The afterpulse effect is an intrinsic characteristic of the single-photon avalanche photodiode that has been widely used in quantum key distribution (QKD). As QKD systems move into the gigahertz regime, the afterpulse effect is no longer ignorable, which will lead to a great deviation compared with the existing analytical model. Here we develop an analytical model to make QKD systems more afterpulse compatible. In addition, we obtain the secure key rate for our model with the analysis of statistical fluctuation using Hoeffding’s inequality and Azuma’s inequality. Our results show that the optimized parameters of the afterpulse-compatible model can provide a much higher key rate than the optimized parameters of the previous afterpulse-omitted model in the same situation.

We experimentally demonstrate long-distance continuous-variable quantum key distribution over a 50-km standard optical fiber based on continuous-variable Einstein-Podolsky-Rosen entangled states. The entanglement survives despite being distributed over a high-loss optical fiber channel. At a channel excess noise level of 0.01 shot-noise units, we achieve an asymptotic secret key rate of 0.03 bit per sample, which is superior to the optimized coherent-state protocol. The superiority is even more evident at a high-channel excess noise level of 0.1 shot-noise units. Our work paves the way toward practical applications of continuous-variable quantum key distribution under high amounts of excess channel noise.

The distributed absorption of photons in photodiodes induces an excess noise in continuous-wave photodetection above the transit-time roll-off frequency. We show that it can be treated as a frequency-dependent excess optical loss in homodyne detection. This places a limit on the bandwidth of high-accuracy homodyne detection, even if an ideal photodetector circuit is available. We experimentally evaluate the excess loss in two ways: a comparison of signal gain and shot-noise gain of one-port homodyne detection and a balanced homodyne detection of squeezed light at a 500-MHz sideband. These results agree with an analytic expression we develop, where the randomness of the photoabsorption is directly modeled by an intrusion of vacuum field. At 500 MHz, we estimate the excess loss at 14% for a $\mathrm{Si}$ photodiode with 860-nm incident light, while the numerical simulation predicts much smaller excess loss in $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ photodiodes with 1550 nm light.

The prospect of computational hardware with quantum advantage relies critically on the quality of quantum-gate operations. Imperfect two-qubit gates are a major bottleneck for achieving scalable quantum-information processors. Here, we propose a generalizable and extensible scheme for a two-qubit tunable coupler that controls the qubit-qubit coupling by modulating the coupler frequency. Two-qubit gate operations can be implemented by operating the coupler in the dispersive regime, which is noninvasive to the qubit states. We investigate the performance of the scheme by simulating a universal two-qubit gate on a superconducting quantum circuit, and find that errors from known parasitic effects are strongly suppressed. The scheme is compatible with existing high-coherence hardware, thereby promising a higher gate fidelity with current technologies.

As the hyperentanglement of photon systems presents lots of unique opportunities in high-capacity quantum networking, the hyperentanglement purification protocol (hyper-EPP) becomes a vital project work and the quality of its accomplishment attracts considerable attention recently. Here we present the first theoretical scheme of faithful hyper-EPP for nonlocal two-photon systems in two degrees of freedom (DOFs) by constructing several fidelity-robust quantum circuits for hyper-encoded photons. With this faithful hyper-EPP, the bit-flip errors in both the polarization and spatial-mode DOFs can be efficiently corrected and the maximal hyperentanglement in two DOFs could in principle be achieved by performing the hyper-EPP multiple rounds. Moreover, the fidelity-robust quantum circuits, parity-check quantum nondemolition detectors, and swap gates make this hyper-EPP works faithfully as the errors coming from practical scattering, in these quantum circuits, are converted into a detectable failure rather than infidelity. Furthermore, this hyper-EPP can be directly extended to purify photon systems entangled in single polarization or spatial-mode DOF and that hyperentangled in polarization and multiple-spatial-mode DOFs.

Nonadiabatic geometric quantum computation is promising as it is robust against certain types of local noises. However, its experimental implementation is challenging due to the need for complex control on multilevel and/or multiple quantum systems. Here, we propose to implement it on a two-dimensional square superconducting qubit lattice. In the construction of our geometric quantum gates, we only use the simplest and experimentally accessible control over the qubit states of the involved quantum systems, without introducing any auxiliary state. Specifically, our scheme is achieved by parametrically tunable all-resonant interaction, which leads to high-fidelity quantum gates. Meanwhile, this simple implementation can be conveniently generalized to a composite scenario, which can further suppress the systematic error during the gate operations. In addition, universal nonadiabatic geometric quantum gates in decoherence-free subspace can also be realized based on the tunable coupling between only two transmon qubits, without consulting multiple qubits and only using two physical qubits to encode a logical qubit. Therefore, our proposal provides a promising way to achieve high-fidelity geometric manipulation for robust solid-state quantum computation.

Active research on mesoscopic quantum systems has increased our understanding of and ability to control quantum objects, allowing the construction of a universal form for quantum networks that consist of more than one physical system. This kind of quantum network is anticipated to enable the building of quantum infrastructure, such as long-distance quantum communication and distributed quantum computers, and motivates the establishment of photonic quantum interfaces that are compatible with physical systems. Here, a universal photonic quantum interface is experimentally developed with the benefit of a unique, specially designed entangled photon source. The detailed experimental results show that this configuration can satisfy all the urgent demands for a photonic quantum interface, including the accurate matching of the working wavelength and bandwidth and specifically, the entanglement ability ($F=89.6\mathrm{\%}$, $S=2.36\pm 0.03$). The realization of this universal photonic quantum interface is expected to expedite the construction of much more complex quantum networks and to be a major step in the area of optical engineering and control.

Based on standard silicon-on-insulator platforms, we propose an interferometer structure operating at telecom wavelengths that is driven by pump-induced nonlinear Kerr-effect. Special attention is given to the waveguide design in achieving the smallest full-vectorial effective mode area corresponding to the largest nonlinear phase shift. We present that this work is so far, in theory, the most promising approach for high-speed optical-driven quantum interference, whose realizability is reflected in two advantages: (1) the proposed strategy mitigates the pump-induced noise from the sideband, spontaneous four-wave mixing, and spontaneous Raman scattering and (2) the pulsed pump at $2.2\mu \mathrm{m}$ facilitates a relatively higher nonlinear response and negligible free-carrier absorption.

We present efficient methods to reliably characterize and tune gate-defined semiconductor spin qubits. Our methods are developed for double quantum dots in $\mathrm{Ga}\mathrm{As}$ heterostructures, but they can easily be adapted to other quantum-dot-based qubit systems. These tuning procedures include the characterization of the interdot tunnel coupling, the tunnel coupling to the surrounding leads, and the identification of various fast initialization points for the operation of the qubit. Since semiconductor-based spin qubits are compatible with standard semiconductor process technology and hence promise good prospects of scalability, the challenge of efficiently tuning the dot’s parameters will only grow in the near future, once the multiqubit stage is reached. With the anticipation of being used as the basis for future automated tuning protocols, all measurements presented here are fast-to-execute and easy-to-analyze characterization methods. They result in quantitative measures of the relevant qubit parameters within a couple of seconds and require almost no human interference.

We present a quantum-limited Josephson-junction-based three-wave-mixing parametric amplifier, the superconducting nonlinear asymmetric inductive element (SNAIL) parametric amplifier (SPA), which uses an array of SNAILs as the source of tunable nonlinearity. We show how to engineer the nonlinearity over multiple orders of magnitude by varying the physical design of the device. As a function of design parameters, we systematically explore two important amplifier nonidealities that limit dynamic range: the phenomena of gain compression and intermodulation distortion, whose minimization are crucial for high-fidelity multiqubit readout. Through a comparison with first-principles theory across multiple devices, we demonstrate how to optimize both the nonlinearity and the input-output port coupling of these SNAIL-based parametric amplifiers to achieve higher saturation power, without sacrificing any other desirable characteristics. The method elaborated in our work can be extended to improve all forms of parametrically induced mixing that can be employed for quantum-information applications.

Subnatural-linewidth single photons are generated from atomic ensembles and temporally reshaped as desired so that their temporal modes are eligible for constructing qubits. However, the photon-counting measurement scheme most commonly used in the time-bin basis does not directly give the complex coefficients between time-bins. On the other hand, a mode-sensitive measurement technique, such as homodyne tomography, is hard to implement since a mode-restricting cavity is not allowed in this reshaped photon source. Here, we utilize a cavity-free homodyne detection scheme for reconstructing the temporal-density matrix for the subnatural-linewidth single photons generated from a cold atomic cloud. The characterization of the pure temporal-spectral state of subnatural-linewidth single photons paves the way for exploiting the temporal-spectral degree of freedom to develop photonic-quantum information processing.

Cubic silicon carbide is an excellent platform for integration of defect qubits into established wafer-scale device architectures for quantum information and sensing applications, where a divacancy qubit, which is similar to the negatively charged nitrogen-vacancy (NV) center in diamond, has favorable coherence properties. We demonstrate by means of density-functional-theory calculations that for most types of distortion the $3C$ divacancy exhibits slightly smaller spin-strain coupling parameters but greater spin-stress coupling parameters in comparison with the diamond NV. We predict that high-quality $3C$-$\mathrm{Si}\mathrm{C}$ thin films hosting divacancy qubits are prospective platforms for quantum-enhanced pressure-sensor devices.

Faithfully transferring the quantum state is essential for quantum information processing. Here we demonstrate a fast (in 84 ns) and high-fidelity (99.2%) transfer of arbitrary quantum states in a chain of four superconducting qubits with nearest-neighbor coupling. This transfer relies on full control of the effective couplings between neighboring qubits, which is realized only by our parametrically modulating the qubits without increasing circuit complexity. Once the couplings between qubits fulfill a specific ratio, perfect quantum state transfer can be achieved in a single step, and is therefore robust to noise and accumulation of experimental errors. This quantum state transfer can be extended to a larger qubit chain and thus adds a desirable tool for future quantum information processing. The demonstrated flexibility of the coupling tunability is suitable for quantum simulation of many-body physics, which requires different configurations of qubit couplings.

We experimentally investigate the propagation of pulsed magnetostatic surface spin-wave (magnon) signals in an yttrium iron garnet (YIG) waveguide at millikelvin temperatures. Our measurements are performed in a dilution refrigerator at microwave frequencies. The excellent signal-to-noise ratio afforded by the low-temperature environment allows the propagation of the pulses to be observed in detail. The evolution of the envelope shape as the spin wave travels is consistent with calculations based on the known dispersion relation for YIG. We observe a temperature-dependent shift of the ferromagnetic resonance frequency below 4 K, which we suggest is due to the low-temperature properties of the substrate below the film, gallium gadolinium garnet. Our measurement and the accompanying calculations give insight into both low-temperature magnon dynamics in YIG and the feasibility of the use of propagating magnons in solid-state-quantum information processing.

We demonstrate a scalable device architecture that facilitates indirect exchange between singlet-triplet spin qubits, mediated by an intermediate quantum state. The device comprises five quantum dots, which can be independently loaded and unloaded via tunneling to adjacent reservoirs, avoiding charge latch-up common in linear dot arrays. In a step toward realizing two-qubit entanglement based on indirect exchange, the architecture permits precise control over tunnel rates between the singlet-triplet qubits and the intermediate state. We show that by our separating qubits by approximately 1 $\mu \mathrm{m}$, the residual capacitive coupling between them is reduced to approximately 7 $\mu \mathrm{eV}$.

The reconfigurability of integrated photonic chips plays a key role in current experiments in the area of linear-optical quantum computing. We demonstrate a reconfigurable multiport interferometer implemented as a femtosecond-laser-written integrated photonic device. The device includes a femtosecond-laser-written $4\times 4$ multiport interferometer equipped with 12 thermo-optical phase shifters, making it a universal programmable linear-optical circuit. We achieve a fast switching time for a single nested Mach-Zender interferometer of $\sim 10$ ms and quantitatively analyse the reconfigurability of the optical circuit. We believe that our results will improve the current state of quantum-optical experiments utilizing femtosecond-laser-written photonic circuits.

With use of the techniques of optomechanics, a high-$Q$ mechanical oscillator may serve as a link between electromagnetic modes of vastly different frequencies. This approach has successfully been exploited for the frequency conversion of classical signals and has the potential of performing quantum-state transfer between superconducting circuitry and a traveling optical signal. Such transducers are often operated in a linear regime, where the hybrid system can be described with linear response theory based on the Heisenberg-Langevin equations. While these equations are mathematically straightforward to solve, this approach yields little intuition about the dynamics of the hybrid system to aid the optimization of the transducer. As an analysis and design tool for such electrooptomechanical transducers, we introduce an equivalent-circuit formalism, where the entire transducer is represented by an electrical circuit. Thereby we integrate the transduction functionality of optomechanical systems into the toolbox of electrical engineering, allowing the use of its well-established design techniques. This unifying impedance description can be applied for both static (dc) and harmonically varying (ac) bias fields, accommodates arbitrary linear circuits, and is not restricted to the resolved-sideband regime. Furthermore, by establishing the quantized input-output formalism for the equivalent circuit, we obtain the scattering matrix for linear transducers using circuit analysis, and thereby have a complete quantum-mechanical characterization of the transducer. Hence, this mapping of the entire transducer to the language of electrical engineering both sheds light on how the transducer performs and can at the same time be used to optimize its performance by aiding the design of a suitable electrical circuit.

We develop and characterize an atom-guiding technique that loads $3\times {10}^6$ cold rubidium atoms into a hollow-core optical fiber, an order-of-magnitude greater than previously reported results. This result is possible because it is guided by a physically realistic simulation that can provide the specifications for a loading efficiency of 3.0% and a peak optical depth of 600. The simulation further shows that the loading efficiency is limited solely by the geometric overlap of the atom cloud and the optical guide beam, and is thus open to further improvement with experimental modification. The experimental arrangement allows observation of the real-time effects of light-assisted cold-atom collisions and background-gas collisions by tracking the dynamics of the cold-atom cloud as it falls into the fiber. The combination of these observations, and physical understanding from the simulation, allows estimation of the limits to loading cold atoms into hollow-core fibers.

Increasing coherence times of quantum bits is a fundamental challenge in the field of quantum computing. With long-lived qubits it is, however, inefficient to wait until the qubits have relaxed to their ground state after completion of an experiment. Moreover, for error-correction schemes it is important to rapidly reinitialize syndrome qubits. We present a simple pulsed qubit reset protocol based on a two-pulse sequence. A first pulse transfers the excited-state population to a higher excited qubit state and a second pulse transfer it into a lossy environment provided by a low-$Q$ transmission-line resonator, which is also used for qubit read-out. We show that the remaining excited-state population can be suppressed to $(1.7\pm 0.1)\mathrm{\%}$ and that this figure may be reduced by further improving the pulse calibration.