Decoherence is a major challenge for quantum technologies, whose negative effects can be mitigated by quantum optimal control. The decoherence dynamics significantly varies depending on the characteristics of the system’s environment, consequently affecting the optimization outcomes. In this work, we investigate the optimization of a negatively charged nitrogen-vacancy (${\mathrm{NV}}^{-}$) center qubit-spin-inversion control pulse, and observe two error-compensation mechanisms, depending on how long the correlation time is compared to the pulse duration. Additionally, we analyze the effects of control space constraints and optimization options to identify a strategy to improve the optimization performance. Finally, we present experimental realizations of the optimized pulses, exhibiting the applicability of the procedure. Our work serves as a generic yet essential guide to implementing optimal control in the presence of realistic noise.

Efficient coupling between quantum emitters and optical cavities is essential for scalable quantum photonic technologies. Group-IV vacancy centers in diamond, particularly the negatively charged tin-vacancy center, have emerged as promising candidates due to their spectral stability, high Debye-Waller factor, and large orbital splitting in ground states. Here, we demonstrate controlled coupling of a single negatively charged tin-vacancy center hosted in a nanodiamond to a fully tunable Fabry-Pérot microcavity. At cryogenic temperatures, we access the weak-coupling regime and observe a transition from the bad-emitter regime to the bad-cavity regime as the optical transition of the color center narrows. At 4 K, a Purcell factor exceeding 1.7 is measured, confirming cavity-enhanced emission. The obtained results demonstrate the potential of $\mathrm{Sn}\text{−}{V}^{-}$ centers in nanodiamonds as a coherent single-photon source for quantum networks.

Noise resilience of quantum information processing is a crucial precondition to reach the fault-tolerance threshold. While resilience to many types of noise can be achieved through suitable control schemes, resilience to amplitude noise seems to be elusive within the common harmonic approximation for the bus mode of trapped ions. We show that weak anharmonicities admit control schemes that achieve amplitude-noise resilience consistent with state-of-the-art experimental requirements and that the required anharmonicities can be achieved with current standards of microstructured traps or even the intrinsically anharmonic Coulomb interaction. This approach applies broadly to any platform that employs a bosonic bus as a qubit coupler.

We propose a machine-learning-based approach enhanced by quantum reservoir computing (QRC) to estimate the zero-time second-order correlation function $g^{(2)}(0)$. Typically, measuring $g^{(2)}(0)$ requires single-photon detectors and time-correlated measurements. Machine learning may offer practical solutions by training a model to estimate $g^{(2)}(0)$ solely from average intensity measurements. In our method, emission from a given quantum source is first processed in QRC. During the inference phase, only intensity measurements are used, and these are then passed to a software-based decision-tree ensemble model. We evaluate this hybrid quantum-classical approach across a variety of quantum optical systems and demonstrate that it provides accurate estimates of $g^{(2)}(0)$. We further extend our analysis to assess the ability of a trained model to generalize beyond its training distribution, both to the same system under different physical parameters and to fundamentally different quantum sources. While the model may yield reliable estimates within specific regimes, its performance across distinct systems is generally limited.

Anonymity and privacy are two key properties of modern communication networks. In quantum networks, distributed quantum sensing has emerged as a powerful use case, with applications to clock synchronization, detecting gravitational effects, and more. In this work, we develop a new protocol that, for the first time, combines the different cryptographic properties of anonymity and privacy for the task of distributed parameter estimation. That is, we present a protocol that allows a selected subset of network participants to anonymously collaborate in estimating the average of their private parameters. Crucially, this is achieved without disclosing either the individual parameter values or the identities of the participants, neither to each other nor to the broader network.

We report on an industry-grade complementary metal-oxide semiconductor (CMOS)-compatible qubit fabrication approach using a CMOS pilot line, enabling a yield of functional devices that reaches 92.8%, with a resistance spread evaluated across the full wafer 200 mm in diameter of 12.4% and relaxation times $T_1$ approaching $80\text{μ}\mathrm{s}$. Furthermore, we conducted a comprehensive analysis of wafer-scale room-temperature (RT) characteristics collected from multiple wafers and fabrication runs, focusing on RT measurements and their correlation to low-temperature qubit parameters. From defined test structures, an across-wafer Josephson junction (JJ) area variation of 10.1% and oxide barrier variation of 7.2% was calculated. Additionally, from the room-temperature JJ characterization the qubit frequency can be derived at the wafer level by applying the Ambegaokar-Baratoff model before making low-temperature measurements. This sets the stage for precooldown wafer-level JJ evaluation and sorting. In particular, such early device characterization and validation are crucial for increasing the fabrication yield and qubit frequency targeting, which currently represent major scaling challenges. Furthermore, this enables the fabrication of large multichip quantum systems in the future. Our analysis highlights the great potential of CMOS-compatible industry-style fabrication of superconducting qubits for scalable quantum computing in a foundry pilot line cleanroom.

Multipartite entangled states are essential for multiuser quantum cryptography. While large-scale continuous-variable (CV) cluster states, particularly the dual-rail cluster state, have been well studied in measurement-based quantum computation, their cryptographic potential remains underexplored. Here, we propose a three-user conference key protocol using a CV dual-rail cluster state. By applying a node-coloring scheme to the infinite dual-rail graph, we create a six-mode pure graph state ideal for cryptographic tasks. Our results demonstrate near-GHZ (Greenberger-Horne-Zeilinger) performance for quantum conference key agreement (QCKA). Crucially, our protocol uniquely enables bipartite keys post-QCKA, which GHZ states cannot provide. It also surpasses two-mode squeezed vacuum states in generating bipartite keys within downstream-access networks. Considering finite-size effects and impure squeezed states, our scheme remains robust despite experimental imperfections. We also introduce an enhanced method to more accurately estimate bipartite key generation capacity in quantum networks, paving the way for practical multiuser quantum cryptography.

Sensing periodic fields using quantum sensors has been an active field of research. In many of these scenarios, the quantum state of the probe is flipped regularly by the application of $\pi$ pulses to accumulate information about the target periodic field. The emergence of a discrete time crystalline phase, as a nonequilibrium phase of matter, naturally provides oscillations in a many-body system with an inherent controllable frequency. They benefit from long coherence time and robustness against imperfections, which makes them excellent potential quantum sensors. In this paper, through theoretical and numerical analysis, we show that a disorder-free discrete time crystal probe can reach the ultimate achievable precision for sensing a periodic field. As the amplitude of the periodic field increases, the discrete time crystalline order diminishes, and the performance of the probe decreases remarkably. Nevertheless, the obtained quantum enhancement in the discrete time crystal phase, which is experimentally accessible using standard projective measurements, shows robustness against different imperfections and dephasing noise in the protocol. Finally, we propose the implementation of our protocol in ultracold atoms in optical lattices.

The spin qubits of color centers are extensively investigated for quantum sensing, communication, and information processing, with their states generally controlled using lasers and microwaves. However, it is challenging to effectively irradiate both lasers and microwaves onto color centers using small-footprint microwave waveguides or antennas that are compatible with semiconductor devices. We show experimentally that, by introducing a compact coplanar waveguide with a thin slit in its signal line, effective irradiation of both lasers and microwaves is enabled, allowing spin-state control of color centers created around the slit. Microwave magnetic fields parallel to the surface, intrinsically generated by a standard coplanar waveguide, persist even after loading the slit, which is necessary to control the color centers whose spin quantization axes are oriented perpendicular to the surface, while laser light for the initialization and readout of spin states can access the color centers through the slit. Continuous and pulsed optically detected magnetic resonance measurements are performed for the silicon vacancies ($V_{\mathrm{Si}}$) in silicon carbide $4H$-$\mathrm{Si}\mathrm{C}$(0001). Experimental results indicate that the spin states of $V_{\mathrm{Si}}$ are effectively controlled by the microwave magnetic fields parallel to the surface, which agrees with numerical results from electromagnetic field simulations. Our small-footprint waveguide is suitable for integrating color-center-based quantum sensors into semiconductor electronic devices and other small-scale systems.

Realizing scalable quantum interconnects necessitates the integration of solid-state quantum memories with foundry photonics processes. While prior photonic integration efforts have relied upon specialized, laboratory-scale fabrication techniques, this work demonstrates the monolithic integration of a quantum memory platform with low-loss foundry photonic circuits via back-end-of-line deposition. We deposited thin films of titanium dioxide (${\mathrm{Ti}\mathrm{O}}_2$) doped with erbium ($\mathrm{Er}$) onto silicon nitride nanophotonic waveguides and studied $\mathrm{Er}$ optical coherence at subkelvin temperatures with photon echo techniques. We suppressed optical dephasing through ex-situ oxygen annealing and optimized measurement conditions, which yielded an optical coherence time of $64\text{μ}\mathrm{s}$ (a 5-kHz homogeneous linewidth) and slow spectral diffusion of 27 kHz over 4 ms, results that are comparable to state-of-the-art $\mathrm{Er}$ nanophotonic devices. Combined with second-long electron spin lifetimes and demonstrated electrical control of $\mathrm{Er}$ emission, our findings establish $\mathrm{Er}$:${\mathrm{Ti}\mathrm{O}}_2$ on foundry photonics as a manufacturable platform for ensemble and single-ion quantum memories.

Efforts to scale up superconducting processors that employ flux qubits face numerous challenges, among which is the crosstalk created by neighboring flux lines, which are necessary to bias the qubits at the zero-field and ${\mathrm{\Phi }}_0/2$ sweet spots. A solution to this problem is to use symmetric gradiometric loops, which incorporate a flux-locking mechanism that, once a fluxon is trapped during cooldown, holds the device at the sweet spot and limits the need for active biasing. We demonstrate this technique by simultaneously locking multiple gradiometric fluxonium qubits in which an aluminum loop retains the trapped fluxon indefinitely. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux bias within ${\mathrm{\Phi }}_{\mathrm{eff}}=-3\times {10}^{-4}{\mathrm{\Phi }}_0$ from the target, corresponding to only $15\mathrm{\%}$ degradation in $T_{2,E}$ when operated in zero external field. The design strategy demonstrated here reduces integration complexity for flux qubits by minimizing crosstalk and potentially eliminating the need for local flux bias.

Radio-frequency measurements could satisfy DiVincenzo’s readout criterion in future large-scale solid-state quantum processors, as they allow for high bandwidths and frequency multiplexing. However, the scalability potential of this readout technique can only be leveraged if quantum device tuning is performed using exclusively radio-frequency measurements, that is, without resorting to current measurements. We demonstrate an algorithm that performs automatic coarse tuning of double quantum dots with only radio-frequency measurements by exploiting their bandwidth and impedance matching. The tuning was completed within a few minutes with minimal prior knowledge about the device. Our results show that it is possible to eliminate the need for transport measurements for quantum-dot tuning, paving the way for more scalable device architectures.

Cavity-mediated interaction between gate-defined quantum dots (QDs) has emerged as a promising platform to achieve long-distance spin-spin coupling. However, the realization of on-chip quantum networks and scalable quantum computing requires robust and high-fidelity remote quantum operations. In this work, we use Floquet modulation of gate voltages to fine-tune remote couplings between QDs. We show that the long-distance controlled-iswap gate can be well implemented by choosing suitable values of the frequency, amplitude, and phase of Floquet driving. Moreover, we propose a protocol to achieve high-fidelity quantum state transfer using phase modulation and the stimulated Raman adiabatic passage, which exhibits robustness to dephasing noises. Our work provides efficient and feasible solutions to control remote couplings between gate-defined QDs, and will greatly promote related theoretical studies and experimental implementations.

Practical implementations of quantum key distribution (QKD) extending beyond urban areas commonly use satellite links; however, the transmission of quantum states through the Earth’s atmosphere is highly susceptible to noise, restricting its application primarily to nighttime. High-dimensional (HD) QKD offers a promising solution to this limitation by employing high-dimensionally entangled quantum states. Although experimental platforms for HD QKD exist, previous security analyses have been limited to the asymptotic regime and have either relied on impractical measurements or employed computationally demanding convex optimization tasks, restricting the security analysis to low dimensions. In this work, we bridge this gap by presenting a composable finite-size security proof against both collective and coherent attacks for a general HD QKD protocol that uses only experimentally accessible measurements. In addition to the conventional yet impractical “one-shot” key rates, we provide a practical variable-length security argument that yields significantly higher expected key rates. This approach is particularly crucial under rapidly changing and turbulent atmospheric conditions, as encountered in free-space and satellite-based QKD platforms.

The demand for cryogenic memory components is driven by the need for ultrafast, low-power, and highly reliable computing systems. Phase-slip-based devices promise to fulfill all these requirements, with potential applications in both classical and quantum information processing. However, previous implementations have faced challenges due to inefficient writing and readout schemes. In this work, we address these limitations with a simple device design and measurement techniques inspired by circuit quantum electrodynamics. We present a memory element that stores information in the winding of a high-kinetic inductance superconducting loop, inductively coupled to a coplanar waveguide resonator. Using 2-$\text{μ}\mathrm{s}$ single-shot measurements, we bring the resonator to the steady state and achieve a readout fidelity of 99.698% with an active measurement time of just 25 ns.

Nonlinear equations are generally challenging to solve. As analytical solutions typically do not exist, numerical methods have been developed to find their solutions. In this article, building upon the classical Newton method and the recent quantum singular value transformation framework, we give a quantum algorithm for solving a system of nonlinear algebraic equations, in which each equation is a homogeneous polynomial of an even degree. A detailed analysis is then carried out to reveal that our method achieves polylogarithmic time relative to the number of variables. Furthermore, the number of required qubits is logarithmic in the number of variables. In particular, we also show that our method can be generalized to deal with polynomials of various types. Some potential applications stem from various contexts, such as the Gross-Pitaevski equation, the Lotka-Volterra equations, and the intersection of algebraic varieties. Our work thus marks a significant step forward toward quantum advantage in nonlinear science, highlighting a promising avenue for future exploration.

Over recent decades, quantum key distribution (QKD) has arisen as a promising solution for secure communications; however, like all cryptographic protocols, implementations of QKD can open security vulnerabilities. Until now, the study of physical vulnerabilities in QKD setups has focused primarily on the optical channel. In classical cryptoanalysis, power and electromagnetic side-channel analysis are powerful techniques that can be used to access secret information about the encryption key in symmetric-key algorithms. Such attacks have rarely been used in QKD, since they require an eavesdropper to have access to Alice’s or Bob’s setup; however, security proofs of QKD protocols generally assume that these setups are secure, making it crucial to understand the security measures required to ensure this protection. In this work, we propose and implement a power side-channel analysis of a QKD system by exploiting the power consumption of the electronic driver controlling the electro-optical components of the QKD transmitter. QKD modules typically require very precise electronic drivers, such as field-programmable gate arrays (FPGAs). Here, we show that the FPGA’s power consumption can leak information about the QKD operation and consequently the transmitted key. Our results are consistent and show critical information leakage, having reached a maximum accuracy of 73.35% in predicting transmitted qubits at a 100-MHz repetition frequency. We also discuss possible countermeasures to prevent such an attack.