Variational quantum time evolution allows us to simulate the time dynamics of quantum systems with near-term compatible quantum circuits. Due to the variational nature of this method the accuracy of the simulation is a priori unknown. We derive a posteriori global phase agnostic error bounds for the state simulation accuracy with variational quantum time evolution that improve the tightness of fidelity estimates over existing a posteriori error bounds. These analysis tools are practically crucial for assessing the quality of the simulation and making informed choices about simulation hyperparameters. The efficient, a posteriori evaluation of the bounds can be tightly integrated with the variational time simulation and, hence, results in a minor resource overhead, which is governed by the system’s energy variance. The performance of the error bounds is demonstrated on numerical examples.

Gate layouts of spin-qubit devices are commonly adapted from previous successful devices. As qubit numbers and device complexity increase, modeling new device layouts and optimizing for yield and performance become necessary. The simulation tools used in the advanced semiconductor industry need to be adapted for smaller structure sizes and electron numbers. Here, we present a general approach to electrostatically modeling new spin-qubit-device layouts, considering gate voltages, heterostructures, doping, reservoirs, and an applied source-drain bias. We identify key challenges in spin-qubit-device design: validating the impact on quantum-dot parameters, considering cross-coupling among gates and reservoirs, and ensuring robustness of the design to fabrication limits. We select a demanding target potential to investigate and optimize examples of gate layouts under these challenges. We verify our model by fabricating two simulated designs and indirectly probing the potential landscape through transport measurements.

Physical reservoir computing is a computational framework that implements spatiotemporal information processing directly within physical systems. By exciting nonlinear dynamical systems and creating linear models from their state, we can create highly-energy-efficient devices capable of solving machine-learning tasks without building a modular system consisting of millions of neurons interconnected by synapses. To act as an effective reservoir, the chosen dynamical system must have two desirable properties: nonlinearity and memory. We present task-agnostic spatial measures to locally measure both of these properties and exemplify them for a specific physical reservoir based upon magnetic skyrmion textures. In contrast to typical reservoir-computing metrics, these metrics can be resolved spatially and in parallel from a single input signal, allowing for efficient parameter searches to design efficient and high-performance reservoirs. Additionally, we show the natural trade-off between memory capacity and nonlinearity in our reservoir’s behavior, both locally and globally. Finally, by balancing the memory and nonlinearity in a reservoir, we can improve its performance for specific tasks.

The field of two-dimensional magnetic materials has paved the way for the development of spintronics and nanodevices with other functionalities. Utilizing antiferromagnetic materials, in addition to layered van der Waals (vdW) ferromagnetic materials, has garnered significant interest. In this work, we present a theoretical investigation of the behavior of ${\mathrm{Mn}\mathrm{Bi}}_2{\mathrm{Te}}_4$ devices based on the nonequilibrium Green’s function method. Our results show that the current-voltage ($I$-$V$) characteristics can be influenced significantly by controlling the length of the device and bias voltage and thus allow us to manipulate the tunneling magnetoresistance (TMR) with an external bias voltage. This can be further influenced by the presence of the boron nitride layer, which shows significantly enhanced TMR by selectively suppressing specific spin channels for different magnetic configurations. By exploiting this mechanism, the observed TMR value reaches up to 3690%, which can be attributed to the spin-polarized transmission channel and the projected local density of states. Our findings on the influence of structural and magnetic configurations on the spin-polarized transport properties and TMR ratios give the potential implementation of antiferromagnetic vdW layered materials in ultrathin spintronics.

The resonant control of the spin magnetic moment using ferromagnetic resonance (FMR) could enable an energy-efficient route for low-power spintronic devices for neuromorphic and signal processing applications. However, resonant spin control by FMR usually requires a rf magnetic field or high-density spin-polarized current resulting in high power consumption. Instigating FMR directly with an external electric field using magnetoelectric (ME) materials can offer an energy-efficient alternative due to large strain-mediated ME coupling. Yet, this method requires a high-saturation electric field that could cause device malfunction due to dielectric breakdown. Also, the necessity to make electrical contacts in ME materials leads to complexity. In this work, we demonstrate light-induced FMR tunability in $\mathrm{Ni}$/PMN-PT ME heterostructure at room temperature. We show that light generates piezostrain in PMN-PT without any bias voltage and controls the spin dynamics of the ferromagnetic $\mathrm{Ni}$ thin film. Depending on the direction of the external applied magnetic field with respect to the PMN-PT crystalline directions, upward and downward volatile FMR shift is demonstrated. We also show that FMR shift can be induced by increasing the irradiation time at constant illumination power. This technique presents a remote means to control the spin magnetic moment for next-generation light-controlled ME-based rf and microwave devices, allowing easier integration of these devices into complex architectures by eliminating the need for direct electrical contacts or bulky external components.

State-of-the-art transmon qubits rely on large capacitors, which systematically improve their coherence due to reduced surface-loss participation. However, this approach increases both the footprint and the parasitic cross-coupling and is ultimately limited by radiation losses—a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as $36\times 39\text{μ}{\mathrm{m}}^2$ with $\gtrsim 100$-nm-wide vacuum-gap capacitors that are micromachined from commercial silicon-on-insulator wafers and shadow evaporated with aluminum. We achieve a vacuum participation ratio up to 99.6% in an in-plane design that is compatible with standard coplanar circuits. Qubit relaxation-time measurements for small gaps with high zero-point electric field variance of up to 22 V/m reveal a double exponential decay indicating comparably strong qubit interaction with long-lived two-level systems. The exceptionally high selectivity of up to 20 dB to the superconductor-vacuum interface allows us to precisely back out the sub-single-photon dielectric loss tangent of aluminum oxide previously exposed to ambient conditions. In terms of future scaling potential, we achieve a ratio of qubit quality factor to a footprint area equal to $20\text{μ}{\mathrm{m}}^{-2}$, which is comparable with the highest $T_1$ devices relying on larger geometries, a value that could improve substantially for lower surface-loss superconductors.

Extensive research has been devoted recently to the design of metasurfaces. Design strategies based on impedance theory stand out for power-flow regulation. The designed impedance matrix is characteristic of a given pair of incident and modulated wave fields, hence providing opportunities for encrypted information storage. We consider acoustic vortices propagating along a cylindrical shell waveguide as signal carriers. Within the shell waveguide, an annular acoustic impedance metasurface is introduced for encrypted information storage. Information stored in the metasurface would only be retrieved when interrogated with the correct input. Hidden messages are successfully read in numerical simulation when the three-layer model is used to implement the metasurface. A realistic structure based on Helmholtz resonant unit cells is further designed to verify experimentally the encryption storage process. This work is expected to spur more applications of impedance metasurface to acoustic holography and encrypted acoustic communications.

Nowadays quantum key distribution (QKD) represents the most mature quantum technology, and multiple countries as well as private institutions are building their quantum network. However, QKD devices are still far from representing a product within everyone’s reach. Indeed, limitations in terms of compatibility with existing telecom infrastructure and limited performances in terms of secret key rate, using noncryogenic detection systems, are still critical. In this work, we implemented a quantum key distribution link between Sicily (Italy) and Malta utilizing two different single-photon avalanche diode (SPAD) detectors. The performances of a standard commercial SPAD have been compared with the results achieved with an alternative prototype of fast-gated system in a package (SIP) SPAD; the SIP detector has shown to be able to accomplish a 14 times higher key rate compared with the commercial device over the channel showing 20 dB of losses.

Metamaterials based on regular two-dimensional arrays of thin wires have attracted renewed attention in light of a recently proposed strategy to search for dark-matter axions. The metamaterial-based resonator concept was specifically motivated by recent stringent predictions for the axion mass in one cosmological scenario, which are expected to improve further in the near future. When placed in the external magnetic field, such metamaterials facilitate resonant conversion of axions into plasmons near their plasma frequency. Since the axion mass is not known a priori, a practical way to tune the plasma frequency of metamaterial is required. In this work, we have studied a system of two interpenetrating rectangular wire lattices where their relative position is varied. The plasma frequency as a function of their relative position in two dimensions has been mapped out experimentally and compared with both a semianalytical theory of wire-array metamaterials and numerical simulations. Theory and simulation yield essentially identical results, which in turn are in excellent agreement with experimental data. Over the range of translations studied, the plasma frequency can be tuned over a range of 16%. This dynamic range is well suited for a search amply encompassing the predicted mass range of postinflation axion, enabling discovery or exclusion within a few years.

We demonstrate a transmon-qubit readout based on the nonlinear response to a drive of polaritonic meters in situ coupled to the qubit. Inside a three-dimensional readout cavity, we place a transmon molecule consisting of a transmon qubit and an ancilla mode interacting via nonperturbative cross-Kerr-coupling. The cavity couples strongly only to the ancilla mode, leading to hybridized lower and upper polaritonic meters. Both polaritons are anharmonic and dissipative, as they inherit a self-Kerr nonlinearity $U$ from the ancilla and effective decay $\kappa$ from the open cavity. Via the ancilla, the polariton meters also inherit the nonperturbative cross-Kerr-coupling to the qubit. This results in a high qubit-dependent displacement $2\chi >\kappa ,U$ that can be read out via the cavity without causing Purcell decay. Moreover, the polariton meters, being nonlinear resonators, present bistability, and bifurcation behavior when the probing power increases. In this work, we focus on the bifurcation at low power in the few-photon regime, called the mesoscopic regime, which is accessible when the self-Kerr and decay rates of the polariton meter are similar, $U\sim \kappa$. Capitalizing on a latching mechanism by bifurcation, the readout is sensitive to transmon-qubit relaxation error only in the first tens of nanoseconds. We thus report a single-shot fidelity of 98.6% while having an integration time of 500 ns and no requirement for an external quantum-limited amplifier.

Fock states are quantized states of electromagnetic waves with diverse applications in quantum optics and quantum communication. However, the generation of arbitrary optical Fock states still remains elusive. Most Fock-state-generation proposals rely on precisely controlling the atom-cavity interactions and are experimentally challenging. We propose a scheme to distill an optical Fock state from a coherent state. A conditional phase flip with arbitrary phase is implemented between the atom and light. The conditional phase flip along with unitary rotations and measurements on the atoms enables us to distill the required Fock state. As an example, we show the distillation of Fock state $|100⟩$.

We report on the theoretical, numerical, and experimental investigation of secure acoustic holography. This secure holography is achieved via an encoded hologram generated by a transducer array decoded by a transmissive acoustic metakey. An unreadable image is received if directly using the encoded hologram, while the desired image can be obtained only with the aid of a decoding metastructure, i.e., a correctly decoding metakey. The proposed metakey consists of a series of unit cells, each of them is composed of a straight channel decorated with several opening panels. By adjusting the opening size of unit cells within the metakey, its transmitted phase profile can be effectively controlled; hence, the desired decoding metakey can be successfully constructed. The patterns of a butterfly, sunglasses, and the letters NJUST are chosen as examples to demonstrate the performance of the secure acoustic holography. Vulnerability tests of the metakey further confirm the safety and reliability of secure acoustic holography. With the advantages of simple design, flexible functionality, and high quality for reconstructed images, our work opens an alternative avenue for acoustic holography and may find applications in secure acoustic communications based on holograms.

We study pressure acoustic propagation in asymmetric spoof-fluid-spoof acoustic waveguides and its potential application in acoustic gas sensors. First, a stable and efficient analytical method is established for rapid calculation of the dispersion curves based on spectral expansion and the enforcement of continuity between segments at suitable collocation points. The analysis is validated by using commercial finite-element software. The geometric design of the waveguide is then optimized for the emergence of a nearly flat dispersion curve associated with vertical geometric asymmetry. The waveguide is fabricated using three-dimensional-printing technology, and the measurement results corroborate the numerical simulations. Based on the nearly flat dispersion curve supported by this waveguide, a ${\mathrm{CO}}_2$ sensor is proposed, allowing the phase difference measured between two points in the waveguide to be related to the composition of the gas in the waveguide. The proposed sensor is experimentally validated in a controlled environment and the measurement results match the computational predictions well. The sensor is robust with respect to noise and signal-recording duration due to fast phase measurements and shows high sensitivity to gas concentration due to reliance on the second, nearly flat, dispersion curve. In addition, the sensor is label free and low cost, while exhibiting a rapid response, low-maintenance requirements, and potential for measurements over a wide range of ${\mathrm{CO}}_2$ concentrations without saturation issues.

The outcome of excited-state processes is strongly dependent on initial conditions, i.e., the nature of the state arising from excitation. The physics of photoexcitation and the nature of the excited states generated are well established and applied routinely in the exploration of photochemical mechanisms and the interpretation of laser experiments across a broad range of molecules and materials. However, despite its importance in emerging technologies such as organic light-emitting diodes (OLEDs), the nature of excited states formed by electrical excitation is much less understood. In this paper, we use kinetic Monte Carlo (KMC) simulations to reveal that during charge recombination there is a substantial probability of forming higher-lying excited states with $>0.5$ eV excess energy compared to the lowest excited state. The probability of these hot-exciton pathways depends on the energy gap between the states and the intrinsic energetic disorder in the system. Our simulations also reveal the concept of electrical selection, in that only higher-lying excited states exhibiting $\mathrm{HOMO}\to x$ or $x\to \mathrm{LUMO}$ character (where “HOMO” refers to the highest occupied molecular orbital and “LUMO” refers to the lowest unoccupied molecular orbital) can be formed. Importantly, the potential to form these higher-lying excited states is likely to have significant implications for the lifetime of OLEDs, especially those operating in the blue, as the excess energy will push the core components closer to the damage thresholds, increasing the probability of degradation.

The negatively charged nitrogen-vacancy center (NVC) in diamond has been utilized in a wide variety of sensing applications. The long spin coherence and relaxation times ($T_2^{\ast }$, $T_2$ and $T_1$) of the center at room temperature are crucial to this, as they often limit sensitivity. Using NVCs in nanodiamonds allows for operations in environments inaccessible to bulk diamond, such as intracellular sensing. We report long spin coherence and relaxation times at room temperature for single NVCs in isotopically purified polycrystalline ball-milled nanodiamonds. Using a spin-locking pulse sequence, we observe spin coherence times, $T_2$, up to $786\pm 200\text{μ}\mathrm{s}$. We also measure $T_2^{\ast }$ times up to $2.06\pm 0.24\text{μ}\mathrm{s}$ and $T_1$ times up to $4.32\pm 0.60\mathrm{ms}$. These results represent improvements on the leading literature values for $T_2$ and $T_1$ times in nano- and microdiamond and for $T_2^{\ast }$ times in nanodiamond. Scanning-electron-microscopy and atomic force microscopy measurements show that the diamond containing the NVC with the longest $T_1$ time is smaller than 100 nm. EPR measurements give an $\mathrm{N}$${}_s^0$ concentration of $0.15\pm 0.02$ parts per million for the nanodiamond sample.

Precise understanding of the Coulomb interaction between single electrons is vital to achieve accurate single-electron control toward quantum current standards and quantum information processing. Since the strength of the Coulomb interaction increases with decreasing distance, a collision experiment of single electrons would be an ideal way to investigate it. It would be useful to study such a Coulomb collision in silicon single-electron pumps, which can accurately transfer single electrons one by one, while silicon systems have not been used for making Coulomb collisions at the single-electron level. Here, we made two series-coupled tunable-barrier single-electron pumps in silicon and used one to inject a hot single electron into the other pump in each pumping cycle. The hot single electron collides with a cold single electron confined in the other single-electron pump. We observed a current flow due to ejection not only of the hot single electron but also of the confined cold single electron. The latter leads to an excess current at a current plateau at a certain voltage range. We also found that increasing the number of cold electrons from one to two increased the cold-electron current by at least twofold. These results can be explained by a charging effect due to the Coulomb interaction. This observation is valuable to understand single-electron dynamics in the silicon single-electron devices toward accurate current generation and quantum manipulation of flying single electrons.

Topological quantum catalysts are developing rapidly due to the emergence of exotic quantum materials and their corresponding catalytic performance. Although tens of thousands of topological semimetals have been developed, their low topological surface density of states (DOSs) remains a hindrance to the development of high-performance catalysts for electrochemical hydrogen evolution reactions (HERs). In this work, we investigate the potential of double dual-nodal line (DDNL) semimetals, which exhibit large surface DOSs and fair-low Gibbs free energies, as ideal topological quantum catalysts (TQCs) for HER. Using the $\mathrm{Na}\mathrm{Al}\mathrm{Ge}$ compound as a representative example, we demonstrate that its DDNLs and high surface DOSs around the Fermi level provide advantages for achieving exceptionally high catalytic activity in the electrochemical HER process. Through a comparison of catalytic performance under different (electron and hole) doping and uniaxial strain conditions, we establish a linear correlation between the Gibbs free energy ($\mathrm{\Delta }{G}_{{\mathrm{H}}^{\ast }}$) and the projected surface DOSs on the (001) semi-infinite surface of the DDNL semimetal $\mathrm{Na}\mathrm{Al}\mathrm{Ge}$, specifically for the hydrogen evolution process. Our work introduces an alternative category of high-performance TQCs free of noble metals and contributes to a better understanding of the relationship between catalytic performance for HER and surface DOSs in DDNL semimetals.