Neutral-atom arrays serve as an ideal platform to study quantum simulation and quantum logic gates, where intense efforts have been devoted to improving the fidelity of two-qubit gates. We report our recent findings in constructing a different type of two-qubit controlled phase gate protocol with neutral atoms enabled by the Rydberg blockade, which aims at both robustness and high fidelity. It relies upon a modulated driving pulse with a specially tailored smooth waveform to gain appropriate phase accumulations for quantum gates. The major features include finishing a gate operation within a single pulse, not necessarily requiring individual site addressing, not being sensitive to the exact value of the blockade shift, and suppressing the population leakage error and rotation error. Building upon this progress, we further develop an upgrade in the form of dual-pulse off-resonant modulated driving with the major distinct feature of Doppler insensitivity, in order to address the challenge to the fidelity of two-qubit gates caused by residual thermal motion of the cold atoms. In principle, the gate fidelity remains reasonably high over a relatively significant velocity range of the qubit atoms. Moreover, we anticipate that this will inspire future improvements in gate protocols for other types of qubit platforms, and the strategies used here may find applications in the area of quantum optimal control.

The shot-noise unit in continuous-variable quantum key distribution plays an important and fundamental role in experimental implementation, as it is used as a normalization parameter that contributes to performing security analyses and distilling key information. However, the traditional calibration procedure and detector model cannot cover all system noise in practical applications, which results in loopholes and influences the practical security. What is more, the traditional procedure is also rather complicated and has difficulty in providing compatibility with automatic operating systems. In this paper, we propose a calibration model based on a proposed trusted detector model, which could naturally close the loopholes in practical applications. It can help to identify the shot-noise unit in only one step, which can not only effectively simplify the evaluation process but also reduce the statistical fluctuations, while two steps are needed in the traditional method. We prove its feasibility and derive a complete version of the corresponding entanglement-based model. A detailed security analysis against arbitrary collective attacks and numerous simulation results in both the asymptotic-limit regime and the finite-size regime are provided. A proof-of-principle experiment is implemented, and the results indicate that our one-time calibration model can be employed as a powerful alternative method for calibrating the shot-noise unit. Our method paves the way for the deployment of continuous-variable quantum key distribution with real-time calibration and automatic operation.

Millimeter-wave superconducting devices offer a platform for quantum experiments at temperatures above 1 K, and new avenues for studying light-matter interactions in the strong coupling regime. Using the intrinsic nonlinearity associated with kinetic inductance of thin-film materials, we realize four-wave mixing at millimeter-wave frequencies, demonstrating a key component for superconducting quantum systems. We report on the performance of niobium nitride resonators around 100 GHz, patterned on thin (20–50-nm) films grown by atomic layer deposition, with sheet inductances up to $212\mathrm{pH}/◻$ and critical temperatures up to 13.9 K. For films thicker than 20 nm, we measure quality factors from $1\times {10}^4$ to $6\times {10}^4$, and explore potential loss mechanisms. Finally, we measure degenerate parametric conversion for a 95-GHz device with a forward efficiency up to $+16\mathrm{dB}$, paving the way for the development of nonlinear quantum devices at millimeter-wave frequencies.

Silicon spin qubits show great promise as a scalable qubit platform for fault-tolerant quantum computing. However, fast high-fidelity readout of charge and spin states, which is required for quantum error correction, has remained elusive. Radio-frequency reflectometry enables rapid high-fidelity readout of $\mathrm{Ga}\mathrm{As}$ spin qubits, but the large capacitances between accumulation gates and the underlying two-dimensional electron gas in accumulation-mode $\mathrm{Si}$ quantum-dot devices, as well as the relatively low two-dimensional electron gas mobilities, have made radio-frequency reflectometry challenging in these platforms. In this work, we implement radio-frequency reflectometry in a $\mathrm{Si}$/$\mathrm{Si}\text{−}\mathrm{Ge}$ quantum-dot device with overlapping gates by making minor device-level changes that eliminate these challenges. We demonstrate charge-state readout with a fidelity above $99.9\mathrm{\%}$ in an integration time of $300\mathrm{ns}$. We measure the singlet and triplet states of a double quantum dot via both conventional Pauli spin blockade and a charge latching mechanism, and we achieve maximum fidelities of $82.9$ and $99.0\mathrm{\%}$ in $2.08$- and $1.6$-$\mu \mathrm{s}$ integration times, respectively. We also use radio-frequency reflectometry to perform single-shot readout of single-spin states via spin-selective tunneling in microsecond-scale integration times.

Determining the state of a qubit on a time scale much shorter than its relaxation time is an essential requirement for quantum information processing. With the aid of a nondegenerate parametric amplifier, we demonstrate the continuous detection of quantum jumps of a transmon qubit with $90\mathrm{\%}$ fidelity of state discrimination. Entirely fabricated by standard two-step optical-lithography techniques, this type of parametric amplifier consists of a dispersion-engineered Josephson-junction (JJ) array. By using long arrays, containing ${10}^3\mathrm{JJs}$, we can obtain amplification in multiple eigenmodes with frequencies below $10\mathrm{GHz}$, which is the typical range for qubit readout. Moreover, if a moderate flux tunability of each mode is introduced, employing superconducting-quantum-interference-device junctions, a single amplifier device could potentially cover the entire frequency band between 1 and 10 GHz.

We introduce a near-quantum-limited amplifier with a large tunable bandwidth and high dynamic range—the Josephson array-mode parametric amplifier (JAMPA). The signal and idler modes involved in the amplification process are realized by the array modes of a chain of 1000 flux-tunable Josephson-junction-based nonlinear elements. The frequency spacing between the array modes is comparable to the flux tunability of the modes, ensuring that any desired frequency can be occupied by a resonant mode, which can further be pumped to produce high gain. We experimentally demonstrate that the device can be operated as a nearly quantum-limited parametric amplifier with 20 dB of gain at almost any frequency within a 4–12-GHz band. On average, it has a 3-dB bandwidth of 11 MHz and an input 1-dB compression power of $-108\mathrm{dBm}$, which can go as high as $-93\mathrm{dBm}$. We envision the application of such a device to the time- and frequency-multiplexed readout of multiple qubits, as well as to the generation of continuous-variable cluster states.

To obtain a complete description of a quantum system, one usually employs standard quantum state tomography, which, however, requires an exponential number of measurements to be performed and hence is impractical when the system’s size grows large. In this paper, we introduce a self-learning tomographic scheme based on the variational hybrid quantum-classical method. The key part of the scheme is a learning procedure, in which we learn a control sequence capable of driving the unknown target state coherently to a simple fiducial state, so that the target state can be directly reconstructed by applying the control sequence in reverse. In this manner, the state tomography problem is converted to a state-to-state transfer problem. To solve the latter problem, we use a closed-loop-learning control approach. Our scheme is experimentally tested using techniques of four-qubit nuclear magnetic resonance. The experimental results indicate that the proposed tomographic scheme can handle a broad class of states, including entangled states in the field of quantum information, as well as dynamical states of quantum many-body systems common to condensed-matter physics.

The performance of many control tasks with Rydberg atoms can be improved via suppression of the motion-induced dephasing between the ground state and Rydberg states of neutral atoms. This dephasing often occurs during the gap time when the atom is shelved in a Rydberg state before its deexcitation. This paper presents two theories about how to suppress this dephasing. First, by using laser fields to induce a specific extra phase change in the Rydberg state during the gap time, it is possible to faithfully transfer the Rydberg state back to the ground state after the gap. Although the Rydberg state transitions back and forth between different eigenstates during the gap time, it preserves the blockade interaction between the atom of interest and a nearby Rydberg excitation. This simple method of suppressing the motional dephasing of a flying Rydberg atom can be used in a broad range of methods of quantum control of neutral atoms. Second, we find that the motional dephasing can also be suppressed by using a transition in a “V”-type dual-rail configuration. The left (right) arm of this “V” represents a transition to a Rydberg state $|r_{1(2)}⟩$ with a Rabi frequency $\mathrm{\Omega }{e}^{ikz}(\mathrm{\Omega }{e}^{-ikz})$, where $z$ is frozen without atomic drift, but changes linearly in each experimental cycle. Such a configuration is equivalent to a transition between the ground state and a hybrid time-dependent Rydberg state with a Rabi frequency $\sqrt{2}\mathrm{\Omega }$, such that there is no phase error whenever the state returns to the ground state. We study two applications of the second theory: (i) it is possible to faithfully transfer the atomic state between a hyperfine ground state and Rydberg states $|r_{1(2)}⟩$ with no gap time between the excitation and deexcitation; and (ii) if infrared laser fields are added to induce a transition between $|r_{1(2)}⟩$ and a nearby Rydberg state $|r_3⟩$ via a greatly detuned low-lying intermediate state during the gap time, the atom can keep its internal state in the Rydberg level as well as adjust the population branching in $|r_{1(2)}⟩$ during the gap time. This allows an almost perfect Rydberg deexcitation after the gap time, making it possible to recover a high fidelity in a Rydberg blockade gate. These theories pave the way for high-fidelity quantum control over neutral Rydberg atoms without cooling qubits to motional ground states in optical traps.

A phononic crystal formed in a suspended membrane provides full confinement of hypersonic waves and thus realizes a range of chip-scale manipulations. In this paper, we demonstrate mode-resolved real-space characterization of mechanical-vibration properties in cavities and waveguide systems. Multiple resonant modes in various designed cavities are independently characterized, and wavelength-scale high-$Q$ resonances up to $Q=4200$ under atmospheric conditions are confirmed. This also reveals that the waveguide allows us to observe mode-resolved wave transmission and thereby drive evanescently coupled cavities. The methods offer a significant tool with which to build compact and low-power microwave phononic circuitry for applications to signal processing and hybrid quantum systems.

Superconducting-nanowire single-photon detectors promise efficient (approximately $100\mathrm{\%}$) and fast (approximately ${10}^9$ counts/s) detection of light at the single-photon level. They constitute one of the building blocks to realize integrated quantum optical circuits in a waveguide architecture. The optical response of single-photon detectors, however, is limited to measure only the presence of photons. It lacks the capability to resolve the spectrum of a possible broadband illumination. In this work, we propose the optical design for a superconducting-nanowire single-photon spectrometer in an integrated optical platform. We use a cascade of cavities with different resonance wavelengths side-coupled to a photonic crystal bus waveguide. This allows us to demultiplex different wavelengths into different spatial regions, where individual superconducting nanowires that measure the presence of single photons are placed next to these cavities. We use temporal coupled-mode theory to derive the optimal conditions to achieve a high absorption efficiency in the nanowire with fine spectral resolution. It is shown that the use of a mirror at the end of the cascaded system that terminates the photonic crystal bus waveguide increases the absorption efficiency up to unity, in principle, in the absence of loss. The expected response is demonstrated by full-wave simulations for both two-dimensional and three-dimensional structures. Absorption efficiencies of about $80\mathrm{\%}$ are achieved both in two-dimensional structures for four cascaded cavities and in three-dimensional structures for two cascaded cavities. The spectral resolution achieved is about $1\mathrm{nm}$. We expect that the proposed setup, both analytically studied and numerically demonstrated in this work, will offer a great impetus for future quantum nanophotonic on-chip technologies.

It is well known that surface codes are favorable candidates toward realizing large-scale fault-tolerant quantum computation. While methods to reduce errors by surface codes have been well explored, it is still challenging to achieve small-enough errors as stated by the threshold theorem in practical experimental settings. This prompts the integration of surface codes and protected quantum operations to further mitigate the negative effect of errors, creating the challenge of executing protected surface codes. In this work, we propose one scheme to perform holonomic quantum operations in a physical system by use of auxiliary qubits. The system Hamiltonian describes a quantum spin model with adjustable coupling strength. Various noncommutable single-qubit and nontrivial two-qubit holonomic gates are applicable on the basis of the interaction between the auxiliary qubits and physical qubits. By considering a two-dimensional lattice of qubits, we explore the execution of surface codes. The utilization rate of physical qubits as data qubits is about $24.7\mathrm{\%}$ in executing the protected surface codes. Our scheme is based on an experimentally achievable Hamiltonian and thus it brings us closer to realizing protected quantum operations on quantum-error-correction codes with a modest resource.

The coupling between a superconducting qubit and a control line inevitably results in radiative decay of the qubit into the line. We propose a Josephson quantum filter (JQF), which protects the data qubit (DQ) from radiative decay through the control line without reducing the gate speed on the DQ. The JQF consists of a qubit strongly coupled to the control line of the DQ and its working principle is a subradiance effect that is characteristic of waveguide quantum electrodynamics setups. The JQF is a passive circuit element and is therefore suitable for integration in a scalable superconducting qubit system.

Focusing microcavities for surface acoustic waves (SAWs) produce highly localized strain and piezoelectric fields that can dynamically control excitations in nanostructures. Focusing transducers (FIDTs) that generate SAW beams that match nanostructure dimensions require pattern correction due to diffraction and wave-velocity anisotropy. The anisotropy correction is normally implemented by adding a quadratic term to the dependence of the wave velocity on the propagation angle. We show that a SAW focusing to a diffraction-limited size in $\mathrm{Ga}\mathrm{As}$ requires corrections that more closely follow the group-velocity wave front, which is not a quadratic function. Optical interferometric mapping of the resultant SAW displacement field reveals tightly focused SAW beams on $\mathrm{Ga}\mathrm{As}$ with a minimal beam waist. An additional set of Gouy-phase–corrected passive fingers creates an acoustic microcavity in the focal region with a small volume and a high quality factor. Our ${\lambda }_{\mathrm{SAW}}=5.6\mu \mathrm{m}$ FIDTs are expected to scale well to the approximately 500-nm wavelength regime needed to study strong coupling between vibrations and electrons in electrostatic $\mathrm{Ga}\mathrm{As}$ quantum dots.

We report cavity-enhanced Raman scattering from a single-crystal diamond membrane embedded in a highly miniaturized fully tunable Fabry-Perot cavity. The Raman intensity is enhanced 58.8-fold compared to the corresponding confocal measurement. The strong signal amplification results from the Purcell effect. We show that the cavity-enhanced Raman scattering can be harnessed as a narrow-band high-intensity internal light source. The Raman process can be triggered in a simple way by using an optical excitation frequency outside the cavity stop band and is independent of the lateral positioning of the cavity mode with respect to the diamond membrane. The strong Raman signal emerging from the cavity output facilitates in situ mode matching of the cavity mode to single-mode collection optics; it also represents a simple way of measuring the dispersion and spatial intensity profile of the cavity modes. The optimization of the cavity performance via the strong Raman process is extremely helpful in achieving efficient cavity outcoupling of the relatively weak emission of single color centers such as nitrogen-vacancy centers in diamond or rare-earth ions in crystalline hosts with a low emitter density.

The successes of superconducting quantum circuits at local manipulation of quantum information and photonics technology at long-distance transmission of the same have spurred interest in the development of quantum transducers for efficient, low-noise, and bidirectional frequency conversion of photons between the microwave and optical domains. We propose to realize such functionality through the coupling of electrical, piezoelectric, and optomechanical resonators. The coupling of the mechanical subsystems enables formation of a resonant mechanical supermode that provides a mechanically mediated, efficient single interface to both the microwave and optical domains. The conversion process is analyzed by applying an equivalent circuit model that relates device-level parameters to overall figures of merit for conversion efficiency $\eta$ and added noise $N$. These can be further enhanced by proper impedance matching of the transducer to an input microwave transmission line. The performance of potential transducers is assessed through finite-element simulations, with a focus on geometries in $\mathrm{Ga}\mathrm{As}$, followed by considerations of the $\mathrm{Al}\mathrm{N}$, ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$, and $\mathrm{Al}\mathrm{N}$-on-$\mathrm{Si}$ platforms. We present strategies for maximizing $\eta$ and minimizing $N$, and find that simultaneously achieving $\eta >50\mathrm{\%}$ and $N<0.5$ should be possible with current technology. We find that the use of a mechanical supermode for mediating transduction is a key enabler for high-efficiency operation, particularly when paired with an appropriate microwave impedance-matching network. Our comprehensive analysis of the full transduction chain enables us to outline a development path for the realization of high-performance quantum transducers that will constitute a valuable resource for quantum information science.

The continuous development of superconducting electronics is encouraging several studies on hybrid Josephson junctions (JJs) based on superconductor-ferromagnet-superconductor (SFS) heterostructures, as either spintronic devices or switchable elements in quantum and classical circuits. Recent experimental evidence of macroscopic quantum tunneling and of an incomplete 0-$\pi$ transition in tunnel-ferromagnetic spin-filter JJs could also enhance the capabilities of SFS JJs as active elements. Here, we provide a self-consistent electrodynamic characterization of $\mathrm{Nb}\mathrm{N}$/$\mathrm{Gd}\mathrm{N}$/$\mathrm{Nb}\mathrm{N}$ spin-filter JJs as a function of the barrier thickness, disentangling the high-frequency dissipation effects due to the environment from the intrinsic low-frequency dissipation processes. The fitting of the $I$-$V$ characteristics at 4.2 K and at 300 mK by using the tunnel-junction-microscopic model allows us to determine the subgap resistance $R_{\mathrm{sg}}$, the quality factor $Q$, and the junction capacitance $C$. These results provide the scaling behavior of the electrodynamic parameters as a function of the barrier thickness, which represents a fundamental step for the feasibility of tunnel-ferromagnetic JJs as active elements in quantum and classical circuits, and are of general interest for tunnel junctions other than conventional SIS JJs.