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.

We propose nonreciprocal phonon lasing in a coupled cavity system composed of an optomechanical resonator and a spinning resonator. We show that the optical Sagnac effect leads to significant modifications in both the mechanical gain and the power threshold for phonon lasing. More importantly, the phonon lasing in this system is unidirectional; that is, the phonon lasing occurs when the coupled system is driven in one direction but not the other. Our work establishes the potential of spinning optomechanical devices for low-power mechanical isolation and unidirectional amplification. This provides a new route, well within the reach of current experimental abilities, to operate cavity optomechanical devices for a wide range of applications such as directional phonon switches, invisible sound sensing, and topological or chiral acoustics.

We experimentally demonstrate long-distance continuous-variable quantum key distribution over a 50-km standard optical fiber based on continuous-variable Einstein-Podolsky-Rosen entangled states. The entanglement survives despite being distributed over a high-loss optical fiber channel. At a channel excess noise level of 0.01 shot-noise units, we achieve an asymptotic secret key rate of 0.03 bit per sample, which is superior to the optimized coherent-state protocol. The superiority is even more evident at a high-channel excess noise level of 0.1 shot-noise units. Our work paves the way toward practical applications of continuous-variable quantum key distribution under high amounts of excess channel noise.

The distributed absorption of photons in photodiodes induces an excess noise in continuous-wave photodetection above the transit-time roll-off frequency. We show that it can be treated as a frequency-dependent excess optical loss in homodyne detection. This places a limit on the bandwidth of high-accuracy homodyne detection, even if an ideal photodetector circuit is available. We experimentally evaluate the excess loss in two ways: a comparison of signal gain and shot-noise gain of one-port homodyne detection and a balanced homodyne detection of squeezed light at a 500-MHz sideband. These results agree with an analytic expression we develop, where the randomness of the photoabsorption is directly modeled by an intrusion of vacuum field. At 500 MHz, we estimate the excess loss at 14% for a $\mathrm{Si}$ photodiode with 860-nm incident light, while the numerical simulation predicts much smaller excess loss in $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ photodiodes with 1550 nm light.

Routing electromagnetic energy at a scale smaller than the wavelength is a highly sought functionality in a variety of applications, including compact lightweight satellite communications, slow-waves sensors, all-optical information processing, and energy harvesting. Unfortunately, strong field confinement at this scale requires the use of coupled subwavelength resonators, implying a large sensitivity to geometrical imperfections and disorder-induced backscattering. We propose a very unconventional solution to this problem by exploiting the interface modes occurring at the boundary between two chiral metamaterials composed of resonant metamolecules with opposite chirality. Our numerical and experimental results demonstrate the inherent robustness of these interface states to disorder in both the position and resonance frequency of the metamaterial’s meta-atoms. By computing transmission averages over many realizations of disorder, we quantitatively demonstrate the superiority of this form of subwavelength routing over previously proposed designs, including frequency-defect lines, symmetry-based topological edge modes, and Valley-Hall interface states.

We provide an in-depth characterization of the dislocation distribution in partially relaxed ${\mathrm{Si}}_{0.92}{\mathrm{Ge}}_{0.08}/\mathrm{Si}(001)$ films. This is achieved by an innovative and general method, combining two state-of-the-art characterization techniques through suitable modeling. After having inferred the dislocation positions from transmission-electron-microscopy images, we theoretically reproduce scanning-x-ray-diffraction-microscopy tilt maps measured on the very same region of the sample. We obtain a nearly perfect match between model predictions and experimental data. As a result, we claim that it is possible to establish a local, direct correlation between the dislocations revealed by the transmission-electron-microscopy analysis and the measured lattice tilt distribution.

We introduce a multiscale framework that combines a time-dependent nonequilibrium Green-function (TDNEGF) algorithm, scaling linearly in the number of time steps and describing quantum-mechanically the conduction electrons in the presence of time-dependent fields of arbitrary strength or frequency, with classical time evolution of localized magnetic moments described by the Landau-Lifshitz-Gilbert (LLG) equation. The TDNEGF+LLG framework can be applied to a variety of problems where current-driven spin torque induces the dynamics of magnetic moments as the key resource for next-generation spintronics. Previous approaches to such nonequilibrium many-body systems (like the steady-state-NEGF+LLG framework) neglect noncommutativity of a quantum Hamiltonian of conduction electrons at different times and, therefore, the impact of time-dependent magnetic moments on electrons leading to the pumping of spin and charge currents. The pumped currents can, in turn, self-consistently affect the dynamics of magnetic moments themselves. Using the magnetic domain wall (DW) as an example, we predict that its motion will pump time-dependent spin and charge currents (on top of the unpolarized dc charge current injected through normal-metal leads to drive the DW motion), where the latter can be viewed as a realization of quantum charge pumping due to the time dependence of the Hamiltonian and the left-right symmetry breaking of the two-terminal device structure. The conversion of ac components of spin current, whose amplitude increases (decreases) as the DW approaches (recedes from) the normal-metal lead, into ac voltage via the inverse spin Hall effect offers a tool to precisely track the DW position along magnetic nanowire. We also quantify the DW transient inertial displacement due to its acceleration and deceleration by pulse current and the entailed spin and charge pumping. Finally, TDNEGF+LLG as a nonperturbative (i.e., numerically exact) framework allows us to establish the limits of validity of the so-called spin-motive force (SMF) theory for pumped charge current by time-dependent magnetic textures—the perturbative analytical formula of SMF theory becomes inapplicable for large frequencies (but unrealistic in a magnetic system) and, more importantly, for increasing noncollinearity when the angles between neighboring magnetic moments exceed approximately ${10}^{\circ }$.

Active research on mesoscopic quantum systems has increased our understanding of and ability to control quantum objects, allowing the construction of a universal form for quantum networks that consist of more than one physical system. This kind of quantum network is anticipated to enable the building of quantum infrastructure, such as long-distance quantum communication and distributed quantum computers, and motivates the establishment of photonic quantum interfaces that are compatible with physical systems. Here, a universal photonic quantum interface is experimentally developed with the benefit of a unique, specially designed entangled photon source. The detailed experimental results show that this configuration can satisfy all the urgent demands for a photonic quantum interface, including the accurate matching of the working wavelength and bandwidth and specifically, the entanglement ability ($F=89.6\mathrm{\%}$, $S=2.36\pm 0.03$). The realization of this universal photonic quantum interface is expected to expedite the construction of much more complex quantum networks and to be a major step in the area of optical engineering and control.

We consider the phenomenon of electric Mott transition (EMT), which is an electrically induced insulator-to-metal transition. Experimentally, it is observed that depending on the magnitude of the electric excitation, the final state may show a short-lived or a long-lived resistance change. We extend a previous model for the EMT to include the effect of local structural distortions through an elastic energy term. We find that by strong electric pulsing, the induced metastable phase may become further stabilized by the electroelastic effect. We present a systematic study of the model by numerical simulations and compare the results to experiments in Mott insulators of the ${\mathrm{AM}}_4{\mathrm{Q}}_8$ family. Our work significantly extends the scope of our recently introduced leaky-integrate-and-fire Mott neuron [P. Stoliar et al., Adv. Funct. Mat. 27, 1604740 (2017)] to provide a better insight into the physical mechanism of its relaxation. This is a key feature for future implementations of neuromorphic circuits.

The ideal solar-cell conversion efficiency limit known as the Shockley-Queisser (SQ) limit, which is based on a detailed balance between absorption and radiation, has long been a target for solar-cell researchers. While the theory for this limit uses several assumptions, the requirements in real devices have not been discussed fully. Given the current situation in which research-level cell efficiencies are approaching the SQ limit, a quantitative argument with regard to these requirements is worthwhile in terms of the understanding of the remaining loss mechanisms in current devices and the device characteristics of solar cells that are operating outside the detailed balance conditions. Here we examine two basic assumptions: (1) that the photogenerated carriers lose their kinetic energy via phonon emission in a moment (fast thermalization), and (2) that the photogenerated carriers are extracted into carrier reservoirs in a moment (fast extraction). Using a model that accounts for the carrier relaxation and extraction dynamics, we reformulate the nonequilibrium theory for solar cells in a manner that covers both the equilibrium and nonequilibrium regimes. Using a simple planar solar cell as an example, we address the parameter regime in terms of the carrier extraction time and then consider where the conventional SQ theory applies and what could happen outside the applicable range.

We experimentally demonstrate generation of coherent propagating magnons in ultrathin magnetic insulator films by spin-orbit torque induced by a direct electric current. We show that this challenging task can be accomplished by use of magnetic insulator films with large perpendicular magnetic anisotropy. We demonstrate simple and versatile spin-orbit-torque devices that can be used as highly efficient nanoscale sources of coherent propagating magnons for insulator-based spintronic applications.

With use of the techniques of optomechanics, a high-$Q$ mechanical oscillator may serve as a link between electromagnetic modes of vastly different frequencies. This approach has successfully been exploited for the frequency conversion of classical signals and has the potential of performing quantum-state transfer between superconducting circuitry and a traveling optical signal. Such transducers are often operated in a linear regime, where the hybrid system can be described with linear response theory based on the Heisenberg-Langevin equations. While these equations are mathematically straightforward to solve, this approach yields little intuition about the dynamics of the hybrid system to aid the optimization of the transducer. As an analysis and design tool for such electrooptomechanical transducers, we introduce an equivalent-circuit formalism, where the entire transducer is represented by an electrical circuit. Thereby we integrate the transduction functionality of optomechanical systems into the toolbox of electrical engineering, allowing the use of its well-established design techniques. This unifying impedance description can be applied for both static (dc) and harmonically varying (ac) bias fields, accommodates arbitrary linear circuits, and is not restricted to the resolved-sideband regime. Furthermore, by establishing the quantized input-output formalism for the equivalent circuit, we obtain the scattering matrix for linear transducers using circuit analysis, and thereby have a complete quantum-mechanical characterization of the transducer. Hence, this mapping of the entire transducer to the language of electrical engineering both sheds light on how the transducer performs and can at the same time be used to optimize its performance by aiding the design of a suitable electrical circuit.

Phase-estimation protocols provide a fundamental benchmark for the field of quantum metrology. The latter represents one of the most relevant applications of quantum theory, potentially enabling the capability of measuring unknown physical parameters with improved precision over classical strategies. Within this context, most theoretical and experimental studies have focused on determining the fundamental bounds and how to achieve them in the asymptotic regime where a large number of resources are employed. However, in most applications, it is necessary to achieve optimal precision by performing only a limited number of measurements. To this end, machine-learning techniques can be applied as a powerful optimization tool. Here, we implement experimentally single-photon adaptive phase-estimation protocols enhanced by machine learning, showing the capability of reaching optimal precision after a small number of trials. In particular, we introduce an approach for Bayesian estimation that exhibits best performance for a very low number of photons $N$. Furthermore, we study the resilience to noise of the tested methods, showing that the optimized Bayesian approach is very robust in the presence of imperfections. Application of this methodology can be envisaged in the more general multiparameter case, which represents a paradigmatic scenario for several tasks, including imaging or Hamiltonian learning.

Inorganic cesium-lead-halide perovskites ($\mathrm{Cs}\mathrm{Pb}{X}_3$, where $X=\mathrm{Cl}$, $\mathrm{Br}$, and $\mathrm{I}$) have recently emerged as promising semiconductors for photovoltaic and light-emitting devices. In this study, we investigate electronic and vibrational properties of $\mathrm{Cs}\mathrm{Pb}{X}_3$ using the density-functional-theory calculations. We explicitly evaluate k-dependent electron-phonon scattering rates of the polar longitudinal-optical phonon modes. The transport property at room temperature is then computed based on Boltzmann transport theory within the relaxation-time approximation. The computational results identify the fundamental limit of the carrier mobility and its dependence on the halide species. Our results show that different choices of $X$ lead to the variation in the mobility by a factor of 3 to 5 depending on the carrier concentration between ${10}^{15}$ and ${10}^{18}{\mathrm{cm}}^{-3}$. The preferred carrier type (electron or hole) in terms of the mobility also varies with $X$. Through the detailed analysis on the band structures and scattering rates, we provide insights into the role of halide species in the transport properties.

Improving the write efficiency of magnetic tunnel junctions (MTJs) by using spin-orbit torque (SOT) is essential for realizing high-density spintronic memory. Here, we investigate a voltage-controlled spintronic memory (VoCSM) with a $\mathrm{Ta}\text{−}\mathrm{B}$ spin Hall electrode. The magnetic properties of MTJs with a storage layer such as $\mathrm{Fe}\text{−}\mathrm{B}$ or $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ on an amorphous boride spin Hall electrode are found to exhibit an extremely small magnetic dead layer, a small saturation magnetization, and a large magnetic anisotropy. The spin Hall angle estimated by spin Hall magnetoresistance is −0.18 for the amorphous $\mathrm{Ta}\text{−}\mathrm{B}$ spin Hall electrode, which is a magnitude twice that for a $\mathrm{Ta}$ spin Hall electrode. The write current density using SOT from the $\mathrm{Ta}\text{−}\mathrm{B}$ spin Hall electrode is small compared with that from the $\mathrm{\beta }\text{−}\mathrm{Ta}$ spin Hall electrode. By combining the self-aligned fabrication technique with the $\mathrm{Ta}\text{−}\mathrm{B}$ electrode, a small value of critical switching current $(I_c\approx 79\mu \mathrm{A})$ is achieved despite a large MTJ size $(60\times 150{\mathrm{nm}}^2)$. We successfully reduce the write current to 48 µA, utilizing a voltage-controlled magnetic anisotropy by applying the voltage to a MTJ: $V_{\mathrm{MTJ}}=-1\mathrm{V}$. Moreover, the device also exhibits a low write error rate $(<1\times {10}^{-8})$, high endurance ($>1\times {10}^{12}$ cycles), and large break-down voltage (>2.5 V). These results indicate that VoCSM with the $\mathrm{Ta}\text{−}\mathrm{B}$ spin Hall electrode could open a path to realizing high-density nonvolatile memories with low power consumption and high-speed read and write operations.

Despite their ubiquity in nanoscale electronic devices, the physics of tunnel barriers has not been developed to the extent necessary for the engineering of devices in the few-electron regime. This problem is of urgent interest, as this is the specific regime into which current extreme-scale electronics fall. Here, we propose theoretically and validate experimentally a compact model for multielectrode tunnel barriers, suitable for design-rules-based engineering of tunnel junctions in quantum devices. We perform transport spectroscopy at approximately $T=4$ K, extracting effective barrier heights and widths for a wide range of biases, using an efficient Landauer-Büttiker tunneling model to perform the analysis. We find that the barrier height shows several regimes of voltage dependence, either linear or approximately exponential. Effects on threshold, such as metal-insulator transition and lateral confinement, are included because they influence parameters that determine barrier height and width (e.g., the Fermi energy and local electric fields). We compare these results to semiclassical solutions of Poisson’s equation and find them to agree qualitatively. Finally, this characterization technique is applied to an efficient lateral tunnel barrier design that does not require an electrode directly above the barrier region in order to estimate barrier heights and widths.

High-performance organic field-effect transistors (OFETs) are an essential building block for future flexible electronics. Although there has been steady progress in the development of high-mobility organic semiconductors, the performance of lateral state-of-the-art OFETs still falls short, especially with regard to the transition frequency. One candidate to overcome the shortcomings of the lateral OFET is its vertical embodiment, the vertical organic field-effect transistor (VOFET). However, the detailed mechanism of VOFET operation is poorly understood and a matter of discussion. Proposed descriptions of the formation and geometry of the vertical channel vary significantly. In particular, values for lateral depth of the vertical channel reported so far show a large variation. This is an important question for the transistor integration, though, since a channel depth in the micrometer range would severely limit the possible integration density. Here, we investigate charge transport in such VOFETs via drift-diffusion simulations and experimental measurements. We use a (vertical) organic light-emitting transistor [(V)OLET] as a means to map the spatial distribution of charge transport within the vertical channel. Comparing simulation and experiment, we can conclusively describe the operation mechanism which is mainly governed by an accumulation of charges at the dielectric interface and the channel formation directly at the edge of the source electrode. In particular, we quantitatively describe how the channel depth depends on parameters such as gate-source voltage, drain-source voltage, and lateral and vertical mobility. Based on the proposed operation mechanism, we derive an analytical estimation for the lateral dimensions of the channel, helping to predict an upper limit for the integration density of VOFETs.

We introduce a concept to generate spin-polarized currents in graphene nanoribbons by using light radiation and a significant electron-electron interaction. For this purpose, we design a phototransistor based on the sawtoothlike graphene nanoribbons (SGNRs). The structures, with their intrinsic magnetic moments, have a narrow spin-dependent band around their Fermi energy that can enhance interband transitions and produce a spin photocurrent at room temperature without applying any magnetic field and without using any element. Interestingly, the changes in the size parameters and the gate voltage modify the magnitude and position of optical absorption peaks and optical spin polarization and the gate voltage individually switches the sign of the spin photocurrent. Finally, the fully spin-polarized photocurrent, the high quantum efficiency with a maximum of approximately 40%, the wide-wavelength-range operation from ultraviolet to infrared and optical spin-filtering effects, that are tunable with size and gate voltage, pave the way toward the improved design and performance of spin-optoelectronic devices based only on carbon atoms.

Cleaning pollution from a heterogeneous flow environment is far from simple. We consider the flow capture problem, which has flows and sinks in a heterogeneous environment, and investigate the problem of positioning pollutant capture units. We show that arrays of capture units carry a high risk of failure without accounting for environmental heterogeneity and chaos in their placement, design, and operation. Our idealized two-dimensional models reveal salient features of the problem. Maximum capture efficiency depends on the required capture rate: long-term efficiency decreases as the number of capture units increases, whereas short-term efficiency increases. If efficiency is important, the capture process should begin as early as feasible. Knowledge of transport controlling flow structures offers predictability for unit placement. We demonstrate two heuristic approaches to near-optimally position capture units.

We demonstrate a vector magnetometer that simultaneously measures all Cartesian components of a dynamic magnetic field using an ensemble of nitrogen-vacancy (NV) centers in a single-crystal diamond. Optical NV-diamond measurements provide high-sensitivity, broadband magnetometry under ambient or extreme physical conditions and the fixed crystallographic axes inherent to this solid-state system enable vector sensing free from heading errors. In the present device, multichannel lock-in detection extracts the magnetic-field-dependent spin-resonance shifts of NVs oriented along all four tetrahedral diamond axes from the optical signal measured on a single detector. The sensor operates from near dc (5 Hz) up to a 12.5-kHz measurement bandwidth and simultaneously achieves approximately $50\mathrm{pT}/\sqrt{\mathrm{Hz}}$ magnetic-field sensitivity for each Cartesian component, which is, to date, the highest demonstrated sensitivity of a full vector magnetometer employing solid-state spins. Compared to optimized devices interrogating the four NV orientations sequentially, the simultaneous vector magnetometer enables a $4\times$ measurement speedup. This technique can be extended to pulsed-type sensing protocols and parallel wide-field magnetic imaging.