The Hodgkin-Huxley model describes the behavior of the cell membrane in neurons, treating each part of it as an electric circuit element; namely, capacitors, memristors, and voltage sources. We focus on the activation channel of potassium ions, due to its simplicity, while keeping most of the features displayed by the original model. This reduced version is essentially a classical memristor, a resistor whose resistance depends on the history of electric signals that have crossed it, coupled to a voltage source and a capacitor. We consider a quantized Hodgkin-Huxley model based on a quantum-memristor formalism. We compare the behavior of the membrane voltage and the potassium-channel conductance when the circuit is subjected to ac sources, in both the classical realm and the quantum realm. Numerical simulations show an expected adaptation of the considered channel conductance depending on the signal history in all regimes. Remarkably, the computation of higher moments of the voltage shows purely quantum features related to the circuit zero-point energy. Finally, we study the implementation of the Hodgkin-Huxley quantum memristor as an asymmetric rf superconducting quantum-interference device in superconducting circuits. This study may allow the construction of quantum neuron networks inspired by the brain function, as well as the design of neuromorphic quantum architectures for quantum machine learning.

We introduce a method for generating benchmark problem sets for Ising machines—devices designed to solve discrete-optimization problems cast as Ising models. In our approach, linear systems of equations are cast as Ising cost functions. While linear systems are easily solvable, the corresponding optimization problems are known to exhibit some of the salient features of nondeterministic polynomial-time hardness, such as strong exponential scaling of heuristic solvers’ runtimes and extensive distances between ground and low-lying excited states. We show how the proposed technique, which we refer to as “equation planting,” can serve as a useful tool for evaluating the utility of Ising solvers functioning either as optimizers or as ground-state samplers. We further argue that equation-planted problems can be used to probe the mechanisms underlying the operation of Ising machines.

The characteristic impedance of a microwave transmission line is typically constrained to a value $Z_0=50\mathrm{\Omega }$, in part because of the low impedance of free space and the limited range of permittivity and permeability realizable with conventional materials. Here we suggest the possibility of constructing high-impedance transmission lines by exploiting the plasmonic response of edge states associated with the quantum Hall effect in gated devices. We analyze various implementations of quantum Hall transmission lines based on distributed networks and lumped-element circuits, including a detailed account of parasitic capacitance and Coulomb drag effects, which can modify device performance. We additionally conceive of a metamaterial structure comprising arrays of quantum Hall droplets and analyze its unusual properties. The realization of such structures holds promise for efficiently wiring-up quantum circuits on chip, as well as engineering strong coupling between semiconductor qubits and microwave photons.

We implement a technique for measuring the singlet-triplet energy splitting responsible for spin-to-charge conversion in semiconductor quantum dots. This method, which requires fast, single-shot charge measurement, reliably extracts both large and small splittings. We perform this technique on an undoped, accumulation-mode $\mathrm{Si}$/$\mathrm{Si}\text{−}\mathrm{Ge}$ triple-quantum dot and find that the measured splitting varies smoothly as a function of confinement gate biases. Not only does this demonstration prove the value of having an in situ excited-state measurement technique as part of a standard tune-up procedure, but it also suggests that in typical $\mathrm{Si}$/$\mathrm{Si}\text{−}\mathrm{Ge}$ quantum-dot devices, spin blockade can be limited by lateral orbital excitation energies rather than valley splitting.

Phonons at gigahertz frequencies interact with electrons, photons, and atomic systems in solids, and therefore, have extensive applications in signal processing, sensing, and quantum technologies. Surface acoustic wave (SAW) resonators that confine surface phonons can play a crucial role in such integrated phononic systems due to small mode size, low dissipation, and efficient electrical transduction. To date, it has been challenging to achieve a high quality (Q) factor and small phonon mode size for SAW resonators at gigahertz frequencies. We present a methodology to design compact high-Q SAW resonators on lithium niobate operating at gigahertz frequencies. We experimentally verify designs and demonstrate Q factors in excess of $2\times {10}^4$ at room temperature ($6\times {10}^4$ at 4 Kelvin) and mode size as low as 1.87 ${\lambda }^2$. This is achieved by phononic band structure engineering, which provides high confinement with low mechanical loss. The frequency Q products (fQ) of our SAW resonators are greater than ${10}^{13}$. These high-fQ and small mode size SAW resonators could enable applications in quantum phononics and integrated hybrid systems with phonons, photons, and solid-state qubits.

We study the optical properties of tetravalent-vanadium impurities in $4H$ silicon carbide. Light emission from two crystalline sites is observed at wavelengths of 1.28 and $1.33\mu \mathrm{m}$, with optical lifetimes of 163 and 43 ns, respectively, which remains stable up to 50 and 20 K, respectively. Moreover, spectrally broad photoluminescence is observed up to room temperature. Group-theory and ab initio density-functional supercell calculations enable unequivocal site assignment and shed light on the spectral features of the defects. Specifically, our numerical simulations indicate that the site assignment is reversed with respect to previous assumptions. Our calculations show that vanadium in silicon carbide has highly favorable properties for the generation of single photons in the telecommunication wavelength regime. Combined with the available electronic and nuclear degrees of freedom, vanadium presents all the ingredients required for a highly efficient spin-photon interface.

We investigate the structural and quantum transport properties of isotopically enriched ${}^{28}\mathrm{Si}/{}^{28}{\mathrm{Si}\mathrm{O}}_2$ stacks deposited on 300-mm $\mathrm{Si}$ wafers in an industrial CMOS fab. Highly uniform films are obtained with an isotopic purity greater than 99.92%. Hall-bar transistors with an oxide stack comprising 10 nm of ${}^{28}\mathrm{Si}{\mathrm{O}}_2$ and 17 nm of ${\mathrm{Al}}_2{\mathrm{O}}_3$ (equivalent oxide thickness of 17 nm) are fabricated in an academic cleanroom. A critical density for conduction of $1.75\times {10}^{11}{\mathrm{cm}}^{-2}$ and a peak mobility of $9800{\mathrm{cm}}^2/\mathrm{Vs}$ are measured at a temperature of 1.7 K. The ${}^{28}\mathrm{Si}/{}^{28}{\mathrm{Si}\mathrm{O}}_2$ interface is characterized by a roughness of $\mathrm{\Delta }=0.4\mathrm{nm}$ and a correlation length of $\mathrm{\Lambda }=3.4\mathrm{nm}$. An upper bound for valley splitting energy of $480\mu \mathrm{eV}$ is estimated at an effective electric field of 9.5 MV/m. These results support the use of wafer-scale ${}^{28}\mathrm{Si}/{}^{28}{\mathrm{Si}\mathrm{O}}_2$ as a promising material platform to manufacture industrial spin qubits.

Superconducting quantum-computing architectures comprise resonators and qubits that experience energy loss due to two-level systems (TLSs) in bulk and interfacial dielectrics. An understanding of these losses is critical to improving performance in superconducting circuits. In this work, we present a method for quantifying the TLS losses of different bulk and interfacial dielectrics present in superconducting coplanar-waveguide (CPW) resonators. By combining statistical characterization of sets of specifically designed CPW resonators on isotropically etched silicon substrates with detailed electromagnetic modeling, we determine the separate loss contributions from individual material interfaces and bulk dielectrics. This technique for analyzing interfacial TLS losses can be used to guide targeted improvements to qubits, resonators, and their superconducting fabrication processes.

There have been multiple attempts to demonstrate that quantum annealing and, in particular, quantum annealing on quantum-annealing machines, has the potential to outperform current classical optimization algorithms implemented on CMOS technologies. The benchmarking of these devices has been controversial. Initially, random spin-glass problems were used, however, these were quickly shown to be not well suited to detect any quantum speedup. Subsequently, benchmarking shifted to carefully crafted synthetic problems designed to highlight the quantum nature of the hardware while (often) ensuring that classical optimization techniques do not perform well on them. Even worse, to date a true sign of improved scaling with the number of problem variables remains elusive when compared to classical optimization techniques. Here, we analyze the readiness of quantum-annealing machines for real-world application problems. These are typically not random and have an underlying structure that is hard to capture in synthetic benchmarks, thus posing unexpected challenges for optimization techniques, both classical and quantum alike. We present a comprehensive computational scaling analysis of fault diagnosis in digital circuits, considering architectures beyond D-Wave quantum annealers. We find that the instances generated from real data in multiplier circuits are harder than other representative random spin-glass benchmarks with a comparable number of variables. Although our results show that transverse-field quantum annealing is outperformed by state-of-the-art classical optimization algorithms, these benchmark instances are hard and small in the size of the input, therefore representing the first industrial application ideally suited for testing near-term quantum annealers and other quantum algorithmic strategies for optimization problems.

Silicon spin qubits are a promising quantum-computing platform offering long coherence times, small device sizes, and compatibility with industry-backed device-fabrication techniques. In recent years, high-fidelity single-qubit and two-qubit operations have been demonstrated in $\mathrm{Si}$. Here we demonstrate coherent spin control in a quadruple quantum dot fabricated from isotopically enriched ${}^{28}\mathrm{Si}$. We tune the ground-state charge configuration of the quadruple dot down to the single-electron regime and demonstrate tunable interdot tunnel couplings as large as 20 GHz, which enables exchange-based two-qubit gate operations. Site-selective single spin rotations are achieved with the use of electric dipole spin resonance in a magnetic field gradient. We execute a resonant controlled-not gate between two adjacent spins in 270 ns.

Quantum networks are the ultimate target in quantum communication, where many connected users can share information carried by quantum systems. The keystones of such structures are the reliable generation, transmission, and manipulation of quantum states. Two-dimensional quantum states, qubits, are steadily adopted as information units. However, high-dimensional quantum states, qudits, constitute a richer resource for future quantum networks, exceeding the limitations imposed by the ubiquitous qubits. The generation and manipulation of such $D$-level systems have been improved over the last ten years, but their reliable transmission between remote locations remains the main challenge. Here, we show how a recent air-core fiber supporting orbital angular momentum (OAM) modes can be exploited to faithfully transmit $D$-dimensional states. Four OAM quantum states and their superpositions are created, propagated in a 1.2-km-long fiber and detected with high fidelities. In addition, three quantum-key-distribution protocols are implemented as concrete applications to assert the practicality of our results. This experiment enhances the distribution of high-dimensional quantum states, attesting the orbital angular momentum as a vessel for the future quantum network.

Superconducting coplanar-waveguide resonators that can operate in strong magnetic fields are important tools for a variety of high-frequency superconducting devices. Magnetic fields degrade resonator performance by creating Abrikosov vortices that cause resistive losses and frequency fluctuations or suppress the superconductivity entirely. To mitigate these effects, we investigate lithographically defined artificial defects in resonators fabricated from $\mathrm{Nb}$-$\mathrm{Ti}\text{−}\mathrm{N}$ superconducting films. We show that by controlling the vortex dynamics, the quality factor of resonators in perpendicular magnetic fields can be greatly enhanced. Coupled with the restriction of the device geometry to enhance the superconductors critical field, we demonstrate stable resonances that retain quality factors $\simeq {10}^5$ at the single-photon power level in perpendicular magnetic fields up to $B_{\perp }\simeq 20\mathrm{mT}$ and parallel magnetic fields up to $B_{\parallel }\simeq 6\mathrm{T}$. We demonstrate the effectiveness of this technique for hybrid systems by integrating an $\mathrm{In}$-$\mathrm{Sb}$ nanowire into a field-resilient superconducting resonator and use it to perform fast charge readout of a gate-defined double quantum dot at $B_{\parallel }=1\mathrm{T}$.

Universal unitary photonic devices can apply arbitrary unitary transformations to a vector of input modes and provide a promising hardware platform for fast and energy-efficient machine learning using light. We simulate the gradient-based optimization of random unitary matrices on universal photonic devices composed of imperfect tunable interferometers. If device components are initialized uniform randomly, the locally interacting nature of the mesh components biases the optimization search space toward banded unitary matrices, limiting convergence to random unitary matrices. We detail a procedure for initializing the device by sampling from the distribution of random unitary matrices and show that this greatly improves convergence speed. We also explore mesh architecture improvements such as adding extra tunable beam splitters or permuting waveguide layers to further improve the training speed and scalability of these devices.

We report the design of a diamond-based honeycomblike phononic network in which a mechanical resonator couples to three distinct phononic crystal waveguides. This two-dimensional (2D) phononic network extends an earlier study on one-dimensional (1D) phononic networks with closed mechanical subsystems. With a special design for the phononic band structures of the waveguides, any two neighboring resonators in the 2D network and the waveguide between them can form a closed mechanical subsystem, which enables nearest neighbor coupling and at the same time circumvents the scaling problems inherent in typical large mechanical systems. The nearest neighbor mechanical coupling allows the theoretical implementation of Molmer-Sorensen quantum gates between spins in adjacent mechanical resonators through phonon-assisted spin transitions. Honeycomblike phononic networks of spin qubits with nearest neighbor coupling can serve as an experimental platform for quantum computing and especially topological quantum error corrections.

Dispersive gate sensing (DGS) uses radio-frequency (rf) reflectometry to locally probe the quantum capacitance of a gate electrode. Applying DGS to heterostructure-based qubit devices, we report the repeated observation of anomalous signals that we attribute to pockets of charge in the potential landscape likely under, and surrounding, the surface gates that define quantum-dot qubits. Interestingly, these charge pockets appear to evade detection with conventional charge sensors but manifest strongly in the response of the gate sensor. Configuring a quantum point contact (QPC) as a highly localized heat source, we show how these charge pockets likely form close to the end of the gate electrodes, in close proximity to gate-defined qubits. The presence of uncontrolled charge may lead to offsets in gate voltage and further contribute to charge noise that produces decoherence in semiconductor qubits.

Optical detection of solid-state spin state has been widely used in research in quantum-information processing. The signal contrast is crucial for various applications. Here we demonstrate a time-gated fluorescence-detection method for enhancing the spin-state signal contrast of the nitrogen-vacancy (N-V) center in diamond. By adjusting the delay between the time gate and the excitation-laser pulse, we increase both the signal contrast and the signal-to-noise ratio for N-V spin detection. An enhancement ratio of approximately 1.9 times is achieved for the signal contrast. Using time-gated fluorescence detection, we extract the N-V-center information from high-level background, and demonstrate high-contrast quantum imaging of a nanoparticle’s stray magnetic field. Without any additional manipulation of the quantum state, we expect that this method can be used to improve the performance of various applications with the N-V center.

We exploit the nonlinearity arising from the spin-photon interaction in an $\mathrm{In}\mathrm{As}$ quantum dot to demonstrate phase shifts of scattered light pulses at the single-photon level. Photon phase shifts of close to ${90}^{\circ }$ are achieved using a charged quantum dot in a micropillar cavity. We also demonstrate a photon phase switch by using a spin-pumping mechanism through Raman transitions in an in-plane magnetic field. The experimental findings are supported by a theoretical model that explores the dynamics of the system. Our results demonstrate the potential of quantum-dot-induced nonlinearities for quantum information processing.

Radio-frequency (rf) reflectometry is implemented in hybrid semiconductor-superconductor nanowire systems designed to probe Majorana zero modes. Two approaches are presented. In the first, hybrid nanowire-based devices are part of a resonant circuit, allowing conductance to be measured as a function of several gate voltages approximately $40$ times faster than using conventional low-frequency lock-in methods. In the second, nanowire devices are capacitively coupled to a nearby rf single-electron transistor made from a separate nanowire, allowing rf detection of charge, including charge-only measurement of the crossover from $2e$ interisland charge transitions at zero magnetic field to $1e$ transitions at axial magnetic fields above 0.6 T, where a topological state is expected. Single-electron sensing yields a signal-to-noise ratio exceeding 3 and a visibility of 99.8% for a measurement time of $1\mu \mathrm{s}$.

Yttrium orthosilicate (${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$, or YSO) has proved to be a convenient host for rare-earth ions used in demonstrations of microwave quantum memories and optical memories with microwave interfaces and shows promise for coherent microwave-optical conversion owing to its favorable optical and spin properties. The strong coupling required by such microwave applications could be achieved using superconducting resonators patterned directly on ${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$ and hence we investigate here the use of ${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$ as an alternative to sapphire or silicon substrates for superconducting-hybrid-device fabrication. A $\mathrm{Nb}\mathrm{N}$ resonator with frequency 6.008 GHz and low-power quality factor $Q\approx 400000$ is fabricated on a ${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$ substrate doped with isotopically enriched ${}^{145}\mathrm{Nd}$. Measurements of dielectric loss yield a loss tangent $\tan \delta =4\times {10}^{-6}$, comparable to that of sapphire. Electron spin resonance (ESR) measurements performed using the resonator show the characteristic angular dependence expected from the anisotropic ${}^{145}\mathrm{Nd}$ spin and the coupling strength between the resonator and the electron spins is in the high-cooperativity regime ($C=30$). These results demonstrate that ${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$ is an excellent substrate for low-loss, high-$Q$ microwave resonators, especially in applications for coupling to optically accessible rare-earth spins.

When probed with current pulses, Josephson junctions and superconducting wires exhibit stochastic switching from a superconducting to a stable nonzero-voltage state. The electrical current dependence of the switching probability (the so-called $\mathsf{S}$ curve) or the switching-current distribution is a fingerprint of the physics governing the escape process. This work addresses the criterion of the independent switching event, which is important for the credibility of the switching measurements of superconducting wires and various Josephson junctions involving superconductor-insulator-superconductor tunnel junctions, proximity junctions, and Dayem nanobridges. Treating the Josephson junction as an electrical coin with a current-tuned switching probability, we investigate the effect of correlation between switching events on the switching statistics. We show that such a correlation originates from the thermal dynamics of the superconducting wire.