Currently, quantum-key-distribution (QKD) systems are evolving toward high-speed implementations to meet the demands of large-scale data encryption. High-performance, high-speed gated single-photon detectors (SPDs) are essential for enabling such systems. However, the practical security of these detectors remains a critical concern. Recently, an attack targeting high-speed sine-gated SPDs was reported, in which the detector was blinded by deceiving the width discrimination mechanism. Inspired by this work, we further optimize the attack strategy by customizing the blinding pulse and modifying the temporal offset between the blinding pulse and the gate signal, thereby achieving a more effective attack. Experimental and simulation results show that in short-distance communication scenarios, the additional quantum bit error rate (QBER) introduced by our proposed attack strategy is less than 0.5%. Of note, we propose and evaluate three practical countermeasures to defend against such attacks, which include auxiliary conventional discrimination monitoring, avalanche current monitoring, and temporal distribution analysis of detection events. These defense strategies enhance the robustness of high-speed SPDs and contribute to the development of more secure and reliable QKD systems.

To encode quantum information in semiconductor spin qubits, voltage pulses are necessary for initialization, gate operation, and readout. However, these pulses dissipate heat, shifting spin-qubit frequencies and reducing gate fidelities. The cause of this pulse heating in quantum dot devices is unknown. Here, we measure pulse heating using charged two-level fluctuators (TLFs) in $\mathrm{Si}$/($\mathrm{Si}$-$\mathrm{Ge}$) quantum dots. Specifically, we observe that voltage pulses on nearby gates tend to increase TLF switching rates and occupation biases. The amount of heating depends on the pulse amplitude and frequency, but not on the distance between the pulsed gates and the TLFs. The amount of heating also generally depends on the idling voltage of the pulsed gates, suggesting that electrons accumulated under or near the gates contribute to the heating. We hypothesize that reducing the area of the gates with electrons nearby could mitigate the heating.

To exploit the subnanometer dimensions of qubits for large-scale quantum information processing, corresponding control architectures require both energy and space efficiency, with the on-chip footprint of unit-cell electronics ideally being micrometer scale; however, the spin coherence of qubits in close packing is severely deteriorated by microwave crosstalk from neighboring control sites. Here, we present a crosstalk-mitigation scheme using foundry microelectronics to address solid-state spins at sub-100-$\text{μ}\mathrm{m}$ spacing without the need for qubit detuning. Using nitrogen-vacancy centers in nanodiamonds as qubit prototypes, we first demonstrate 10-MHz Rabi oscillations at milliwatt-level microwave power. By implementing active cancellation, we then demonstrate that the crosstalk field from neighboring lattice sites can be reduced to undetectable levels. We finally extend the scheme to show increased qubit control, or, effectively, enhanced spin coherence under crosstalk mitigation. Compatible with integrated optics, our results present a step toward scalable control across quantum platforms using silicon microelectronics.

Ultracold atoms coupled to optical cavities offer a powerful platform for studying strongly correlated many-body physics. Here, we propose an experimental scheme for creating diatomic molecules via cavity-enhanced photoassociation from an atomic condensate. The resulting system features long-range three-body interactions mediated by tripartite cavity-atom-molecule coupling. Beyond a critical pump strength, a self-organized square lattice phase for molecular condensate emerges, resulting in hybrid atom-molecule superradiance with spontaneous $U(1)$ symmetry breaking. Distinct from previously reported ultracold bosonic (fermionic) atomic superradiance, our findings demonstrate bosonic enhancement with a cubic scaling of steady-state photon number with total atom number. Additionally, strong photon-matter entanglement serves as a sensitive probe of superradiant quantum phase transition and may enable the study of super-Heisenberg-limited metrology enhanced by cavity-mediated three-body interactions. Our findings advance the frontier of quantum superchemistry and nonequilibrium many-body dynamics in cavity-coupled quantum gases.

Quantum key distribution (QKD) can distribute secret keys between two parties over a long distance. In practical applications, the demand for multiplexing QKD and conventional data communication through wavelength-division multiplexing (WDM) in the same fiber is increasing. However, the in-band noise that is difficult to filter out through experimental methods has become a key factor limiting the performance of WDM-QKD systems. In this work, we establish a practical decoy-state WDM-QKD model with advantage distillation (AD). We theoretically investigate the enhancement of AD technology on WDM-QKD systems and experimentally verify it. When the classical launch power is fixed at 8.6 dBm, the transmission distance can be extended by 80.3 km. When the classical received power is fixed at $-27$ dBm, the tolerable classical launch power can be extended by 5.4 dBm. In the experiment, we obtained positive key rates at the optimal block lengths of 2 and 3, respectively. Our results demonstrate the enormous potential of AD in WDM-QKD applications, providing a way for reducing the impact of noise on practical WDM-QKD systems.

Free-space continuous-variable quantum key distribution (CV-QKD) is attractive for its low cost, operational simplicity, and deployment flexibility. Discrete modulation further enhances practicality, yet rigorous security analysis over free-space channels is challenging because turbulence-induced fluctuations are compounded by the non-Gaussian nature of discrete modulation. We perform a fully numerical characterization of the free-space quantum channel, avoiding the simplifying assumptions inherent in existing analytical models. Together with a semidefinite-programming method, we establish an asymptotic security analysis framework of free-space discrete modulated (DM) CV-QKD without invoking Gaussian optimality. Unlike the cutoff behavior implied by the analytical models of transmittance probability distribution, our numerical results exhibit a pronounced long tail, indicating that the models assign larger probability mass to low transmittance and might overestimate turbulence-driven fluctuations. Based on the precise channel characterization, we compute tight lower bounds on the asymptotic secret key rate (SKR) under weak and moderate turbulence, and systematically assess postselection strategies. We find that optimized amplitude postselection increases the SKR, whereas phase postselection consistently degrades it. Additionally, in the absence of finite-size effects, transmittance-based postselection yields further gain.

Passive-state-preparation continuous-variable quantum key distribution (CVQKD) leverages the inherent field fluctuations of thermal source for quantum state preparation, thereby eliminating the need for modulators and quantum random number generators. It not only contributes to the development of low-cost, miniaturized CVQKD systems, but also inherently closes all modulation-related security loopholes. Despite its advantages in facilitating scalable and cost-effective QKD networks, current passive-state-preparation CVQKD implementations remain confined to point-to-point configurations, constrained by low secret key rates and the lack of rigorous security proofs for multiuser protocols. In this work, we propose a point-to-multipoint (PTMP) CVQKD protocol with passive-state preparation, supported by a comprehensive security framework that enables simultaneous and independent key generation between the passive state source and all end users. Simulation results demonstrate that the protocol can support hundreds of users over transmission distances of tens of kilometers, making it highly suitable for metropolitan-area and access-network applications. The simplified source preparation, combined with concurrent key generation, lays the foundation for a high-performance and scalable CVQKD network, offering a practical and efficient pathway toward the realization of large-scale QKD infrastructures.

Electrons floating on solid neon exhibit long charge-coherence times, making them attractive for hybrid quantum systems. When combined with high-quality high-impedance superconducting resonators and a local magnetic field gradient, this platform enables strong charge-photon and spin-charge coupling—key ingredients for scalable spin-qubit architectures. In this work, we demonstrate that $\mathrm{Nb}$-$\mathrm{Ti}$-$\mathrm{N}$ nanowire resonators maintain high quality factors around ${10}^5$ after depositing solid neon onto the resonators and subsequently loading electrons onto the neon surface, validating their suitability for electrons-on-neon platforms. Building on these experimental results, we theoretically analyze micromagnet designs and coupling strategies that can enable spin-photon interactions in this platform. Our analysis outlines performance targets for next-generation devices, showing that, at the charge sweet spot, spin-qubit gate fidelities exceeding 99.99% for single-qubit operations and 99.9% for two-qubit operations are achievable with natural neon.

Lithium niobate (LN) microring resonators, characterized by an exceptionally high second-order nonlinear coefficient and superior electro-optic tunability, serve as an outstanding platform for the precise control of integrated quantum frequency combs (QFCs). In this study, we introduce a bipartite entanglement criterion to investigate the pairwise entanglement characteristics of QFCs generated via the spontaneous parametric down-conversion (SPDC) process in lithium niobate microring resonators operating below threshold. Furthermore, we propose a universal framework for analyzing multimode squeezing in quadratic frequency combs, enabling the realization of ultrabroadband and high-degree multimode squeezing. We further reveal the underlying physical mechanism: modulation instability (MI), regulated by temporal walk-off control, not only enables the formation of frequency combs but also induces multimode squeezing in the corresponding resonant modes. This study uncovers the previously unexplored role of on-chip multimode squeezing in quadratic frequency combs while facilitating collective noise suppression across multiple modes, thus holding substantial potential for advancing quantum precision measurement and quantum information processing.

Demonstrating quantum speedup for approximate optimization of classical spin glasses is of current interest. Such a demonstration must be done with respect to the best-known scaling of classical heuristics at a given optimality gap of a given problem. For cubic-lattice classical Ising spin glasses, recent theoretical and experimental developments open up the possibility of showing quantum speedup for approximate optimization with quantum annealing. It is therefore desirable to understand the optimality-gap range over which such a speedup should be searched for. Here we show that on cubic-lattice tile-planting models, classical metaheuristics that are linear-time by construction can reach optimality gaps at which simulated annealing and parallel tempering exhibit superlinear scaling. This implies that the optimality gaps achieved by linear-time classical metaheuristics can serve as useful upper bounds for the optimality-gap range over which quantum speedups in approximate optimization should be searched for. We also explain how classical heuristics with fixed scaling that is beyond cubic can provide upper bounds to optimality-gap ranges for beyond-quadratic quantum speedups in approximate optimization. These results encourage the development of classical heuristics with fixed scaling that achieve optimality gaps as small as possible.

Hybrid quantum architectures that integrate matter and photonic degrees of freedom present a promising pathway toward scalable fault-tolerant quantum computing. This approach needs to combine well-established entangling operations between distant registers using photonic degrees of freedom with direct interactions between matter qubits within a solid-state register. The high-fidelity control of such a register, however, poses significant challenges. In this work, we address these challenges with pulsed control sequences that modulate all interspin interactions to preserve the nearest-neighbor couplings while eliminating unwanted long-range interactions. We derive pulse sequences, including broadband and selective gates, using composite-pulse and shaped-pulse techniques as well as optimal-control methods. This ensures a general pulse sequence in the presence of spin-position bias, robustness against static offset detunings, and Rabi-frequency fluctuations of the control fields. The control techniques developed here apply well beyond the present setting to a broad range of physical platforms. We demonstrate the efficacy of our methods for the resource-state generation for fusion-based quantum computing in four- and six-spin systems encoded in the electronic ground states of nitrogen-vacancy centers or other molecular solid-state qubits. We also outline other elements of the proposed architecture, highlighting its potential for advancing quantum computing technology.

Modern superconducting and semiconducting quantum hardware uses external charge and microwave flux drives to both tune and operate devices. However, each external drive is susceptible to low-frequency (e.g., $1/f$) noise that can drastically reduce the coherence time of the device unless the drive is placed at specific operating points that minimize the sensitivity to fluctuations. We show that operating a qubit in a driven frame using two periodic drives of distinct commensurate frequencies can have advantages over both monochromatically driven frames and static frames with constant offset drives. Employing Floquet theory, we analyze the spectral and lifetime characteristics of a two-level system under weak and strong bichromatic drives, identifying drive-parameter regions with high coherence (“sweet spots”) and highlighting regions where coherence is limited by additional sensitivity to noise at the drive frequencies (“sour spots”). We present analytical expressions for quasienergy gaps and dephasing rates, demonstrating that bichromatic driving can alleviate the trade-off between dc and ac noise robustness observed in monochromatic drives. This approach reveals continuous manifolds of doubly dynamical sweet spots, along which drive parameters can be varied without compromising coherence. Our results motivate further study of bichromatic Floquet engineering as a powerful strategy for maintaining tunability in high-coherence quantum systems.

Superconducting traveling-wave parametric amplifiers (TWPAs) have emerged as highly versatile devices, offering broadband amplification with quantum-limited noise performance. They hold significant potential for addressing the readout bottleneck in prototype quantum computers, enabling scalability. Key challenges with this technology include achieving sufficient gain with minimal gain ripple while maintaining low noise performance. Efficient phase matching between a weak signal and a strong pump over the entire length of the TWPA is critical to overcoming these challenges. We present an experimental demonstration of the inverse Kerr phase-matching technique in a TWPA, first proposed by Bell and Samolov [Phys. Rev. Appl. 4, 024014]. This method addresses several limitations of conventional dispersion engineering approaches of phase matching in the four-wave mixing parametric process in TWPAs, most notably the existence of an unusable region of gain near the pump frequency which typically corresponds to the region of most optimal phase matching and maximum gain. The inverse Kerr phase-matching approach allows for greater frequency separation between the region of optimal gain and pump, in situ tunability of the pump, minimal gain ripple, and a compact footprint which reduces losses. A TWPA employing the inverse Kerr phase-matching technique experimentally demonstrated 20 dB of gain over a 3-GHz instantaneous bandwidth, with a tunable bandwidth of 8 GHz, minimal gain ripple, and near quantum-limited noise performance, with 1.5 photons of added noise.

Superconducting resonators with high kinetic inductance play a central role in hybrid quantum circuits, enabling strong coupling to systems with small electric dipole moment and providing the nonlinear response required for parametric amplification. However, simultaneously achieving these resonators with high internal quality factors ($Q_i$) and resilience to strong magnetic fields remains challenging. In this work, we present a systematic and quantitative comparison between niobium nitride ($\mathrm{Nb}\mathrm{N}$) and granular aluminum (grAl) nanowire resonators, two widely used high-kinetic-inductance superconductors that offer complementary properties in terms of magnetic robustness and intrinsic nonlinearity. The devices are fabricated to exhibit comparable sheet kinetic inductance ($L_k\sim 100$ pH/sq), to allow a direct assessment of material-dependent behavior under identical experimental conditions. At zero magnetic field, grAl resonators display higher $Q_i$ compared with their $\mathrm{Nb}\mathrm{N}$ counterparts, whereas under applied magnetic fields, $\mathrm{Nb}\mathrm{N}$ resonators demonstrate significantly better resilience, maintaining $Q_i>{10}^4$ up to several tesla. A nonmonotonic enhancement of $Q_i$ in $\mathrm{Nb}\mathrm{N}$ near 1 T is observed, providing new insight into dissipation mechanisms in disordered superconductors. In contrast, grAl resonators show monotonic $Q_i$ evolution and a distinct critical field above which $Q_i$ rapidly decreases. Characterization of nonlinear properties at zero magnetic field reveals that the self-Kerr coefficient in grAl is more than an order of magnitude higher than in $\mathrm{Nb}\mathrm{N}$, making grAl particularly attractive for applications requiring pronounced nonlinear interactions. This comparative study clarifies how microscopic disorder and superconducting energy scales influence field resilience and nonlinearity in high-$L_k$ materials providing practical guidelines for designing magnetic-field compatible, high-impedance resonators for hybrid quantum circuits.

Parametric amplification offers a powerful means to enhance quantum interactions through field squeezing, yet it typically introduces additional noise, which accelerates quantum decoherence, a major obstacle for scalable quantum information processing. The squeezing field is implemented in cavities rather than continuous waveguides, thereby limiting its scalability for applications in quantum simulation. Giant atoms, which couple to waveguides at multiple points, provide a promising route to mitigate dissipation via engineered interference, enabling decoherence-free interactions. We extend the squeezing-amplified interaction to an alternative quantum platform combining giant atoms with traveling-wave parametric waveguides based on ${\chi }^{(2)}$ nonlinearity. By exploiting destructive interference between different coupling points, the interaction between giant atoms is not only significantly enhanced but also becomes immune to squeezed noise. Unlike conventional waveguide quantum electrodynamics without a squeezing pump, the giant emitters exhibit both exchange and pairing interactions, making this platform particularly suitable for simulating many-body quantum physics. More intriguingly, the strengths of these interactions can be smoothly tuned by adjusting the squeezing and coupling parameters. Our architecture thus provides a versatile and scalable platform for quantum simulation of strongly correlated physics and paves the way toward robust quantum control in many-body regimes.