Continuous-variable (CV) quantum communication is an important branch of quantum information technology and promises information-theoretical secure communication. High-performance and lightweight CV quantum devices are one of the keys to realizing quantum networks. Among them, the Gaussian-modulation module still has problems such as cumbersome photoelectric calibration and inconvenient use in the experimental stage, which introduces modulation noise and brings additional excess noise to the system, reducing the signal-to-noise ratio of the system and thus reducing system performance. To this end, by designing a field-programmable gate array and integrating all optical components (except the laser source), we achieved a miniaturized and low-cost Gaussian-modulation unit compatible with the existing fiber-optic communication infrastructure, realizing quantum signal modulation under adaptive photoelectric calibration. Taking CV quantum key distribution as the verification object, we analyzed the signal characteristics and the stability of the unit experimentally. The experimental results show that the proposed scheme exhibits quality Gaussian properties and correlations. Moreover, the temporal stability of the device is relatively high (the mean fluctuation is on the order of ${10}^{-31}$ and the variance fluctuation is 0.0363), which provides more possibilities for the subsequent popularization of quantum communication and the development of quantum information networks.

Gate-defined quantum dots in silicon-germanium heterostructures have become a compelling platform for quantum computation and simulation. Thus far, developments have been limited to quantum dots defined in a single plane. Here, we propose to advance beyond planar systems by exploiting heterostructures with multiple quantum wells. We demonstrate the operation of a gate-defined double quantum dot in a strained germanium double quantum well, where both quantum dots are tunnel coupled to both reservoirs and parallel transport occurs. We analyze the capacitive coupling to nearby gates and find both quantum dots to be accumulated under the central plunger gate. We extract their position and size, from which we conclude that the double quantum dots are vertically stacked in the two quantum wells. We discuss the challenges and opportunities of multilayer devices and outline some potential applications in quantum computing and quantum simulation.

The cross-resonance gate (cr gate) architecture that exploits fixed-frequency transmon qubits and fixed couplings is a leading candidate for quantum computing. Nonetheless, without the tunability of qubit parameters such as qubit frequencies and couplings, gate operations can be limited by the presence of quantum crosstalk arising from the always-on couplings. When increasing system sizes, this can become even more serious, considering frequency collisions caused by fabrication uncertainties. Here, we introduce a cr-gate-based transmon architecture with passive mitigation of both quantum crosstalk and frequency collisions. Assuming typical parameters, we show that $ZZ$ crosstalk can be suppressed while maintaining $XY$ couplings to support fast, high-fidelity cr gates. The architecture also allows one to go beyond the existing literature by extending the operating regions in which fast, high-fidelity cr gates are possible, thus alleviating the frequency-collision issue. To examine the practicality, we analyze the cr-gate performance in multiqubit lattices and provide an intuitive model for identifying and mitigating the dominant source of error. For the state-of-the-art precision in setting frequencies, we further investigate its impact on the gates. We find that $ZZ$ crosstalk and frequency collisions can be largely mitigated for neighboring qubits, while interactions beyond near-neighbor qubits can introduce new frequency collisions. As the strength is typically at the submegahertz level, adding weak off-resonant drives to selectively shift qubits can mitigate the collisions. This work could be useful for suppressing quantum crosstalk and improving gate fidelities in large-scale quantum processors based on fixed-frequency qubits and fixed couplings.

We investigated the heralded generation of two-qubit dual-rail-encoded states by programmable linear optics. Two types of scheme generating the states from four single photons, which is the minimum possible to accomplish the task, were considered. The schemes have different detection patterns heralding successful generation events; namely, one-mode heralding, in which the two auxiliary photons are detected in one mode, and two-mode heralding, in which single photons are detected in each of the two modes simultaneously. We show that the dependencies of the schemes’ success probabilities on the target state’s degree of entanglement are essentially different. In particular, one-mode heralding yields greater efficiency for highly entangled states if the programmable interferometers can explore the full space of the unitary transfer matrices. The reverse is the case for weakly entangled states, where two-mode heralding is better. We found a minimal decomposition of the scheme with two-mode heralding that is programmed by one variable phase shift. We infer that the linear optical schemes designed specifically for the generation of two-qubit states are more efficient than schemes implementing known linear optical gate–based circuits. Our results reveal a substantial reduction of physical resources needed to generate some types of nonmaximally entangled multiqubit states used in quantum computing.

High-dimensional entangled states in an orbital-angular-momentum (OAM) basis offer several unique advantages for quantum information applications. However, for the optimal performance of a given application, one requires a generation technique for OAM entangled states that is completely postselection-free and fully controllable. Nonetheless, despite several efforts in the past, no such technique currently exists. In this paper, we propose just such a technique and experimentally demonstrate postselection-free generation of up to about 150-dimensional OAM entangled states. We report the generation accuracy, which is a measure of the control, to be more than $98\mathrm{\%}$ for states with Gaussian and triangular OAM Schmidt spectra, and up to $90\mathrm{\%}$ for the maximally entangled OAM states, which have rectangular spectra.

Thin-film lithium niobate (LN) has become an enabling platform in the development of quantum integrated circuits, for applications including microwave-to-optical converters and electromechanical transducers. Incorporation of high-quality planar superconducting resonators will further elevate the performance of these devices. However, alongside the desired interactions originated from LN, its piezoelectric nature also introduces parasitic coupling between microwave and phononic modes, resulting in excessive dielectric loss. In this work, we investigate such coupling by studying niobium nitride resonators directly deposited on patterned LN thin films. By varying device configuration, we pinpoint that overlapping electric field with LN can inadvertently excite bulk acoustic modes within the substrate, inducing microwave loss independent of the acoustic dissipation rate. The experimental results are supported by simulation and theoretical modeling. Our approach can be applied to different piezoelectric systems and provide insights into the design of microwave components for quantum transduction devices.

We detect correlations in qubit-energy fluctuations of non-neighboring qubits defined in isotopically purified Si/Si-Ge quantum dots. At low frequencies (where the noise is strongest), the correlation coefficient reaches 10% for a next-nearest-neighbor qubit-pair separated by 200 nm. Correlations with the charge-sensor signal reach up to 70%, proving that the observed noise is of electrical origin. A simple theoretical model quantitatively reproduces the measurements and predicts a polynomial decay of correlations with interqubit distance. Our results quantify long-range correlations of noise in quantum-dot spin-qubit arrays, essential for their scalability and fault tolerance.

We present a blueprint for building a fault-tolerant quantum computer using the spin states of electrons on the surface of liquid helium. We propose to use ferromagnetic micropillars to trap single electrons on top of them and to generate a local magnetic field gradient. Introducing a local magnetic field gradient hybridizes charge and spin degrees of freedom, which allows us to benefit from both the long coherence time of the spin state and the long-range Coulomb interaction that affects the charge state. We present concrete schemes to realize single- and two-qubit gates and quantum nondemolition readout. In our framework, the hybridization of charge and spin degrees of freedom is large enough to perform fast qubit gates and small enough not to degrade the coherence time of the spin state significantly, which leads to the realization of high-fidelity qubit gates.

A distributed quantum computing system requires a quantum communication channel between spatially separated processing units. In superconducting circuits, such a channel can be realized by using propagating microwave photons to encode and transfer quantum information between an emitter and a receiver node. Here we experimentally demonstrate a superconducting circuit that deterministically transfers the state of a data qubit into a propagating microwave mode, with a process fidelity of 94.5%. We use a time-varying parametric drive to shape the temporal profile of the propagating mode to be time symmetric and with constant phase, so that reabsorption by the receiving processor can be implemented as a time-reversed version of the emission. We demonstrate a self-calibrating routine to correct for time-dependent shifts of the emitted frequencies due to the modulation of the parametric drive. Our work provides a reliable method to implement high-fidelity quantum state transfer and remote entanglement operations in a distributed quantum computing network.

Time-dependent drives play a crucial role in quantum computing efforts with circuit quantum electrodynamics. They enable single-qubit control and entangling logical operations, as well as qubit readout. However, their presence can lead to deleterious effects such as large ac-Stark shifts and unwanted qubit transitions ultimately reflected into reduced control or readout fidelities. Qubit cloaking was introduced by Lledó et al. [C. Lledó, R. Dassonneville, A. Moulinas et al., Nat. Commun. 14, 6313 (2023)] to temporarily decouple the qubit from the coherent photon population of a driven cavity, allowing for the application of arbitrary displacements to the cavity field while avoiding the deleterious effects on the qubit. For qubit readout, cloaking permits us to prearm the cavity with an, in principle, arbitrarily large number of photons, in anticipation of the qubit-state-dependent evolution of the cavity field, allowing for improved readout strategies. Here, we take a closer look at two such strategies: first, arm-and-release readout, introduced together with qubit cloaking, where after arming the cavity, the cloaking mechanism is released and the cavity field evolves under the application of a constant drive amplitude; and, second, an arm-and-longitudinal readout scheme, where the cavity drive amplitude is slowly modulated after the release. We show that the two schemes complement each other, offering an improvement over standard dispersive readout for any values of the dispersive interaction and the cavity decay rate, as well as any target measurement integration time. Our results provide a recommendation for improving qubit readout without changes to the standard circuit-QED architecture.

Interfacing the different building blocks of a future large-scale quantum network will require efficient and noiseless frequency conversion of quantum light. Nitrogen-vacancy centers in diamond are a leading candidate to form the nodes of such a network. However, the performance of a suitable converter remains a bottleneck, with existing demonstrations severely limited by parasitic noise arising at the target telecommunication wavelength. Here we demonstrate a platform for efficient low-noise quantum frequency conversion based on a monolithic bulk periodically poled potassium titanyl phosphate cavity and show its suitability for the conversion of 637-nm single photons from nitrogen-vacancy centers in diamond to telecommunication wavelengths. By resonantly enhancing the power of an off-the-shelf pump laser, we achieve an internal conversion efficiency of $(72.3\pm 0.4)\mathrm{\%}$ while generating noise of only $(110\pm 4)\mathrm{kHz}/\mathrm{nm}$ at the target wavelength without the need for any active stabilization. This constitutes a fivefold reduction in noise over existing state-of-the-art single-step converters at these wavelengths. We verify the almost-ideal preservation of nonclassical correlations by converting photons from a spontaneous-parametric-down-conversion source and moreover show the preservation of time-energy entanglement via Franson interferometry.

Determining Hamiltonian parameters from noisy experimental measurements is a key task for the control of experimental quantum systems. An interesting experimental platform where precise knowledge of device parameters is useful is the quantum-dot-based Kitaev chain. In these systems, the fine tuning of Hamiltonian parameters is crucial in order to reach the desired regime with stable midgap modes. In this work, we demonstrate an adversarial machine-learning algorithm to determine the parameters of a quantum-dot-based Kitaev chain. We train a convolutional conditional generative adversarial neural network (CCGAN) with simulated differential conductance data and use the model to predict the parameters at which Majorana bound states are predicted to appear. In particular, the CCGAN model facilitates a rapid, numerically efficient exploration of the phase diagram describing the transition between elastic co-tunneling and crossed Andreev reflection regimes. We verify the theoretical predictions of the model by applying it to experimentally measured conductance obtained from a minimal Kitaev chain consisting of two spin-polarized quantum dots coupled by a superconductor-semiconductor hybrid. Our model accurately predicts, with an average success probability of 97%, whether the measurement was taken in the elastic co-tunneling or crossed Andreev reflection-dominated regime. Our work constitutes a stepping stone towards fast, reliable parameter prediction for tuning quantum dot systems into distinct Hamiltonian regimes. Ultimately, our results yield a strategy to support Kitaev-chain tuning that is scalable to longer chains.

The initialization of superconducting qubits is one of the essential techniques for the realization of quantum computation. In previous research, initialization above 99% fidelity has been achieved at 280 ns. Here, we demonstrate the rapid initialization of a superconducting qubit with a quantum circuit refrigerator (QCR). Photon-assisted tunneling of quasiparticles in the QCR can temporally increase the relaxation time of photons inside the resonator and helps release energy from the qubit to the environment. Experiments using this protocol have shown that 99% of initialization time is reduced to 200 ns. This initialization time depends strongly on the relaxation rate of the resonator, and faster initialization is possible by reducing the resistance of the QCR, which limits the on:off ratio, and by strengthening the coupling between the QCR and the resonator.

Noisy mean-field annealing (NMFA) is an algorithm that mimics a coherent Ising machine (CIM), which is an optical system for solving Ising problems. The NMFA has reproduced the solver performance of the CIM for systems of limited size even though it simplifies the interaction between spins with a mean-field approximation. However, recent experiments observing various thermodynamic quantities have revealed that the CIM can capture the phase transitions of the two-dimensional Ising models that the mean field cannot capture. This situation leads to a fundamental question as to how well the NMFA can capture the features of the thermodynamic quantities around the phase transition. This paper answers that the NMFA reproduces the thermodynamic features of the mean field, but cannot reproduce the CIM results. This suggests that, in terms of sampling, the level of performance of the CIM is beyond that of the NMFA.

A central goal in quantum technologies is to maximize $G{T}_2$, where $G$ stands for the coupling of a qubit to control and readout signals and $T_2$ is the qubit’s coherence time. This is challenging, as increasing $G$ (e.g., by coupling the qubit more strongly to external stimuli) often leads to deleterious effects on $T_2$. Here, we study the coupling of pure and magnetically diluted crystals of ${\mathrm{Ho}\mathrm{W}}_{10}$ magnetic clusters to microwave superconducting coplanar waveguides. Absorption lines give a broadband picture of the magnetic energy level scheme and, in particular, confirm the existence of level anticrossings at equidistant magnetic fields determined by the combination of crystal field and hyperfine interactions. Such “spin clock transitions” are known to shield the electronic spins against magnetic field fluctuations. The analysis of the microwave transmission shows that the spin-photon coupling also becomes maximum at these transitions. The results show that engineering spin-clock states of molecular systems offers a promising strategy to combine sizable spin-photon interactions with a sufficient isolation from unwanted magnetic noise sources.

The generation of localized magnetic field gradients by on-chip nanomagnets is useful for a variety of technological applications, in particular, for spin qubits. To advance beyond the empirical design of these nanomagnets, we propose a systematic and general approach based on the micromagnetic formulation of an optimal field gradient source. We study the different field configurations that can be realized and find out quantitatively the most suitable ferromagnetic layer geometries. Using micromagnetic simulations, we then investigate the minimum requirements for reaching magnetic saturation in these nanomagnets. In terms of either longitudinal- or transverse-field gradient, the results provide an optimal solution for uniform, saturated nanomagnets, where the magnetic material can be selected according to the strength of the external fields that can be used.

Variational quantum time evolution allows us to simulate the time dynamics of quantum systems with near-term compatible quantum circuits. Due to the variational nature of this method the accuracy of the simulation is a priori unknown. We derive a posteriori global phase agnostic error bounds for the state simulation accuracy with variational quantum time evolution that improve the tightness of fidelity estimates over existing a posteriori error bounds. These analysis tools are practically crucial for assessing the quality of the simulation and making informed choices about simulation hyperparameters. The efficient, a posteriori evaluation of the bounds can be tightly integrated with the variational time simulation and, hence, results in a minor resource overhead, which is governed by the system’s energy variance. The performance of the error bounds is demonstrated on numerical examples.

Gate layouts of spin-qubit devices are commonly adapted from previous successful devices. As qubit numbers and device complexity increase, modeling new device layouts and optimizing for yield and performance become necessary. The simulation tools used in the advanced semiconductor industry need to be adapted for smaller structure sizes and electron numbers. Here, we present a general approach to electrostatically modeling new spin-qubit-device layouts, considering gate voltages, heterostructures, doping, reservoirs, and an applied source-drain bias. We identify key challenges in spin-qubit-device design: validating the impact on quantum-dot parameters, considering cross-coupling among gates and reservoirs, and ensuring robustness of the design to fabrication limits. We select a demanding target potential to investigate and optimize examples of gate layouts under these challenges. We verify our model by fabricating two simulated designs and indirectly probing the potential landscape through transport measurements.

State-of-the-art transmon qubits rely on large capacitors, which systematically improve their coherence due to reduced surface-loss participation. However, this approach increases both the footprint and the parasitic cross-coupling and is ultimately limited by radiation losses—a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as $36\times 39\text{μ}{\mathrm{m}}^2$ with $\gtrsim 100$-nm-wide vacuum-gap capacitors that are micromachined from commercial silicon-on-insulator wafers and shadow evaporated with aluminum. We achieve a vacuum participation ratio up to 99.6% in an in-plane design that is compatible with standard coplanar circuits. Qubit relaxation-time measurements for small gaps with high zero-point electric field variance of up to 22 V/m reveal a double exponential decay indicating comparably strong qubit interaction with long-lived two-level systems. The exceptionally high selectivity of up to 20 dB to the superconductor-vacuum interface allows us to precisely back out the sub-single-photon dielectric loss tangent of aluminum oxide previously exposed to ambient conditions. In terms of future scaling potential, we achieve a ratio of qubit quality factor to a footprint area equal to $20\text{μ}{\mathrm{m}}^{-2}$, which is comparable with the highest $T_1$ devices relying on larger geometries, a value that could improve substantially for lower surface-loss superconductors.