealing.

The time-domain technique for impedance spectroscopy consists of computing the excitation voltage and current response Fourier images by fast or discrete Fourier transformation and calculating their relation. Here we propose an alternative method for excitation voltage and current response processing for deriving a system impedance spectrum based on a fast and flexible adaptive filtering method. We show the equivalence between the problem of adaptive filter learning and deriving the system impedance spectrum. To be specific, we express the impedance via the adaptive filter weight coefficients. The noise-canceling property of adaptive filtering is also justified. Using the RLC circuit as a model system, we experimentally show that adaptive filtering yields correct admittance spectra and elements ratings in the high-noise conditions when the Fourier-transform technique fails. Providing the additional sensitivity of impedance spectroscopy, adaptive filtering can be applied to otherwise impossible-to-interpret time-domain impedance data. The advantages of adaptive filtering are justified with practical living-cell impedance measurements.

We propose and theoretically study a parity-time ($\mathcal{P}\mathcal{T}$)-symmetric photonic-crystal coupled-resonator optical waveguide (CROW) based on buried heterostructure nanocavities which has potential scalability and controllability. We analytically reveal its spectral transport properties with a tight-binding model and show the possibility of the wide-range control of its group velocity using the $\mathcal{P}\mathcal{T}$ phase transition. While the group velocity at the $\mathcal{P}\mathcal{T}$ phase-transition point diverges, the group-velocity dispersion converges. A numerical estimation of the system response to temporal pulse inputs shows that the pulse broadening is not severe in a device of hundreds of micrometers in size. Furthermore, a longer pulse duration results in a higher upper limit of the pulse peak velocity, which can be, in principle, superluminal. We next perform numerical simulations on the considered photonic-crystal slab structures with the finite-element method, and we successfully observe $\mathcal{P}\mathcal{T}$ phase transitions. In the simulated parameter range, gain and loss coefficients of the order of $100{\mathrm{cm}}^{-1}$ meet the condition for the maximum group-velocity coefficient in the context of the tight-binding approach. A 9.3-fold increase in the group velocity at 1502 nm is obtained in a three-dimensional device by switching between the conventional and $\mathcal{P}\mathcal{T}$-symmetric CROWs. Meanwhile, we also encounter band smoothing around the phase transition, which hampers the group-velocity divergence. Our simulation result indicates that it arises from interfering evanescent waves decaying out of the device structure, and we discuss ways to suppress this effect.

Implementing holonomic quantum computation is a challenging task as it requires complicated interaction among multilevel systems. Here we propose to implement nonadiabatic holonomic quantum computation based on dressed-state qubits in circuit QED. An arbitrary holonomic single-qubit gate can be conveniently achieved using external microwave fields and tuning their amplitudes and phases. Meanwhile, nontrivial two-qubit gates can be implemented in a coupled-cavities scenario assisted by a grounding superconducting quantum-interference device (SQUID) with tunable interaction, where the tuning is achieved by modulating the ac flux threaded through the SQUID. In addition, our proposal is directly scalable, up to a two-dimensional lattice configuration. In the present scheme, the dressed states involve only the lowest two levels of each transmon qubit, and the effective interactions exploited are all of resonant nature. Therefore, we release the main difficulties for physical implementation of holonomic quantum computation on superconducting circuits.

We show that a weak external magnetic field affects significantly nonequilibrium quasiparticle (QP) distributions under the conditions of the inverse proximity effect, using the single-electron hybrid turnstile as a generic example. Inverse proximity suppresses the superconducting gap in superconducting leads in the vicinity of turnstile junctions, thus, trapping hot QPs in this region. An external magnetic field creates additional QP traps in the leads in the form of vortices or regions with a reduced superconducting gap resulting in the release of QPs away from the junctions. We present clear experimental evidence of the interplay of the inverse proximity effect and magnetic field revealing itself in the superconducting gap enhancement and significant improvement of the turnstile characteristics. The observed interplay and its theoretical explanation in the context of QP overheating are important for various superconducting and hybrid nanoelectronic devices, which find applications in quantum computation, photon detection, and quantum metrology.

The speed of quantum gates and measurements is a decisive factor for the overall fidelity of quantum protocols when performed on physical qubits with a finite coherence time. Reducing the time required to distinguish qubit states with high fidelity is, therefore, a critical goal in quantum-information science. The state-of-the-art readout of superconducting qubits is based on the dispersive interaction with a readout resonator. Here, we bring this technique to its current limit and demonstrate how the careful design of system parameters leads to fast and high-fidelity measurements without affecting qubit coherence. We achieve this result by increasing the dispersive-interaction strength, by choosing an optimal linewidth of the readout resonator, by employing a Purcell filter, and by utilizing phase-sensitive parametric amplification. In our experiment, we measure 98.25% readout fidelity in only 48 ns, when minimizing readout time, and 99.2% in 88 ns, when maximizing the fidelity, limited predominantly by the qubit lifetime of $7.6\mu \mathrm{s}$. The presented scheme is also expected to be suitable for integration into a multiplexed readout architecture.

Auto-oscillation of a ferromagnet due to spin-orbit torques in response to a dc current is of wide interest as a flexible mechanism for generating controllable high-frequency magnetic dynamics. However, degeneracies of the spin-wave modes and nonlinear magnon-magnon scattering impede coherent precession. Discretization of the spin-wave modes can reduce this scattering. Spatial localization of the spin-wave modes by the strongly inhomogeneous dipole magnetic field of a nearby spherical micromagnet provides variable spatial confinement, thus offering the option of systematic tunability of magnon spectrum for studying multimode interactions. Here we demonstrate that field localization generates a discrete spin-wave spectrum observable as a series of well-resolved localized modes in the presence of imposed spin currents arising from the spin Hall effect in a permalloy-platinum ($\mathrm{Py}/\mathrm{Pt}$) microstrip. The observation of linewidth reduction through damping control in this micromagnetically engineered spectrum demonstrates that localized modes can be controlled efficiently, an important step toward continuously tunable spin Hall effect–driven auto-oscillators.

An approach to quantum random number generation based on unambiguous quantum state discrimination is developed. We consider a prepare-and-measure protocol, where two nonorthogonal quantum states can be prepared, and a measurement device aims at unambiguously discriminating between them. Because the states are nonorthogonal, this necessarily leads to a minimal rate of inconclusive events whose occurrence must be genuinely random and which provide the randomness source that we exploit. Our protocol is semi-device-independent in the sense that the output entropy can be lower bounded based on experimental data and a few general assumptions about the setup alone. It is also practically relevant, which we demonstrate by realizing a simple optical implementation, achieving rates of $16.5\mathrm{Mbits}/\mathrm{s}$. Combining ease of implementation, a high rate, and a real-time entropy estimation, our protocol represents a promising approach intermediate between fully device-independent protocols and commercial quantum random number generators.

Carbon nanotube quantum dots allow accurate control of electron charge, spin, and valley degrees of freedom in a material which is atomically perfect and can be grown isotopically pure. These properties underlie the unique potential of carbon nanotubes for quantum information processing, but developing nanotube charge, spin, or spin-valley qubits requires efficient readout techniques as well as understanding and extending quantum coherence in these devices. Here, we report on microwave spectroscopy of a carbon nanotube charge qubit in which quantum information is encoded in the spatial position of an electron. We combine radio-frequency reflectometry measurements of the quantum capacitance of the device with microwave manipulation to drive transitions between the qubit states. This approach simplifies charge-state readout and allows us to operate the device at an optimal point where the qubit is first-order insensitive to charge noise. From these measurements, we are able to quantify the degree of charge noise experienced by the qubit and obtain an inhomogeneous charge coherence of 5 ns. We use a chopped microwave signal whose duty-cycle period is varied to measure the decay of the qubit states, yielding a charge relaxation time of 48 ns.

Recent studies reveal that domain walls in magnetic nanostructures can serve as compact, energy-efficient spin-wave waveguides for building magnonic devices that are considered promising candidates for overcoming the challenges and bottlenecks of today’s CMOS technologies. However, imprinting long strip-domain walls into magnetic nanowires remains a challenge, especially in bent geometries. Here, through micromagnetic simulations, we present a method for writing strip-domain walls into bent magnetic nanowires using spin-orbit torque. We employ $\mathsf{Y}$-shaped magnetic nanostructures as well as an $\mathsf{S}$-shaped magnetic nanowire to demonstrate the injection process. In addition, we verify that the $\mathsf{Y}$-shaped nanostructures that incorporate strip-domain walls can function as superb spin-wave multiplexers and that spin-wave propagation along each conduit can be controllably manipulated. This spin-wave multiplexer based on strip-domain walls is expected to become a key signal-processing component in magnon spintronics.

Accessing intrinsic properties of a graphene device can be hindered by the influence of contact electrodes. Here, we capacitively couple graphene devices to superconducting resonant circuits and observe clear changes in the resonance frequency and widths originating from the internal charge dynamics of graphene. This allows us to extract the density of states and charge relaxation resistance in graphene $p\text{−}n$ junctions without the need for electrical contacts. The presented characterization paves a fast, sensitive, and noninvasive measurement of graphene nanocircuits.

Reservoir computing is a bioinspired computing paradigm for processing time-dependent signals. Its hardware implementations have received much attention because of their simplicity and remarkable performance on a series of benchmark tasks. In previous experiments, the output was uncoupled from the system and, in most cases, simply computed off-line on a postprocessing computer. However, numerical investigations have shown that feeding the output back into the reservoir opens the possibility of long-horizon time-series forecasting. Here, we present a photonic reservoir computer with output feedback, and we demonstrate its capacity to generate periodic time series and to emulate chaotic systems. We study in detail the effect of experimental noise on system performance. In the case of chaotic systems, we introduce several metrics, based on standard signal-processing techniques, to evaluate the quality of the emulation. Our work significantly enlarges the range of tasks that can be solved by hardware reservoir computers and, therefore, the range of applications they could potentially tackle. It also raises interesting questions in nonlinear dynamics and chaos theory.

An atomic rubidium beam formed in a 70-mm-long two-dimensional magneto-optical trap (2D MOT), directly loaded from a collimated Knudsen source, is analyzed using laser-induced fluorescence. The longitudinal velocity distribution, the transverse temperature, and the flux of the atomic beam are reported. The equivalent transverse reduced brightness of an ion beam with properties similar to the atomic beam is calculated because the beam is developed to be photoionized and applied in a focused ion beam. In a single two-dimensional magneto-optical trapping step, an equivalent transverse reduced brightness of $({1.0}_{-0.4}^{+0.8})\times {10}^6\mathrm{A}/({\mathrm{m}}^2\mathrm{sr}\mathrm{eV})$ is achieved with a beam flux equivalent to $({0.6}_{-0.2}^{+0.3})\mathrm{nA}$. The temperature of the beam is further reduced with an optical molasses after the 2D MOT. This optical molasses increases the equivalent brightness to $(6_{-2}^{+5})\times {10}^6\mathrm{A}/({\mathrm{m}}^2\mathrm{sr}\mathrm{eV})$. For currents below 10 pA, for which disorder-induced heating can be suppressed, this number is also a good estimate of the ion-beam brightness that can be expected. Such an ion-beam brightness would be a $6\times$ improvement over the liquid-metal ion source and could improve the resolution in focused ion-beam nanofabrication.

In this paper, we investigate the lithiation of pristine amorphous silicon nanoclusters with diameters from 0.8 to 1.4 nm using first-principles molecular dynamics. It is found that this process occurs in two stages. In the first one, Li atoms accumulate at the surface of the nanocluster. A transfer of electrons from Li to the Si atoms takes place, which is accompanied by moderate structural changes. At a given Li coverage threshold that is related to the nanoparticle size, it becomes energetically favorable for any additional Li atoms to penetrate into the nanoparticle. Our results suggest that the driving force for this transition is the increasing electrostatic repulsion between positively charged surface Li ions. Beyond the threshold a second lithiation stage starts where the nanocluster is progressively filled with Li atoms. This leads to significant structural changes, but no sign of fracture is observed during the first lithiation cycle, suggesting that the amorphous silicon cluster remains intact at this size.

${\mathrm{SiO}}_2$, ${\mathrm{P}}_2{\mathrm{O}}_5$, ${\mathrm{B}}_2{\mathrm{O}}_3$, and ${\mathrm{Al}}_2{\mathrm{O}}_3$ are all well-known network formers in glasses, but the structure and properties of mixed ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses without the presence of network modifiers are poorly understood. The relatively low atomic packing density of these glasses should favor network densification when subjected to high local stress (e.g., indentation) at room temperature, and it is therefore interesting to examine their structural response to high-pressure treatment. In the present study, we investigate the pressure-induced changes in volume, structure, and mechanical properties (hardness and crack resistance) of five ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses with varying $\mathrm{Si}:\mathrm{P}$ ratio. The glasses are isostatically compressed at 1 GPa at the glass transition temperature, enabling permanent densification of large (approximately ${\mathrm{cm}}^2$) sample specimens. In the as-prepared glasses, boron atoms become partially converted from the threefold- to the fourfold-coordinated state when $[{\mathrm{P}}_2{\mathrm{O}}_5]>[{\mathrm{Al}}_2{\mathrm{O}}_3]$, with all ${\mathrm{Al}}_2{\mathrm{O}}_3$ maintained in tetrahedral groups. For $[{\mathrm{P}}_2{\mathrm{O}}_5]>([{\mathrm{Al}}_2{\mathrm{O}}_3]+[{\mathrm{B}}_2{\mathrm{O}}_3]$), boron is exclusively found in fourfold coordination, while the aluminum coordination number increases, and all aluminum atoms are preferentially associated with phosphorus as next-nearest-neighbor cations compared to silicon. Upon isostatic compression, the glasses permanently densify up to approximately 6%, leading to an increase in hardness and a change in the indentation cracking pattern. We discuss these pressure-induced changes in glass properties in relation to the structural changes quantified through Raman and ${}^{11}\mathrm{B}$, ${}^{27}\mathrm{Al}$, and ${}^{31}\mathrm{P}$ NMR spectroscopy.

Silicon-vacancy color centers in nanodiamonds are promising as fluorescent labels for biological applications, with a narrow, nonbleaching emission line at 738 nm. Two-photon excitation of this fluorescence offers the possibility of low-background detection at significant tissue depth with high three-dimensional spatial resolution. We measure the two-photon fluorescence cross section of a negatively charged silicon vacancy ($\mathrm{Si}\text{−}{V}^{-}$) in ion-implanted bulk diamond to be $0.74(19)\times {10}^{-50}{\mathrm{cm}}^4\mathrm{s}/\mathrm{photon}$ at an excitation wavelength of 1040 nm. Compared to the diamond nitrogen-vacancy center, the expected detection threshold of a two-photon excited $\mathrm{Si}\text{−}V$ center is more than an order of magnitude lower, largely due to its much narrower linewidth. We also present measurements of two- and three-photon excitation spectra, finding an increase in the two-photon cross section with decreasing wavelength, and we discuss the physical interpretation of the spectra in the context of existing models of the $\mathrm{Si}\text{−}V$ energy-level structure.

The nitrogen-vacancy ($\mathrm{N}\text{−}V$) color center in diamond is an enormously important platform for the development of quantum sensors, including for single-spin and single-molecule NMR. Detection of weak single-spin signals is greatly enhanced by repeated sequences of microwave pulses; in these dynamical-decoupling techniques, the key control parameters swept in the experiment are the time intervals, $\tau$, between pulses. Here, we show that, in fact, the pulse duration $t_p$ offers a powerful additional control parameter. While a non-negligible $t_p$ was previously considered simply a source of experimental error, we elucidate here the underlying quantum dynamics: we identify a landscape of quantum-state crossings which are usually inactive (closed) but may be controllably activated (opened) by adjusting $t_p$ from zero. We identify these crossings with recently observed but unexpected dips (so-called spurious dips) seen in the quantum coherence of the $\mathrm{N}\text{−}V$ spin. With this new understanding, both the position and the strength of these sharp features may be accurately controlled; they coexist with the usual broader coherence dips of short-duration microwave pulses, but their sharpness allows for higher-resolution spectroscopy with quantum diamond sensors, or their analogs.

Two-pulse photon-echo-based quantum-memory applications require a precise control over the echo strength and appearance time. We describe a numerical investigation of observation and control of photon echo in a room-temperature $\mathrm{InAs}/\mathrm{InP}$–based quantum-dot (QD) semiconductor optical amplifier (SOA). We address an important case where the spectral excitation is narrower than the inhomogeneous broadening of the SOA. It is revealed that, in such a QD SOA, the amplitude of the echo pulse depends not only on the excitation-to-rephasing pulse temporal separation but also on the interference among the rephrasing pulse and the echo pulses generated during the propagation along the amplifier. More importantly, the appearance time and amplitude of the echo pulse can be precisely controlled by shaping the first (excitation) pulse. We also assert that deviations in the echo pulse stemming from the SOA gain inhomogeneity can be compensated for so as to be utilized in quantum coherent information processing.