Recent advances in nanotechnology have enabled researchers to manipulate small collections of quantum-mechanical objects with unprecedented accuracy. In semiconductor quantum-dot qubits, this manipulation requires controlling the dot orbital energies, the tunnel couplings, and the electron occupations. These properties all depend on the voltages placed on the metallic electrodes that define the device, the positions of which are fixed once the device is fabricated. While there has been much success with small numbers of dots, as the number of dots grows, it will be increasingly useful to control these systems with as few electrode voltage changes as possible. Here, we introduce a protocol, which we call the “compressed optimization of device architectures” (CODA), in order both to efficiently identify sparse sets of voltage changes that control quantum systems and to introduce a metric that can be used to compare device designs. As an example of the former, we apply this method to simulated devices with up to 100 quantum dots and show that CODA automatically tunes devices more efficiently than other common nonlinear optimizers. To demonstrate the latter, we determine the optimal lateral scale for a triple quantum dot, yielding a simulated device that can be tuned with small voltage changes on a limited number of electrodes.

In the last 60 years conventional solid and electrolyte gating has allowed sizable modulations of the surface carrier concentration in metallic superconductors, resulting in tuning of their conductivity and change of their critical temperature. Recent conventional gating experiments on superconducting metal nanostructures showed full suppression of the critical current without variation of the normal-state resistance and the critical temperature. These results still lack a microscopic explanation. In this article, we describe a complete set of gating experiments on $\mathrm{Ti}$-based superconducting Dayem-bridges and a proposed classical thermodynamic model that seems to account for several of our experimental findings. In particular, zero-bias resistance and critical-current $I_C$ measurements highlight the following: the suppression of $I_C$ with both polarities of gate voltage, the surface nature of the effect, the independence of critical temperature from the electric field, and the gate-induced growth of a subgap dissipative component. In addition, the temperature dependence of the Josephson critical current seems to show a transition from ballistic Kulik-Omelyanchuk behavior to Ambegaokar-Baratoff tunnellike behavior on increase of the electric field. Furthermore, the $I_C$ suppression persists in the presence of sizable perpendicular-to-plane magnetic fields. We propose a classical thermodynamic model able to describe some of the experimental observations of the present and previous work. Above all, the model grabs the bipolar-electric-field-induced suppression of $I_C$ and the emergence of a subgap dissipative component near full suppression of the supercurrent. Finally, applications using the effect discussed are proposed.

We present exact numerical calculations of the supercurrent density, inductance, and impurity-induced flux noise of cylindrical superconducting wires in the nonlocal Pippard regime, which occurs when the Pippard coherence length is greater than the London penetration depth. In this regime, the supercurrent density displays a peak away from the surface and changes sign inside the superconductor, signaling a breakdown of the usual approximation of local London electrodynamics with a renormalized penetration depth. Our calculations show that the internal inductance and the bulk flux noise power are enhanced in nonlocal superconductors. In contrast, the kinetic inductance is reduced and the surface flux noise remains the same. As a result, impurity spins in the bulk may dominate the flux noise in superconducting qubits in the Pippard regime, such as the ones using aluminum superconductors with a large electron mean free path.

With the increasing crossover between quantum information and machine learning, quantum simulation of neural networks has drawn unprecedentedly strong attention, especially for the simulation of associative memory in Hopfield neural networks due to their wide applications and relatively simple structures that allow easier mapping to the quantum regime. Quantum stochastic walk, a strikingly powerful tool to analyze quantum dynamics, has recently been proposed to simulate the firing pattern and associative memory with a dependence on the Hamming distance. We successfully map the theoretical scheme into a three-dimensional photonic quantum chip and realize quantum-stochastic-walk evolution through well-controlled detunings of the propagation constant. We demonstrate a good match rate of the associative memory between the experimental quantum scheme and the expected result for Hopfield neural networks. The ability of quantum simulation for an important feature of a neural network combined with the scalability of our approach through low-loss-integrated-chip and straightforward Hamiltonian engineering provides a primary but steady step toward photonic artificial-intelligence devices for optimization and computation tasks with greatly increased efficiencies.

The silicon monovacancy in $4H$-$\mathrm{Si}\mathrm{C}$ is a promising candidate for solid-state quantum information processing. We perform high-resolution optical spectroscopy on single $V2$ defects at cryogenic temperatures. We find favorable low-temperature optical properties that are essential for optical readout and coherent control of its spin and for the development of a spin-photon interface. The common features among individual defects include two narrow, nearly lifetime-limited optical transitions that correspond to $m_s=\pm 3/2$ and $m_s=\pm 1/2$ spin states with no discernable zero-field-splitting fluctuations. Initialization and readout of the spin states are characterized by time-resolved optical spectroscopy under resonant excitation of these transitions, showing significant differences between the $\pm 3/2$ and $\pm 1/2$ spin states. These results are well described by a theoretical model that strengthens our understanding of the quantum properties of this defect.

Trapped-ion quantum-information processors offer many advantages for achieving high-fidelity operations on a large number of qubits, but current experiments require bulky external equipment for classical and quantum control of many ions. We demonstrate the cryogenic operation of an ion trap that incorporates monolithically integrated high-voltage complementary metal-oxide semiconductor (CMOS) electronics ($\pm 8\mathrm{V}$ full swing) to generate surface-electrode control potentials without the need for external analog voltage sources. A serial bus programs an array of 16 digital-to-analog converters (DACs) within a single chip that apply voltages to segmented electrodes on the chip to control ion motion. Additionally, we present the incorporation of an integrated circuit that uses an analog switch to reduce voltage noise on trap electrodes due to the integrated amplifiers by over 50 dB. We verify the function of our integrated electronics by performing diagnostics with trapped ions and find noise and speed performance similar to those that we observe using external control elements.

Superconducting circuits are a versatile platform to implement a multitude of Hamiltonians that perform quantum computation, simulation, and sensing tasks. A key ingredient for realizing a desired Hamiltonian is the irradiation of the circuit by a strong drive. These strong drives provide an in situ control of couplings, which cannot be obtained by near-equilibrium Hamiltonians. However, as shown in this paper, out-of-equilibrium systems are easily plagued by complex dynamics, leading to instabilities. The prediction and prevention of these instabilities is crucial, both from a fundamental and application perspective. We propose an inductively shunted transmon as the elementary circuit optimized for strong parametric drives. Developing a numerical approach that avoids the built-in limitations of perturbative analysis, we demonstrate that adding the inductive shunt significantly extends the range of pump powers over which the circuit behaves in a stable manner.

The interfacing of quantum platforms via photonic links is a precursor for establishing scalable quantum networks. The connection of different types of quantum memories for hybrid networking requires the overcoming of the disparate photon wavelengths emitted by each quantum memory. Given achievements in quantum information processing with trapped-ion and neutral-atom architectures, a hybrid system with modular interconnectivity is advantageous. Here, we use a trapped ${}^{138}{\mathrm{Ba}}^{+}$ ion and a periodically poled lithium-niobate (PPLN) waveguide, with a fiber-coupled output, to demonstrate 19% end-to-end efficient quantum frequency conversion (QFC) of single photons from 493 to 780 nm and use fluorescence of the ion to produce light at the ${}^{87}\mathrm{Rb}{D}_2$ transition wavelength. To demonstrate the quantum nature of both the unconverted 493 nm photons and the converted photons near 780 nm, we observe strong quantum statics in their respective second-order intensity correlations. This work extends the range of intralaboratory networking between ions and networking between disparate quantum memories.

Dephasing induced by residual thermal photons in the readout resonator is a leading factor limiting the coherence times of qubits in the circuit quantum electrodynamics architecture. This residual thermal photon, on the order of ${10}^{-1}$–${10}^{-3}$, is suspected to arise from noise impinging on the resonator from its input and output ports. To address this problem, we designed and tested a new type of band-pass microwave attenuator that consists of a dissipative cavity well thermalized to the mixing chamber stage of a dilution refrigerator. By adding such a cavity attenuator in-line with a three-dimensional superconducting cavity housing a transmon qubit, we have reproducibly measured increased qubit coherence times. At base temperature, through a Hahn echo experiment, we measured $T_{2e}/2T_1=1.0\begin{array}{c}+0.0\\ -0.1\end{array}$ for two qubits over multiple cooldowns. Through noise-induced dephasing measurement, we obtained an upper bound of $2\times {10}^{-4}$ on the residual photon population in the fundamental mode of the readout cavity, which to our knowledge is the lowest value reported so far. These results validate an effective method for protecting qubits against photon noise, which can be developed into a standard technology for quantum circuit experiments.

Josephson circuits have been ideal systems with which to study complex nonlinear dynamics that can lead to chaotic behavior and instabilities. More recently, Josephson circuits in the quantum regime, particularly in the presence of microwave drives, have demonstrated their ability to emulate a variety of Hamiltonians that are useful for the processing of quantum information. In this paper, we show that these drives lead to an instability that results in the escape of the circuit mode into states that are not confined by the Josephson cosine potential. We observe this escape in a ubiquitous circuit: a transmon embedded in a 3D cavity. When the transmon occupies these free-particle-like states, the circuit behaves as though the junction had been removed and all nonlinearities are lost. This work deepens our understanding of strongly driven Josephson circuits, which is important for fundamental and application perspectives, such as the engineering of Hamiltonians by parametric pumping.

We have studied superconducting coplanar-waveguide (CPW) resonators fabricated from disordered films of aluminum. The very high kinetic inductance of these films, inherent to disordered materials, allows us to implement ultrashort (200 $\mu \mathrm{m}$ at 5 GHz resonance frequency) and high-impedance (up to 5 $\mathrm{k}\mathrm{\Omega }$) half-wavelength resonators. We have shown that the intrinsic losses in these resonators at temperatures $\lesssim 250$ mK are limited by resonator coupling to two-level systems (TLSs) in the environment. The demonstrated internal quality factors are comparable with those for CPW resonators made of conventional superconductors. The high kinetic inductance and well-understood losses make these disordered aluminum resonators promising for a wide range of microwave applications, which include kinetic-inductance photon detectors and superconducting quantum circuits.

We investigate a nonadiabatic holonomic operation that enables us to entangle two fixed-frequency superconducting transmon qubits attached to a common bus resonator. Two coherent microwave tones are applied simultaneously to the two qubits and drive transitions between the first excited resonator state and the second excited state of each qubit. The cyclic evolution within this effective three-level $\mathrm{\Lambda }$-type system gives rise to a holonomic operation entangling the two qubits. Two-qubit states with 95% fidelity, limited mainly by charge noise of the current device, are created within 213 ns. This scheme is a step toward implementation of a swap-type gate directly in an all-microwave controlled hardware platform. By extending the available set of two-qubit operations in the fixed-frequency qubit architecture, the proposed scheme may find applications in near-term quantum applications using variational algorithms to efficiently create problem-specific trial states. We illustrate this point by computing the ground state of molecular hydrogen using the holonomic operation.

Using a two-temperature model coupled with the modified time-dependent Ginzburg-Landau equation, we calculate the delay time ${\tau }_d$ in the appearance of a growing normal domain in the current-biased superconducting strip after absorption of the single photon. We demonstrate that ${\tau }_d$ depends on the place in the strip where the photon is absorbed and monotonically decreases with an increasing current. We argue that the variation of ${\tau }_d$ (timing jitter), connected either with position-dependent response or Fano fluctuations, could be as small as the lowest relaxation time of the superconducting order parameter, approximately $\hslash /k_B{T}_c$ ($T_c$ is the critical temperature of the superconductor), when the current approaches the depairing current.

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.

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.

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.

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.

Surface noise is a detrimental issue for sensing devices based on shallow nitrogen-vacancy (N-$V$) color-center diamonds. A recent experiment indicates that electric-field noise is significant compared to magnetic-field noise. They also found that the electric-field noise can be reduced with a protective surface layer, although the mechanism of noise reduction is not well understood. We examine the effect of a protective surface layer on the noise spectrum, which is caused by surface-charge fluctuations. We use the fluctuation-dissipation theorem to calculate and analyze the noise spectrum for six different surface-layer materials typically used for N-$V$-center diamond devices. We find that four parameters largely affect the noise spectrum: the effective relaxation time, the effective loss tangent, the power-law exponent of the noise spectrum, and the layer thickness. Our results suggest that a surface-covering layer is indeed useful for decreasing surface noise, but which material is most suitable depends on the device operational frequency range.

Small spin ensembles play an important role in many areas of condensed-matter physics. Here we present a method to measure spin densities in very dilute spin systems. We report on surface and bulk spin-density measurements of diamond using ultrasensitive magnetic force microscopy with magnetic field gradients up to 0.5 $\mathrm{T}/\mu \mathrm{m}$. At temperatures between 25 and 800 mK, we measure the shifts in the resonance frequency and quality factor of a cantilever with a micromagnet attached to it. A recently developed theoretical analysis allows us to extract a surface spin density of 0.072 ${\mathrm{spins}/\mathrm{nm}}^2$ and a bulk spin density of 0.4 ppm from these data. In addition, we find an increase of the $T_1$ time of the surface spins in high magnetic field gradients due to the suppression of spin diffusion. Our technique is applicable to a variety of samples other than diamond, and could be of interest for several research fields where surface, interface or impurity bulk spin densities are an important factor.