Optical nanofiber waveguides are widely used for near-field delivery and measurement of light. Despite their versatility and efficiency, nanofibers have a critical drawback—their inability to maintain light’s polarization state on propagation. Here, we design a directional coupler consisting of two crossed nanofibers to probe the polarization state at the waist region. Directionality of coupling occurs due to asymmetric dipolar emission or spin locking when the evanescent field pattern breaks the mirror symmetry of the crossed-nanofiber system. We demonstrate that, by monitoring the outputs from the directional coupler, two nonorthogonal polarization states can be prepared at the nanofiber waist with a fidelity higher than 99%. Based on these states, we devise a simple and reliable method for complete control of the polarization along a nanofiber waveguide.

The availability of large amounts of data and the necessity of processing it efficiently have led to the rapid development of machine-learning techniques. To name a few examples, artificial-neural-network architectures are commonly used for financial forecasting, speech and image recognition, robotics, medicine, and even research. Direct hardware for neural networks is highly sought for overcoming the von Neumann bottleneck of software implementations. Reservoir computing (RC) is a recent and increasingly popular bioinspired computing scheme that holds promise for efficient temporal information processing. We demonstrate the applicability and performance of RC in a general complex Ginzburg-Landau lattice model, which adequately describes the dynamics of a wide class of systems, including coherent photonic devices. In particular, we propose that the concept can be readily applied in exciton-polariton lattices, which are characterized by unprecedented photonic nonlinearity, opening the way to signal processing at rates of the order of $1\mathrm{Tbit}{\mathrm{s}}^{-1}$.

Dielectric laser acceleration (DLA) represents a promising approach to building miniature particle accelerators on a chip. However, similar to conventional rf accelerators, an automatic and reconfigurable control mechanism is needed to scale DLA technology towards high-energy gains and practical applications. We propose a system providing control of the laser coupling to DLA using integrated optics and introduce a component for power distribution using a reconfigurable mesh of Mach-Zehnder interferometers. Using numerical simulations, we show how such a mesh may be sequentially and efficiently tuned to optimize power distribution in the circuit and find that this strategy has favorable scaling properties with respect to size of the mesh.

Surface recombination has a major impact on the open-circuit voltage ($V_{\mathrm{OC}}$) of organic photovoltaics. Here, we study how this loss mechanism is influenced by imbalanced charge transport in the photoactive layer. As a model system, we use organic solar cells with a 2 orders-of-magnitude-higher electron than hole mobility. We find that small variations in the work function of the anode have a strong effect on the light-intensity dependence of $V_{\mathrm{OC}}$. Transient measurements and drift-diffusion simulations reveal that this is due to a change in the surface recombination rather than the bulk recombination. We use our numerical model to generalize these findings and determine the circumstances under which the effect of contacts is stronger or weaker compared to the idealized case of balanced charge transport. Finally, we derive analytical expressions for $V_{\mathrm{OC}}$ in the case that a pile-up of space charge is present due to highly imbalanced mobilities.

Nanomagnetic logic, which makes use of arrays of dipolar-coupled single-domain nanomagnets for computation, holds promise as a low-power alternative to traditional computation with CMOS. Beyond the use of nanomagnets for Boolean logic, nanomagnets can also be exploited for nondeterministic computational schemes such as edge detection in images and for solving the traveling salesman problem. Here, we demonstrate the potential of arrangements of thermally active nanomagnets based on artificial spin ice for both deterministic and probabilistic computation. This is achieved by engineering structures that follow particular thermal relaxation pathways consisting of a sequence of reorientations of magnet moments from an initial field-set state to a final low-energy output state. Additionally, we demonstrate that it is possible to tune the probability of attaining a particular final low-energy state, and therefore the likelihood of a given output, by modifying the intermagnet distance. Finally, we experimentally demonstrate a scheme to connect several computational building blocks for complex computation.

All-dielectric bianisotropic metasurfaces with tailored polarization and spectral responses are studied, and a systematic approach to their design is developed. It is shown that a wide range of polarization transformations over broad bandwidths, as well as multiple bands, can be implemented by cascading subwavelength dielectric gratings. The subwavelength gratings, modeled as anisotropic layers, are cascaded to form metasurfaces that control the polarization of impinging waves. These metasurfaces are a low-loss alternative to their lossy, plasmonic counterparts at optical frequencies, and can replace conventional, bulky optical components. The multifunctional performance and compactness (wavelength-sized thickness) of the proposed devices will also find use at millimeter-wave frequencies. Example metasurfaces are presented and their performance is verified through full-wave simulation. This work demonstrates the flexibility with which cascaded subwavelength gratings can realize unprecedented polarization control with varied spectral responses.

Electronic emulation of the biological synapse, which is the memory and learning element of the brain, is an important step towards the realization of brain-inspired computing systems. However, a complementary metal-oxide-semiconductor−based implementation of a synapse is not scalable due to the power and area inefficiency. It is therefore essential to develop alternative material and device concepts that can mimic the maximum number of synaptic functionalities in an energy-efficient way. Here, we demonstrate a nanosized energy-efficient three-terminal artificial synapse, with a separate read and write path, based on a magnetic material ($\mathrm{Co}$) that could generate both positive and negative synaptic outputs. The $\mathrm{Co}$ magnetization, which is representative of the synaptic weight, is modulated in an energy-efficient way by using an electric field to transport oxygen ions in and out of the $\mathrm{Co}$ layer. A wide range of synaptic functions such as synaptic potentiation and depression, spike magnitude, rate, and timing-dependent plasticity, and the transition from short-term to long-term plasticity are demonstrated. Our results suggest the viability of a spintronic synapse in neuromorphic computing.

Efficient electrical switching of antiferromagnets is at the center of their application in high-density and ultrafast nonvolatile memories. Antiferromagnetic (AFM) ${\mathrm{Mn}}_2\mathrm{Au}$ with opposite-spin sublattices is a unique metallic material in which the Edelstein effect can induce fieldlike torque switching of the AFM moments. However, the antidamping-torque-induced AFM moment switching remains to be proved in metallic AFM materials. Here, we demonstrate current-induced AFM moment switching in both a (103)-oriented ${\mathrm{Mn}}_2\mathrm{Au}$ single layer and ${\mathrm{Mn}}_2\mathrm{Au}/\mathrm{Pt}$ heterojunction, detected by the spin Hall magnetoresistance and anisotropic magnetoresistance at room temperature. In (103)-oriented ${\mathrm{Mn}}_2\mathrm{Au}$, fieldlike torque induces the alignment of AFM moments perpendicular to the writing current direction. Once the (103)-oriented ${\mathrm{Mn}}_2\mathrm{Au}$ film is covered by a thin $\mathrm{Pt}$ layer, the antidamping torque induced by the spin current from the $\mathrm{Pt}$ layer drives the AFM moments of ${\mathrm{Mn}}_2\mathrm{Au}$ switching toward the direction parallel to the writing current, which is in contrast to the case without $\mathrm{Pt}$. The simultaneous realization of fieldlike torque and antidamping torque in metallic ${\mathrm{Mn}}_2\mathrm{Au}$ makes it a promising candidate in AFM spintronics.

Active cooling systems use electrical work to bring a hot device (such as an integrated circuit) down to near ambient temperature by draining heat from the hot area of the device to a passive heat sink. Commercial thermoelectric modules are optimized for refrigeration, and are not ideal for active cooling. Refrigeration maintains a temperature that is below the ambient temperature in a device (such as a kitchen refrigerator) by pumping heat from the cold area of the device to a heat sink. The thermoelectric figure of merit zT is used traditionally to evaluate the performance of thermoelectric modules, including refrigeration modules. But it is not a good indicator of the performance of active cooling materials. Here, we describe an efficient, all-solid-state active cooler based on the Peltier effect in metals with high thermoelectric power factor due to electron correlation effects (${\mathrm{Ce}\mathrm{Pd}}_3$) or magnon drag ($\mathrm{Co}$) and high passive thermal conductivity. We show theoretically and experimentally that the effective thermal conductance under applied current can exceed the limits imposed by Fourier’s heat conduction law. The designed device measures an effective thermal conductance that is an order of magnitude larger than the passive thermal conductance at ΔT = 1 K with the dynamic response of 4 s.

Deterministic switching of polarization vortices in ferroelectrics by a uniaxial mechanical load is challenging. Here, we demonstrate, by using phase field simulations and ferroelectric instability analyses, a deterministic switching of polarization vortices in a compositionally graded ferroelectric (CGFE) nanoplate under a compressive stress. The obtained results suggest that the rational distribution of material constituents in the CGFE nanoplate tailors the distributions of electromechanical coupling and total energy densities of the nanoplate, which modifies the stress-induced vortex multiplication and annihilation behaviors, distinguished from those in a homogeneous ferroelectric nanoplate. The switching of polarization vortices by compressive stress is verified through instability analyses for the CGFE nanoplate, which not only confirms when the polarization switching occurs, but also demonstrates how the polarization responds at the onset of the switching. In addition, the effects of various factors on the feasibility of stress-induced vortex switching, including material gradient, temperature, and width-to-height ratio of the CGFE nanoplate, are investigated and summarized in phase diagrams.

A key requirement to perform simulations of large quantum systems on near-term quantum hardware is the design of quantum algorithms with a short circuit depth that finish within the available coherence time. A way to stay within the limits of coherence is to reduce the number of gates by implementing a gate set that matches the requirements of the specific algorithm of interest directly in the hardware. Here, we show that exchange-type gates are a promising choice for simulating molecular eigenstates on near-term quantum devices since these gates preserve the number of excitations in the system. We report on the experimental implementation of a variational algorithm on a superconducting qubit platform to compute the eigenstate energies of molecular hydrogen. We utilize a parametrically driven tunable coupler to realize exchange-type gates that are configurable in amplitude and phase on two fixed-frequency superconducting qubits. With gate fidelities around 95%, we are able to compute the eigenstates to within an accuracy of 50 mHa (milliHartree) on average, a limit set by the coherence time of the tunable coupler.

The valley Chern effect is theoretically demonstrated with an alternating current circuitry, where closed-loop LC resonators sitting at the nodes of a honeycomb lattice are inductively coupled along the bonds. This enables us to generate a dynamical matrix, which copies identically the Hamiltonian driving the electrons in graphene. The valley Chern effect is generated by splitting the inversion symmetry of the lattice. After a detailed study of the Berry-curvature landscape and of the localization of the interface modes, we derive an optimal configuration of the circuit. Furthermore, we show that $Q$ factors as high as ${10}^4$ can be achieved with reasonable materials and configurations.

Dispersive sensing is a powerful technique that enables scalable and high-fidelity readout of solid-state quantum bits. In particular, gate-based dispersive sensing has been proposed as the readout mechanism for future topological qubits, which can be measured by single electrons tunneling through zero-energy modes. The development of such a readout requires resolving the coherent charge tunneling amplitude from a quantum dot in a Majorana-zero-mode host system faithfully on short time scales. Here, we demonstrate rapid single-shot detection of a coherent single-electron tunneling amplitude between $\mathrm{In}\mathrm{As}$ nanowire quantum dots. We realize a sensitive dispersive detection circuit by connecting a sub-GHz, lumped-element microwave resonator to a high-lever arm gate on one of the dots. The resulting large dot-resonator coupling leads to an observed dispersive shift that is of the order of the resonator linewidth at charge degeneracy. This shift enables us to differentiate between Coulomb blockade and resonance—corresponding to the scenarios expected for qubit-state readout—with a signal-to-noise ratio exceeding 2 for an integration time of $1\mu \mathrm{s}$. Our result paves the way for single-shot measurements of fermion parity on microsecond time scales in topological qubits.

We present a design for a superconducting, on-chip circulator composed of dynamically modulated transfer switches and delays. Design goals are set for the multiplexed readout of superconducting qubits. Simulations of the device show that it allows for low-loss circulation (insertion loss $<0.35$ dB and isolation $>20$ dB) over an instantaneous bandwidth of 2.3 GHz. This design improves on the bandwidth of previous superconducting circulators by more than an order of magnitude, making it ideal for integration with broadband quantum-limited amplifiers.

Combining solid-state single-photon emitters (SPEs) with nanophotonic platforms is a key goal in integrated quantum photonics. In order to realize functionality in potentially scalable elements, suitable SPEs have to be bright, stable, and widely tunable at room temperature. In this work, we show that selected SPEs embedded in a few-layer hexagonal boron nitride ($h$-BN) meet these demands. In order to show the wide tunability of these SPEs we employ an atomic force microscope (AFM) with a conductive tip to apply an electrostatic field to individual $h$-BN emitters sandwiched between the tip and an indium-tin-oxide-coated glass slide. A very large and reversible Stark shift of $(5.5\pm 0.3)\mathrm{nm}$ at a zero-field wavelength of 670 nm is induced by applying just 20 V, which exceeds the typical resonance linewidths of nanodielectric and even nanoplasmonic resonators. Our results help to further understand the physical origin of SPEs in $h$-BN as well as for practical quantum photonic applications where wide spectral tuning and on/off resonance switching are required.

We study the electrocaloric effect (ECE) in a typical relaxor ferroelectric ceramic of ${\mathrm{Pb}}_{0.91}{\mathrm{La}}_{0.06}{\mathrm{Zr}}_{0.8}{\mathrm{Ti}}_{0.2}{\mathrm{O}}_3$ by analyzing the respective contributions of long-range-ordered ferroelectric macrodomains and short-range-ordered polar nanoregions (PNRs) by both direct and indirect characterization. The ECE exhibits two peaks in its dependence on temperature, the former responds to the transition from long-range ferroelectric order to short-range-ordered PNRs and the latter likely to the transition between two kinds of PNRs with different phase structures or motion states. The contributions of different mechanisms are clarified by resolving the overlapping peaks in direct heat flow experimental results. The first peak has a $\mathrm{\Delta }{T}_{\max }=0.7\mathrm{K}$ at 80 °C (approximately $T_d$) and the second a $\mathrm{\Delta }{T}_{\max }=0.98\mathrm{K}$ at 140 °C (approximately $T_m$) under 40 kV/cm. We conclude that the thermodynamic method based on the Maxwell relation is not applicable if PNRs are involved as their signatures are only picked up by direct measurements, but is valid for the case of ferroelectric domain rotation or phase transition where it provides a reasonably accurate ECE result. Besides the impact on aspects related to metrology, our work suggests that the region between the peaks in ΔT-T curves may be used to produce a large electrocaloric ΔT over a broad temperature range, which is highly desirable in cooling applications.

Electromagnetic metasurfaces can be characterized as intelligent if they are able to perform multiple tunable functions, with the desired response being controlled by a computer influencing the individual electromagnetic properties of each metasurface inclusion. In this paper, we present an example of an intelligent metasurface that operates in the reflection mode in the microwave frequency range. We numerically show that, without changing the main body of the metasurface, we can achieve tunable perfect absorption and tunable anomalous reflection. The tunability features can be implemented using mixed-signal integrated circuits (ICs), which can independently vary both the resistance and reactance, offering complete local control over the complex surface impedance. The ICs are embedded in the unit cells by connecting two metal patches over a thin grounded substrate and the reflection property of the intelligent metasurface can be readily controlled by a computer. Our intelligent metasurface can have a significant influence on future space-time modulated metasurfaces and a multitude of applications, such as beam steering, energy harvesting, and communications.

Dynamical decoupling can enforce a symmetry on the dynamics of an open quantum system. Here we develop an efficient dynamical-decoupling-based strategy to create the decoherence-free subspaces for a set of qubits by optimally transferring the states of these qubits. Our method is to transfer the state of each qubit to all of the other qubits in an optimal and efficient manner, so as to produce an effective collective-type noise for all qubits. This collective pseudonoise is essentially derived from arbitrarily independent real baths and needs only nearest-neighbor state transfers for its implementation. Moreover, our scheme requires only $N$ steps to produce the effective collective-type noise for a system of $N$ qubits. It provides an experimentally feasible and efficient approach to achieving the decoherence-free subspaces for encoding the protected logical qubits.

We find an unusually optimal near-field heat transfer, where the maximum heat transfer is reached at experimentally feasible gap separation. We attribute this to the localized zero-energy electronic edge states, which also substantially change the near-field behaviors. We demonstrate these anomalous behaviors in two typical carbon-based nanostructures: zigzag single-walled carbon nanotubes and graphene nanotriangles. For the system of carbon nanotubes, the maximal heat flux in this work surpasses all the previous results reported so far. The underlying mechanism for the peculiar effects is uncovered from a simple Su-Schrieffer-Heeger model. Our findings also offer a route to achieve an active near-field thermal switch, where the heat flux can be modulated through tuning the presence or absence of edge states.

The development of alternative mm-wave high-gradient, >200 MV/m, accelerating structures offers a promising path to reduce the cost and footprint of future TeV-scale linear colliders, as well as linacs for industrial, medical, and security applications. The major factor limiting accelerating gradient is vacuum rf breakdown. The probability of such breakdowns increases with pulse length. For reliable operation, millimeter-wave structures require nanoseconds-long pulses at the megawatt level. This power is available from gyrotrons, which have a minimum pulse length on the order of microseconds. To create shorter pulses and to reliably detect rf breakdowns, we developed the following devices: a laser-based rf switch capable of selecting 10 ns long pulses out of the microseconds long gyrotron pulses, thus enabling the use of the gyrotrons as power sources for mm-wave high-gradient linacs, and a shot-to-shot sub-THz spectrometer with high-frequency resolution, capable of detecting pulse shortening due to rf breakdowns. We will describe the principle of operation of these devices and their achieved parameters. We also report on the experimental demonstration of these devices with the high-power gyrotron at the Massachusetts Institute of Technology. In the experiments, we demonstrate nanosecond rf power modulation, shot-to-shot measurements of the pulse spectra, and detection of rf breakdowns.