Using the time dependent Ginzburg-Landau equations, we simulate the inductive responses of a variety of thin-film systems containing patterned antidots with different sizes and shapes. The results for all shapes show that the kinetic inductance diverges as the applied current approaches a critical current that is below the BCS depairing current. Exploiting the similarity of the observed current-voltage behavior to that of Josephson junctions, we obtain an empirical equation that well fits the inductivity curve as a function of the applied current.

The variational quantum imaginary time-evolution algorithm is efficient in finding the ground state of a quantum Hamiltonian. This algorithm involves solving a system of linear equations in a classical computer and the solution is then used to propagate a quantum wave function. Here, we show that owing to the noisy nature of current quantum processors, such a quantum algorithm or the family of quantum algorithms will require single- and two-qubit gates with very low error probability to produce reliable results. Failure to meet such a condition will result in erroneous quantum data propagation even for a relatively small quantum circuit ansatz. Specifically, we provide the upper bounds on how the relative error in variational parameters’ propagation scales with the probability of noise in quantum hardware. We also present an exact expression of how the relative error in variational parameter propagation scales with the probability of partially depolarizing noise.

The presence of an external magnetic field leads to the loss of anisotropic physical information and is responsible for the broadening of spectra in sensing experiments with solid-state quantum sensors. However, the range of applicable sensing protocols at zero field was limited due to the incomplete manipulations. Here we propose and demonstrate a precise geometric control protocol in a spin-1 quantum sensor for zero-field multiquantum gates. By employing specific phased geometric controls, state leakage is prevented at zero field even under low control power, which allows much more complicated multipulse control especially under living biological conditions. Our method offers an approach to implement an arbitrary multiquantum gate on the spin-1 quantum sensor, rendering it a valuable tool for zero-field multipulse quantum technologies, such as nanoscale zero-field multidimensional nuclear magnetic resonance.

The silicon vacancy in silicon carbide has emerged as a promising quantum system embedded in an industry-friendly platform due to its long-lived spin qubits that can effectively interface with photonic qubits. However, the unique spin quantum number of 3/2 gives rise to a statistical mixture of the optically initialized ground-state spin sublevels, hindering its successful application as a high-fidelity spin-photon interface. Recent experimental breakthroughs have demonstrated a solution to this challenge by achieving pure-state preparation through simultaneous optical initialization and depletion of selected spin sublevels using electron spin resonance. Nonetheless, the underlying mechanism of this process remains poorly understood, and an efficient method for achieving deterministic initialization has not yet been explored. In this work, we present a comprehensive investigation of the selective initialization process by establishing a complete rate model. We offer a detailed explanation of the underlying mechanism and elucidate the trade-off between initialization fidelities and efficiencies, which are strongly influenced by the experimental parameters employed. Through a thorough exploration of a wide range of experimental parameters, we identify the optimal initialization process that allows for pure-state initialization fidelity exceeding 99%. Our study offers valuable insights into achieving high-fidelity spin-photon interface applications, such as quantum repeaters, based on silicon vacancies in silicon carbide.

In the cavity-magnonics system, the coupling between microwave photon and magnon is of beam-splitter (BS) interaction, which in general does not support steady-state entanglement in a vacuum environment. We propose a scheme to generate and enhance steady-state magnon-photon entanglement and one-way steering in cavity-magnonics system. By coupling microwave cavity mode and magnon mode to a common waveguide, the effective dissipative coupling of the cavity-magnonics system is derived. If there is only nondegenerate parametric (NP) dissipative coupling, entanglement is negatively affected by the effective gain of the cavity mode caused by the common waveguide environment. When the coherent coupling exists, effective gain of cavity mode can cancel the intrinsic dissipation. Consequently, the larger the intrinsic dissipation of cavity the better the entanglement. When the effective dissipative coupling with NP and BS coexist, the effective dissipation can be eliminated and the entanglement is significantly improved. In addition, we find that the asymmetric coupling of the two modes to the common waveguide leads to one-way steering, which means that the cavity mode can guide the magnetic mode, and the magnetic mode can not affect the cavity mode through local measurements. This property facilitates the development of the cavity-magnonics system towards quantum information processing.

We investigate the pure dephasing of a Josephson qubit due to the spectral diffusion of two-level systems that are close to resonance with the qubit. We identify the parameter regime in which this pure dephasing rate can be of the order of the energy relaxation rate and, thus, the relation $T_2=2T_1$ is violated for the qubit. This regime is reached if the dynamics of the thermal TLSs responsible for the spectral diffusion is sufficiently slower than the energy relaxation of the qubit. By adding periodic bias modulating the qubit frequency or TLS excitation energies we show that this pure dephasing mechanism can be mitigated, allowing enhancement of superconducting qubits’ coherence time. Mitigating pure dephasing, even if it is subdominant, is of special significance in view of recent suggestions for converting the dominant relaxation process ($T_1$) into erasure errors, leaving pure dephasing as the bottleneck for efficient quantum computation.

Dicke superradiance is essentially a case of correlated dissipation leading to the macroscopic quantum coherence. Superradiance for arrays of inverted emitters in free space requires interactions far beyond the nearest neighbor, limiting its occurrence to small emitter-emitter distances. Epsilon-near-zero (ENZ) materials, which exhibit infinite effective wavelengths, can mediate long-range interactions between emitters. We investigate the superradiance properties of two ENZ structures, namely plasmonic waveguides and dielectric photonic crystals, and demonstrate their potential to support near-ideal Dicke superradiance across expanded spatial domains. We employ a general method that we have developed to assess the occurrence of superradiance, which is applicable to various coupling scenarios and relies only on the decoherence matrix. Furthermore, by numerically examining the emission dynamics of the few-emitter systems, we distinguish the roles of quantum coherence at different stages of emission for the case of all-to-all interaction, and demonstrate that the maximum quantum coherence in the system can be determined using the maximum photon burst rate. The findings of this work have prospective applications in quantum information processing and light-matter interaction.

We investigate superradiant enhancements in the refrigeration performance of a set of $N$ three-level systems that are collectively coupled to a hot and a cold thermal reservoir and are additionally subject to collective periodic (circular) driving. Assuming the system-reservoir coupling to be weak, we explore the regime of stronger periodic driving strengths by comparing collective weak driving, Floquet-Lindblad, and Floquet-Redfield master equations. We identify regimes where the power injected by the periodic driving is used to pump heat from the cold to the hot reservoir and derive analytic sufficient conditions for them based on a cycle analysis of the Floquet-Lindblad master equation. In those regimes, we also argue for which parameters collective enhancements like a quadratic scaling of the cooling current with $N$ can be expected and support our arguments by numerical simulations.

The Toffoli gate takes a special place in the quantum information theory. It opens up a path for efficient implementation of complex quantum algorithms. Despite tremendous progress of the quantum processors based on the superconducting qubits, realization of a high-fidelity three-qubit operation is still a challenging problem. Here, we propose a way to perform a high-fidelity $CCZ$ gate on fluxoniums capacitively connected via a transmon qubit, activated by a microwave pulse on the coupler. The main advantages of the approach are relative quickness, simplicity of calibration and significant suppression of the unwanted longitudinal $ZZ$ interaction. We provide numerical simulation of a 95-ns-long gate of higher than 99.99% fidelity with realistic circuit parameters in the noiseless model and estimate an error of about 0.25% under the conventional decoherence rates.

As the quantum computing community gravitates towards understanding the practical benefits of quantum computers, having a clear definition and evaluation scheme for assessing practical quantum advantage in the context of specific applications is paramount. Generative modeling, for example, is a widely accepted natural use case for quantum computers, and yet has lacked a concrete approach for quantifying success of quantum models over classical ones. In this work, we construct a simple and unambiguous approach to probe practical quantum advantage for generative modeling by measuring the algorithm’s generalization performance. Using the sample-based approach proposed here, any generative model, from state-of-the-art classical generative models such as generative adversarial networks (GANs) to quantum models such as quantum circuit born machines, can be evaluated under the same conditions on a concrete well-defined framework. In contrast to other sample-based metrics for probing practical generalization, we leverage constrained optimization problems (e.g., cardinality-constrained problems) and use these discrete datasets to define specific metrics capable of unambiguously measuring the quality of the samples and the model’s generalization capabilities for generating data beyond the training set, but still within the valid solution space. Additionally, our metrics can diagnose trainability issues such as mode collapse and overfitting, as we illustrate when comparing GANs to quantum-inspired models built out of tensor networks. Our simulation results show that our quantum-inspired models have up to a 68 times enhancement in generating unseen unique and valid samples compared to the GANs explored here, and a ratio of 61:2 for generating samples with better quality than those observed in the training set. We foresee these metrics as valuable tools for rigorously defining practical quantum advantage in the domain of generative modeling, where finding a winning model remains an ongoing challenge as new architectures are developed.

A classical switch or router determines which of several inputs to the device passes through the output. In analogy, a quantum switch that coherently links the input to a superposition of several outputs plays an important role in quantum information processing, especially when operating at the fundamental limit where a single photon can be controlled. The controllable path superposition of propagating photonic qubits enabled by the quantum switch provides the basis for numerous quantum applications, including entanglement generation and distribution, enhanced quantum communication, and quantum random access memory. In this work, we realize such a quantum switch for microwave single photons with superconducting quantum circuits. We use a superconducting qubit to coherently switch the output path of single microwave photons and confirm quantum entanglement between the control qubit and the photons’ propagation path. We further showcase the generation of quantum entanglement between a time-bin-encoded propagating photonic qubit and a superconducting qubit with the quantum switch, thus providing a versatile approach to generate reconfigurable multinode entanglement in one step. As such, our work provides a prime building block for microwave quantum networks and modular superconducting quantum computing.

We investigate atomic chains with different spatial configurations coupled to a ring cavity comprising two counterpropagating traveling modes. We describe the collective atom-light scattering effect with a structure factor of the atomic chain and demonstrate that the interactions between the atoms and the cavity are controlled by the structure factor, resulting in distinctly different collective excitation modes and energy spectrum than for Fabry-Pérot cavities. Remarkably, we observe that a cavity dark mode emerges when the atomic spacings are integer multiples of the half-wavelength. The nodes of this standing-wave dark mode align precisely with the atomic positions, enabling intracavity field conversion without free-space scattering. By adjusting the configuration of the atomic chain, we realize tunable photon routing and a large optical phase shift with almost no photon loss, which can be used to implement versatile building blocks for optical quantum engineering.

The success of a future quantum internet will rest in part on the ability of quantum and classical signals to coexist in the same optical fiber infrastructure, a challenging endeavor given the orders of magnitude differences in flux of single-photon-level quantum fields and bright classical traffic. We theoretically describe and experimentally implement Procrustean entanglement concentration for polarization-entangled states contaminated with classical light, showing significant mitigation of crosstalk noise in dense wavelength-division multiplexing. Our approach leverages a pair of polarization-dependent loss emulators to attenuate highly polarized crosstalk that results from imperfect isolation of conventional signals copropagating on shared fiber links. We demonstrate our technique both on the tabletop and over a deployed quantum local area network, finding a substantial improvement of two-qubit entangled state fidelity from approximately 75% to over 92%. This local filtering technique could be used as a preliminary step to reduce asymmetric errors, potentially improving the overall efficiency when combined with more complex error-mitigation techniques in future quantum repeater networks.

Two-level system (TLS) defects constitute a major decoherence source of quantum information science, but they are generally less understood at material interfaces than in deposited films. Here we study surface TLSs at the metal-air interface, by probing them using a quasiuniform field within vacuum-gap (VG) capacitors of resonators. The VG capacitor has a nanogap that creates an order-of-magnitude larger contribution from the metal-air interface than typical resonators used in circuit QED. We measure three phenomena and find qualitative agreement with an interacting TLS model, where near-resonant TLSs experience substantial frequency jitter from the state switching of far-detuned low-frequency TLSs. First, we find that the loss in all of our VG resonators is weakly or logarithmically power dependent, in contrast to data from deposited dielectric films. Second, we add a saturation tone with power $P_{\mathrm{in}}$ to a transmission measurement and obtain the TLS Rabi frequency ${\mathrm{\Omega }}_0$. These data show a substantially weaker $P_{\mathrm{in}}$ dependence of ${\mathrm{\Omega }}_0$ than the prediction from the standard noninteracting TLS model. Lastly, we increase the temperature and find an increased TLS jitter rate and dephasing rate from power-dependent loss and phase noise measurements, respectively. We also anneal samples, which lowers the low-frequency TLS density and jitter rate, but the single-photon loss is found to be unchanged. The results are qualitatively consistent with a fast-switching interacting TLS model and they contrast the standard model of TLSs that describes TLSs independently.

Fast measurements of quantum devices is useful in areas such as quantum sensing, quantum computing, and nanodevice quality analysis. Here, we develop a superconductor-semiconductor multimodule microwave assembly to demonstrate charge-state readout at the state of the art. The assembly consist of a superconducting readout resonator interfaced to a silicon-on-insulator (SOI) chiplet containing quantum dots (QDs) in a high-$\kappa$ nanowire transistor. The superconducting chiplet contains resonant and coupling elements as well as $LC$ filters that, when interfaced with the silicon chip, result in a resonant frequency $f=2.12$ GHz, a loaded quality factor $Q=850$, and a resonator impedance $Z=470\mathrm{\Omega }$. Combined with the large gate lever arms of SOI technology, we achieve a minimum integration time for single and double QD transitions of 2.77 and 13.5 ns, respectively. We utilize the assembly to measure charge noise over 9 decades of frequency up to 500 kHz and find a $1/f$ dependence across the whole frequency spectrum as well as a charge-noise level of $4\text{μ}\mathrm{eV}/\sqrt{\text{Hz}}$ at 1 Hz. The modular microwave circuitry presented here can be directly utilized in conjunction with other quantum devices to improve the readout performance as well as enable large bandwidth noise spectroscopy, all without the complexity of superconductor-semiconductor monolithic fabrication.

The creation of fluorescent defects in silicon is a key stepping stone toward assuring the integration perspectives of quantum photonic devices into existing technologies. Here, we demonstrate the creation, by femtosecond laser annealing, of W and G centers in commercial silicon on insulator (SOI) previously implanted with ${}^{12}$$\mathrm{C}$${}^{+}$ ions. Their quality is comparable to that found for the same emitters obtained with conventional implantation processes; as quantified by the photoluminescence radiative lifetime, the broadening of their zero-phonon line (ZPL), and the evolution of these quantities with temperature. In addition to this, we show that both defects can be created without carbon implantation and that we can erase the G centers by annealing while enhancing the W-center emission. These demonstrations are relevant to the deterministic and operando generation of quantum emitters in silicon.

In this work, we develop a supervised learning model for implementing robust quantum control in composite-pulse systems, where the training parameters can be either phases, detunings, or Rabi frequencies. This model exhibits great resistance to all kinds of systematic errors, including single, multiple, and time-varying errors. We propose a modified gradient descent algorithm for adapting the training of phase parameters, and show that different sampling methods result in different robust performances. In particular, there is a trade-off between high fidelity and robustness for a given number of training parameters, and both can be simultaneously enhanced by increasing the number of training parameters (pulses). For its applications, we demonstrate that the current model can be used for achieving high-fidelity arbitrary superposition states and universal quantum gates in a robust manner. This work provides a highly efficient learning model for fault-tolerant quantum computation by training various physical parameters.

Quantum entanglement is a fundamental physical resource that enables a range of information-processing tasks with no efficient solution on classical computers. In this paper, we construct a class of general maximum entangled states by parameterizing the coefficients of Bell states. By calculating, we obtain the local unitary operations required to realize the mutual transformation of the constructed states, analogous to Pauli operators acting on one particle of a Bell state. Interestingly, the local unitary operations on the two particles of Bell states are identical, whereas for the constructed states, they must differ in certain cases. To explore the potential applications of these maximum entangled states and the obtained local unitary operations, we design a class of measurement-device-independent quantum secure direct communication protocols based on entanglement swapping. In the class of designed protocols, entanglement swapping is realized between a constructed maximum entangled state and a Bell state. We demonstrate that the class of designed protocols have no information leakage and can resist intercept-and-resend attacks, measure-resend attacks, and collective attacks. In addition, the unconditional security of the class of designed protocols is proved. We believe that the constructed maximum entangled states and the obtained local unitary operations may find applications in other quantum information-processing fields.