It has been shown that transformation elastics or illusion elastics is challenging as the elastodynamic wave equation will change its form after coordinate transformations. By using transformation acoustics based on Fabry-Perot (FP) resonances, we obtain an illusion such that an arbitrary solid object sounds like another solid object in a fluid background. We analytically confirm this illusion elastic effect from Mie theory for both two dimensions and three dimensions. This method is important in underwater acoustics communications.

Using a previously-well-tested numerical model, we demonstrate that good acoustophoresis can be obtained in a microchannel embedded in an acoustically soft, all-polymer chip by excitation of whole-system ultrasound resonances. In contrast to conventional techniques based on a standing bulk acoustic wave inside a liquid-filled microchannel embedded in an elastic, acoustically hard material, such as glass or silicon, the proposed whole-system resonance does not need high acoustic contrast between the liquid and the surrounding solid. Instead, it relies on the very high acoustic contrast between the solid and the surrounding air. In microchannels of usual dimensions, we demonstrate by numerical simulations the existence of whole-system resonances in an all-polymer device that supports acoustophoresis of a quality fully comparable to that of a conventional hard-walled system. Our results open up the possibility of using cheap and easily processable polymers in a controlled manner to design and fabricate microfluidic devices for single-use acoustophoresis.

We study the effects of pulse shape on the write-error rate (WER) of perpendicularly magnetized magnetic tunnel junctions driven by subnanosecond voltage pulses. The WER almost monotonically increases as the rise time of the write pulse is increased. On the other hand, a small fall time is found to reduce the WER. These experimental results are reproduced well by numerical simulations based on macrospin approximation. The numerical simulations reveal that a proper fall time cancels magnetization drift due to intrinsic damping and that this contributes to the WER reduction. We demonstrate that the magnetization dynamics can be precisely manipulated by a controlled voltage-pulse shape and also show the importance of controlling the pulse shape for achieving a low WER.

The Loewner equation (LE) is used to obtain conformal mappings that lead to exact and analytical expressions for several electrostatic properties of realistic quasi-unidimensional nanoelectromechanical systems. The LE approach also embraces curved geometries, which are impossible to address by traditional methods such as the Schwarz-Christoffel transformation, which is often used in this scenario. Among the possible applications of the formalism, we show that it allows an exact evaluation of the field enhancement factor (FEF) in a region close to the apex of different emitters. Despite its key role in the demodulation process for radio-receiver nanodevices, actual FEF values have been mostly obtained by numerical and/or phenomenological approaches. This work extends the already huge universe of applications of the LE and provides an analytical method to evaluate the FEF, even for curved emitters. Furthermore, our results provide a signature of the varying emitted-current’s response due to the nanostructure oscillation, justifying its role in the demodulation process in radio-frequency nanoelectromechanical systems.

The manipulation of magnetism and valley has vital applications in multifunctional spintronic and valleytronic devices. Coupling the valley degree of freedom of a two-dimensional (2D) magnet to the ferroelectricity of multiferroic materials offers exciting opportunities to achieve challenging electrically controlled valleytronics. Meanwhile, the magnetic behavior of 2D materials can also be tailored for magnetoelectric coupling. The proposed valley-ferroelectricity coupling is predicted in monolayer ${\mathrm{Mn}\mathrm{P}\mathrm{Se}}_3$ on top of a ${\mathrm{Y}\mathrm{Mn}\mathrm{O}}_3$ (001) substrate. The results show that the magnitude of valley polarization and the conductivity of monolayer ${\mathrm{Mn}\mathrm{P}\mathrm{Se}}_3$ are closely correlated with ${\mathrm{Y}\mathrm{Mn}\mathrm{O}}_3$ terminations. The valley polarization modulations in $\mathrm{MnO}$-terminated heterostructures can reach 46 meV when the ferroelectric polarization is reversed. Furthermore, owing to the exchange interaction between ${\mathrm{Mn}\mathrm{P}\mathrm{Se}}_3$ and the topmost surface atoms, the magnetism of monolayer ${\mathrm{Mn}\mathrm{P}\mathrm{Se}}_3$ in ${\mathrm{O}}_2$- and $\mathrm{MnO}$-terminated models can be effectively modulated with polarization switching. The electrical control of magnetic behavior and valley polarization in ${\mathrm{Mn}\mathrm{P}\mathrm{Se}}_3\text{−}{\mathrm{Y}\mathrm{Mn}\mathrm{O}}_3$ paves the way for developing nonvolatile data memories, where the magnetization and valley pseudospin are both involved in the memory elements.

Large-scale micromagnetics simulations based on the Landau-Lifshitz-Gilbert equation are performed to clarify the initial magnetization process of a ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ hot-deformed permanent magnet. A simulation model of the permanent magnet having 5302 grains is capable of elucidating the effects of networks among the grains. The calculated initial magnetization curve of the permanent magnet reproduces the two-step structure found in experiments. In the demagnetized state obtained from random magnetic configuration, multi- and single-magnetic-domain structures are formed in tabular grains that constitute the hot-deformed permanent magnet. The external magnetic field displaces the domain wall during the first step in the initial magnetization curve, and the multidomain grains disappear after the first step. In the second step, the magnetization reversal of the single domain occurs in grains with magnetization opposite the external field, and the magnetization reversal tends to occur in the grains with the easy axis tilted from the bulk easy magnetization axis. The average magnetic switching field of these single-domain grains is slightly higher than that of the bulk hot-deformed permanent magnet, and the domain-wall displacement across the grain boundary reduces the coercivity by 3.57 kOe in our simulation model. This discrepancy enables us to estimate the effects of networks among the grains through the dipolar interaction.

High-fidelity gate operations are essential to the realization of a fault-tolerant quantum computer. In addition, the physical resources required to implement gates must scale efficiently with system size. A longstanding goal of the superconducting qubit community is the tight integration of a superconducting quantum circuit with a proximal classical cryogenic control system. Here we implement coherent control of a superconducting transmon qubit using a single-flux quantum (SFQ) pulse driver cofabricated on the qubit chip. The pulse driver delivers trains of quantized flux pulses to the qubit through a weak capacitive coupling; coherent rotations of the qubit state are realized when the pulse-to-pulse timing is matched to a multiple of the qubit oscillation period. We measure the fidelity of SFQ-based gates to be approximately 95% using interleaved randomized benchmarking. Gate fidelities are limited by quasiparticle generation in the dissipative SFQ driver. We characterize the dissipative and dispersive contributions of the quasiparticle admittance and discuss mitigation strategies to suppress quasiparticle poisoning. These results open the door to integration of large-scale superconducting qubit arrays with SFQ control elements for low-latency feedback and stabilization.

Trapped-ion optical clocks are capable of achieving systematic fractional frequency uncertainties of ${10}^{-18}$ and possibly below. However, the stability of current ion clocks is fundamentally limited by the weak signal of single-ion interrogation. We present an operational, scalable platform for extending clock spectroscopy to arrays of Coulomb crystals consisting of several tens of ions while allowing systematic shifts as low as ${10}^{-19}$. We observe three-dimensional excess micromotion amplitudes inside a Coulomb crystal with atomic spatial resolution and subnanometer amplitude uncertainties, and show that in ion Coulomb crystals of length $400\mu \text{m}$ and 2 mm, time-dilation shifts of ${\mathrm{In}}^{+}$ ions due to micromotion can be close to $1\times {10}^{-19}$ and below ${10}^{-18}$, respectively. In previous ion traps, excess micromotion would have dominated the uncertainty budget for spectroscopy of even a few ions. By minimizing its contribution and providing a means to quantify it, we open up a path to precision spectroscopy in many-body ion systems, enabling entanglement-enhanced ion clocks and providing a well-controlled, strongly coupled quantum system.

We examine the symmetry of the pressure and the velocity fields near the acoustic resonators with front-and-back mirror-symmetry structures excited by a single incident wave, and show that there are two types of acoustic monopoles, namely the pressure monopoles (PMs) and the velocity monopoles (VMs). Under one-wave incidence, only the pressure field near a PM at resonance possesses monopolar symmetry, while for a VM at resonance, only the velocity field possesses monopolar symmetry. Counterintuitively, it is the dipolar response function of the monopoles that dictates whether they are PMs or VMs. A hybrid monopolar device with neither velocity nor pressure field symmetry is also demonstrated by combining a VM and a PM in parallel, which exhibits several resonances with neither PM nor VM symmetry, even though it has mirror structural symmetry. The breaking of monopolar symmetry could lead to higher absorption of the subwavelength scale devices.

We present a general theory of exceptional points of degeneracy (EPD) in periodically time-variant systems. We show that even a single resonator with a time-periodic component is able to develop EPDs, contrary to parity-time- (PT) symmetric systems that require two coupled resonators. An EPD is a special point in a system parameter space at which two or more eigenmodes coalesce in both their eigenvalues and eigenvectors into a single degenerate eigenmode. We demonstrate the conditions for EPDs to exist when they are directly induced by time-periodic variation of a system without loss and gain elements. We also show that a single resonator system with zero time-average loss-gain exhibits EPDs with purely real resonance frequencies, yet the resonator energy grows algebraically in time since energy is injected into the system from the time-variation mechanism. Although the introduced concept and formalism are general for any time-periodic system, here, we focus on the occurrence of EPDs in a single LC resonator with time-periodic modulation. These findings have significant importance in various electromagnetic and photonic systems and pave the way for many applications, such as sensors, amplifiers, and modulators. We show a potential application of this time-varying EPD as a highly sensitive sensor.

Hybrid superconducting-spin systems offer the potential to combine highly coherent atomic quantum systems with the scalability of superconducting circuits. To fully exploit this potential requires a high-quality-factor microwave resonator, tunable in frequency and able to operate at magnetic fields optimal for the spin system. Such magnetic fields typically rule out conventional $\mathrm{Al}$-based Josephson-junction devices that have previously been used for tunable high-$Q$ microwave resonators. The larger critical field of $\mathrm{Nb}$ allows microwave resonators with large field resilience to be fabricated. Here we demonstrate how constriction-type weak links, patterned in parallel into the central conductor of a $\mathrm{Nb}$ coplanar resonator with a neon focused ion beam, can be used to implement a frequency-tunable resonator. We study transmission through two such devices and show how they realize high-quality-factor, tunable, field-resilient devices that hold promise for future applications coupling to spin systems.

By direct numerical solution of Maxwell’s equations and the semiconductor drift-diffusion equations, we demonstrate solar-power conversion efficiencies in the 29%–30% range in crystalline-silicon photonic-crystal solar cells. These 3–12-$\mu \mathrm{m}$-thick photonic crystals, consisting of wavelength-scale inverted-pyramid arrays, absorb sunlight considerably in excess of the Lambertian limit due to wave-interference-based light trapping. It is suggested that the resulting optimized balance between bulk Auger recombination and solar absorption occurs for a silicon thickness of 10 $\mu \mathrm{m}$, in contrast to previous estimates of more than 100 $\mu \mathrm{m}$. Optimized $n^{+}p{p}^{+}$ doping profiles involving low $p$ doping throughout most of the bulk and thin $n^{+}$ and $p^{+}$ doping at the emitter and base regions yield an ideal electronic response. For solar absorption restricted to the 300–1100-nm range, we obtain a short-circuit current of 42.5 ${\mathrm{mA}/\mathrm{cm}}^2$, an open-circuit voltage of nearly 0.8 V, and a fill factor of about 87%, provided that the surface recombination velocities are less than 100 cm/s. With inclusion of solar absorption in the 1100–1200-nm range through electronic-band-gap narrowing and the Urbach optical absorption edge, our wave-interference-based light trapping enables an additional photocurrent density of 1.09 ${\mathrm{mA}/\mathrm{cm}}^2$ for an overall power conversion efficiency of $30\mathrm{\%}$. Under solar concentration by factors of $20$ and $150$, the power conversion efficiencies are increased to $32.5\mathrm{\%}$ and $33.5\mathrm{\%}$, respectively.

Magnetic skyrmions are promising information carriers for building future high-density and high-speed spintronic devices. However, to achieve a current-driven high-speed skyrmion motion, the required driving-current density is usually very large, which could be energy inefficient and even destroy the device due to Joule heating. The mechanism of voltage-driven skyrmion motion through equidistant identical electrodes on a uniformly thick nanowire is studied. The high-speed skyrmion motion is realized by utilizing a voltage shift, and the average skyrmion velocity reaches up to 259 m/s under a 0.45-V applied voltage. In comparison with the widely studied vertical current-driven model, the energy dissipation is three orders of magnitude lower in our voltage-driven model for the same speed motion of skyrmions. Our approach uncovers valuable opportunities for building skyrmion racetrack memories and logic devices with both ultra-low power consumption and ultra-high processing speed, which are appealing features for future spintronic applications.

Theoretical predictions indicate that the n-type $\mathrm{Ge}/\mathrm{Si}\text{−}\mathrm{Ge}$ multi-quantum-well system is the most promising material for the realization of a $\mathrm{Si}$-compatible THz quantum cascade laser operating at room temperature. To advance in this direction, we study, both experimentally and theoretically, asymmetric coupled multi-quantum-well samples based on this material system, that can be considered as the basic building block of a cascade architecture. Extensive structural characterization shows the high material quality of strain-symmetrized structures grown by chemical vapor deposition, down to the ultrathin barrier limit. Moreover, THz absorption spectroscopy measurements supported by theoretical modeling unambiguously demonstrate inter-well coupling and wavefunction tunneling. The agreement between experimental data and simulations allows us to characterize the tunneling barrier parameters and, in turn, achieve highly controlled engineering of the electronic structure in forthcoming unipolar cascade systems based on n-type $\mathrm{Ge}/\mathrm{Si}\text{−}\mathrm{Ge}$ multi-quantum-wells.

The increasing need to store large amounts of information with an ultradense, reliable, low-power, and low-cost memory device is driving aggressive efforts to improve on current perpendicular magnetic recording technology. However, the difficulties in fabricating small-grain recording media while maintaining thermal stability and a high signal-to-noise ratio motivate the development of alternative methods, such as the patterning of magnetic nanoislands and use of energy assistance for future applications. In addition, from both a sensor perspective and a memory perspective, three-dimensional spintronic devices are highly desirable to overcome the restrictions on the functionality in planar structures. Here we demonstrate a three-dimensional magnetic memory (magnetic page memory) based on thermally assisted and stray-field-induced transfer of domains in a vertical stack of magnetic nanowires with perpendicular anisotropy. Use of spin-torque-induced domain multiplication in such a device with periodic pinning sites provides additional degrees of freedom by allowing lateral information flow to realize truly-three-dimensional integration.

We report on the challenges and limitations of direct coupling of the magnetic field from a circuit resonator to an electron spin bound to a donor potential. We propose a device consisting of a trilayer lumped-element superconducting resonator and a single donor implanted in enriched ${}^{28}\mathrm{Si}$. The resonator impedance is significantly smaller than the practically achievable limit obtained with prevalent coplanar resonators. Furthermore, the resonator includes a nanoscale spiral inductor to spatially focus the magnetic field from the photons at the location of the implanted donor. The design promises an increase of approximately 2 orders of magnitude in the local magnetic field, and thus the spin-to-photon coupling rate $g$, compared with the estimated rate of coupling to the magnetic field of coplanar transmission line resonators. We show that by use of niobium (aluminum) as the resonator’s superconductor and a single phosphorous (bismuth) atom as the donor, a coupling rate of $g/2\pi =0.24$ MHz (0.39 MHz) can be achieved in the single-photon regime. For this hybrid cavity-quantum-electrodynamic system, such enhancement in $g$ is sufficient to enter the strong-coupling regime.

We investigate the temperature dependence of the magnetic damping in exchange-biased $\mathrm{Pt}/{\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}/{\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}/\mathrm{Si}{\mathrm{O}}_x$ multilayers. In samples having a strong exchange bias, we observed a drastic decrease of the magnetic damping of $\mathrm{Fe}\text{−}\mathrm{Ni}$ with increasing temperature up to the blocking temperature. The results essentially indicate that the nonlocal enhancement of the magnetic damping can be choked by the adjacent antiferromagnet and its temperature-dependent exchange bias. We also point out that such a strong temperature-dependent damping may be very beneficial for spintronic applications.

We investigate thermal properties of a $\mathrm{NbN}$ single-photon detector capable of unit internal detection efficiency. Using an independent calibration of the coupling losses, we determine the absolute optical power absorbed by the $\mathrm{NbN}$ film and, via resistive superconductor thermometry, the temperature dependence of the thermal resistance $Z(T)$ of the $\mathrm{NbN}$ film. In principle, this approach permits simultaneous measurement of the electron-phonon and phonon-escape contributions to the energy relaxation, which in our case is ambiguous because of the similar temperature dependencies. We analyze $Z(T)$ with a two-temperature model and impose an upper bound on the ratio of electron and phonon heat capacities in $\mathrm{NbN}$, which is surprisingly close to a recent theoretical lower bound for the same quantity in similar devices.

Avalanche-photodiode-based single-photon detectors (SPDs), as crucial and practical components, are widely used in quantum key distribution (QKD) systems. For effective detection, most of these SPDs are operated in the gated mode, in which the gate is added to obtain high avalanche gain and is removed to quench the avalanche. The avalanche-transition region (ATR) is certain to exist in the process of adding and removing the gate. We first experimentally investigate the characteristic of the ATR, including in a commercial SPD and a high-speed SPD, and then propose an ATR attack to control the detector. In the experiment on hacking the plug-and-play QKD system, Eve introduces less than 0.5% quantum bit error rate and almost leaves no traces of her presence, including the photocurrent and afterpulse probability. We finally give possible countermeasures against this attack.

Defects in crystals are leading candidates for photon-based quantum technologies, but progress in developing practical devices critically depends on improving defect optical and spin properties. Motivated by this need, we study a new defect qubit candidate, the shallow donor in $\mathrm{Zn}\mathrm{O}$. We demonstrate all-optical control of the electron spin state of the donor qubits and measure the spin coherence properties. We find a longitudinal relaxation time $T_1$ exceeding 100 ms, an inhomogeneous dephasing time $T_2^{\ast }$ of $17\pm 2$ ns, and a Hahn-spin-echo time $T_2$ of $50\pm 13\mu \mathrm{s}$. The magnitude of $T_2^{\ast }$ is consistent with the inhomogeneity of the nuclear hyperfine field in natural $\mathrm{Zn}\mathrm{O}$. Possible mechanisms limiting $T_2$ include instantaneous diffusion and nuclear spin diffusion (spectral diffusion). These dephasing mechanisms suggest that with isotope and chemical purification qubit coherence times can be extended. This work should motivate further research on high-purity material growth, quantum-device fabrication, and high-fidelity control of the donor-$\mathrm{Zn}\mathrm{O}$ system for quantum technologies.