We present an implementation of multicontrolled (mc) quantum gates that provides significant reductions of cost compared to state-of-the-art methods. The operator applied on the target qubit is a unitary, special unitary, or the Pauli $X$ operator (multicontrolled Toffoli), and requires one clean ancilla, no ancilla, or one dirty ancilla, respectively. We generalize our methods for any number of target qubits, and provide further cost reductions if additional ancilla qubits are available. For each type of mc gate, we provide implementations for unrestricted (all-to-all) connectivity and for linear-nearest-neighbor (LNN) connectivity. All of the methods use a linear cost of gates from the Clifford+$T$ (fault-tolerant) set. In the context of LNN architecture, the cost and depth of our circuits scale linearly irrespective of the position of the qubits on which the gate is applied. Our methods directly improve the compilation process of many quantum algorithms, providing optimized circuits. Given the scale of our improvements, for example, the quadratic-to-linear controlled-not count for LNN, they will naturally result in a large reduction of errors.

We present a SNAIL-based parametric amplifier that integrates a lumped-element impedance matching network for increased bandwidth and an on-chip pump-port filter for efficient pump delivery. The amplifier is fabricated using a single-layer optical lithography step, followed by a single-layer electron-beam-lithography step. We measure a flat 20-dB gain profile with less than 1-dB ripple across a bandwidth of up to 250 MHz on multiple devices, demonstrating robust performance against variations arising from fabrication and packaging. We characterize the amplifier’s linearity by analyzing gain compression and intermodulation distortion under simultaneous multi-tone excitation. We show that the intermodulation products remain suppressed by more than 23 dB relative to the signal tones, even at the 1-dB gain compression point. We further validate its utility by performing simultaneous high-fidelity readout of two transmon qubits, achieving state assignment fidelities of $99.51\mathrm{\%}$ and $98.55\mathrm{\%}$. The combination of compact design, fabrication simplicity, and performance robustness makes this amplifier a practical device for quantum experiments with superconducting circuits.

Implementing fast and high-fidelity quantum operations using open-loop quantum optimal control relies on having an accurate model of the quantum dynamics. Any deviations between this model and the complete dynamics of the device, such as the presence of spurious modes or pulse distortions, can degrade the performance of optimal controls in practice. Here, we propose an experimentally simple approach to realize optimal quantum controls tailored to the device parameters and environment while specifically characterizing this quantum system. Concretely, we use physics-inspired machine learning to infer an accurate model of the dynamics from experimentally available data and then optimize our experimental controls on this trained model. We show the power and feasibility of this approach by optimizing arbitrary single-qubit operations in detailed numerical simulations of a superconducting transmon qubit. We demonstrate that this framework produces an accurate description of the device dynamics under arbitrary controls, together with the precise pulses achieving arbitrary single-qubit gates with a high fidelity of $\sim 99.99\mathrm{\%}$.

Scalable coherent manipulation of qubit quantum states is crucial for building large-scale superconducting quantum processors towards fault-tolerant quantum computing. Complete coherent manipulation contains quantum gate control and magnetic flux bias for the qubit, especially for quantum processors with frequency-tunable qubits and tunable couplers. In recent years, efforts have been made to propose quantum gate control in a scalable way. However, proposals and experiments for scalable magnetic flux bias for qubits are rarely reported. Here, we propose a scalable scheme to provide magnetic flux for qubits by using an on-chip direct current source (OCDCS). We demonstrate both theoretically and experimentally that only single pulse is needed to deterministically modulate the magnetic flux in the OCDCS, which then provides the qubit with in situ and constant magnetic flux. With the magnetic flux provided by the OCDCS, the experimental results exhibit high-fidelity single-qubit gate and low noise current of the OCDCS, which shows a 15.6 dB reduction compared with that of the RTE scheme. Additionally, we propose a time-division-multiplex (TDM) scheme combining the OCDCS with switch arrays, which could exponentially reduce the number of cables for flux bias from traditional $n$ to ${\log }_2(n)+1$. Our work thus paves the way for scalable flux bias in large-scale superconducting quantum processors.

Light-matter interaction with a squeezed vacuum has received much interest for the ability to increase the native interaction strength between an atom and a photon with a reservoir assumed to have an infinite bandwidth. Here we study a model of parametrically driven cavity quantum electrodynamics (QED) for enhancing light-matter interaction while subjected to a finite-bandwidth squeezed vacuum drive. Our method is capable of unveiling the effect of relative bandwidth as well as squeezing required to observe the anticipated anticrossing spectrum and enhanced cooperativity without the ideal squeezed bath assumption. Furthermore, we analyze the practicality of said models when including intrinsic photon loss due to resonator imperfection. With these results, we outline the requirements for experimentally implementing an effectively squeezed bath in solid-state platforms such as $\mathrm{In}\mathrm{As}$ quantum dot cavity QED such that in situ control and enhancement of light-matter interaction could be realized.

Coupled spins in semiconductor quantum dots are a versatile platform for quantum computing and simulations of complex many-body phenomena. However, on the path of scale-up, crosstalk from densely packed electrodes poses a severe challenge. While crosstalk onto the quantum dot potentials is nowadays routinely compensated for, crosstalk on the exchange interaction is much more difficult to tackle because it is not always directly measurable. Here we propose and implement a way of characterizing and compensating crosstalk on adjacent exchange interactions by following the singlet-triplet avoided crossing in $\mathrm{Ge}$. We show that we can easily identify the barrier-to-barrier crosstalk element without knowledge of the particular exchange value in a $2\times 4$ quantum dot array. We uncover striking differences among these crosstalk elements that can be linked to the geometry of the device and the barrier gate fan-out. We validate the method by tuning up four-spin Heisenberg chains. The same method should be applicable to longer chains of spins and to other semiconductor platforms in which mixing of the singlet and the lowest-energy triplet is present or can be engineered. Additionally, this procedure is well-suited for automated tuning routines as we obtain a standout feature that can be easily tracked and directly returns the magnitude of the crosstalk.

The narrow band gaps of semiconductors allow for thick, uniform Josephson junction barriers, potentially enabling reproducible, stable, and compact superconducting qubits. We study vertically stacked van der Waals Josephson junctions with semiconducting weak links, whose crystalline structures and clean interfaces offer a promising platform for quantum devices. We observe robust Josephson coupling across 2–12 nm (3–18 atomic layers) of semiconducting $\mathrm{WSe}$${}_2$ and, notably, a crossover from proximity- to tunneling-type behavior with increasing weak-link thickness. Building on these results, we fabricate a prototype all-crystalline merged-element transmon qubit with transmon frequency and anharmonicity closely matching design parameters. We demonstrate dispersive coupling between this transmon and a microwave resonator, highlighting the potential of crystalline superconductor-semiconductor structures for compact, tailored superconducting quantum devices.

Characterizing quantum processes is crucial for the execution of quantum algorithms on available quantum devices. A powerful framework for this purpose is the Quantum Model Learning Agent (QMLA), which characterizes a given system by learning its Hamiltonian via adaptive generations of informative experiments and their validation against simulated models. Identifying the incoherent noise of a quantum device in addition to its coherent interactions is, however, as essential. Precise knowledge of such imperfections of a quantum device allows one to devise strategies to mitigate detrimental effects, for example, via quantum error correction. We introduce the open Quantum Model Learning Agent (oQMLA) framework to account for Markovian noise through the Liouvillian formalism. By simultaneously learning the Hamiltonian and jump operators, oQMLA independently captures both the coherent and incoherent dynamics of a system. The added complexity of open systems necessitates advanced algorithmic strategies. Among these, we implement regularization to steer the algorithm toward plausible models and an unbiased metric to evaluate the quality of the results. We validate our implementation in simulated scenarios of increasing complexity, demonstrating its robustness to hardware-induced measurement errors and its ability to characterize systems using only local operations. Additionally, we develop a scheme to interface oQMLA with a publicly available superconducting quantum computer, showcasing its practical utility. These advancements represent a significant step toward improving the performance of quantum hardware and contribute to the broader goal of advancing quantum technologies and their applications.

Parametric frequency converters (PFCs) play a critical role in bridging the frequency gap between quantum information carriers. PFCs in the microwave band are particularly important for superconducting quantum processors but their operating bandwidth is often strongly limited. Here, we present a multimode kinetic metastructure for parametric frequency conversion between broadly spanning frequency modes. This device comprises a chain of asymmetric kinetic inductance grids designed to deliver efficient three-wave-mixing nonlinearity. We demonstrate highly efficient coherent conversion among broadly distributed modes and the mode frequency is continuously tunable by controlling the external magnetic field strength, making it ideally suited for quantum computing and communication applications requiring flexible and efficient frequency conversion.

The integration of cold atoms with nanophotonic chips is a promising approach for realizing scalable quantum technologies, yet stable trapping of single atoms near chip surfaces remains an outstanding challenge. Here we study the dynamics of single ${}^{87}\mathrm{Rb}$ atoms near the surface of a photonic chip by deterministically delivering single atoms to sub-10-$\text{μ}\mathrm{m}$ distances from the chip surface using optical tweezers. We use a translatable tweezer to transport atoms to the chip surface, revealing that atom loss is dominated by surface-induced evaporation as atoms at temperatures of approximately $0.1\mathrm{mK}$ collide with the 300 K surface. To overcome this loss mechanism, we implement an optical conveyor belt technique that transfers atoms in a standing-wave tweezer, preventing direct surface collisions. This approach enables single-atom lifetimes of up to 4 s in close proximity to the chip without the assistance of additional laser cooling. Our results demonstrate the viability of long-lived atomic qubits when they are integrated with photonic circuits and provide design insights for hybrid quantum systems. These advancements pave the way for the creation of scalable atom-photon interfaces, which could find applications in quantum networking, computation, simulation, and sensing.

Waveguide quantum electrodynamics—the study of atomic systems interacting with propagating electromagnetic fields—is a powerful platform for understanding the complex interplay between light and matter. Qubit control is an indispensable tool in this field, and most experiments thus far have used narrowband control pulses to interact with qubits at specific frequencies. Dynamics can change significantly with fast, broadband pulses, since waveguide properties such as dispersion affect the pulse evolution and its impact on the qubit. Here we use dispersion to achieve spatial addressing of superconducting qubits separated by a subwavelength distance within a microwave waveguide. This approach relies on a self-focusing effect to create a position-dependent interaction between a chirped pulse and an individual qubit. This method emphasizes the importance of dispersion in the design and analysis of quantum experiments, and offers avenues for the rapid control of quantum states.

Superconducting traveling-wave parametric amplifiers (TWPAs) are increasingly used in various applications, including quantum computing, quantum sensing, and dark-matter detection. However, one characteristic of these amplifiers, gain compression, has not received much attention. As a result, there is a lack of comprehensive experimental exploration of this phenomenon in the existing literature. In this study, we present an experimental investigation of gain compression in a Josephson traveling-wave parametric amplifier based on a four-wave mixing process. We have implemented an alternative setup to monitor the complex transmission of both the pump and signal tones, which allows us to simultaneously track pump depletion and signal amplification as functions of signal power and frequency across the entire bandwidth of the device. Our findings indicate that, while pump depletion occurs during gain compression, it is not the only mechanism involved in the saturation of a TWPA. Power-induced phase-matching processes also take place within the device. This study provides valuable insights for optimizing TWPAs for applications that require high total input power, such as multiplexed qubit readout or broadband photon emission.

Vacuum-gap capacitors have recently attracted significant interest in superconducting circuit platforms due to their compact design and exceptionally low dielectric losses in the microwave regime. Their intrinsic ability to support mechanical vibrational modes makes them well suited for circuit optomechanics. However, precise control over the gap size and the realization of high-coherence mechanical modes remain longstanding challenges. Here, we present a detailed and scalable fabrication process for vacuum-gap capacitors that support ultrahigh-coherence mechanical motion, exhibit low microwave loss, and occupy a significantly smaller footprint compared to conventional planar geometries. By employing a planarized ${\mathrm{Si}\mathrm{O}}_2$ sacrificial layer, we achieve vacuum gaps on the order of 150 nm. Using this platform, we have recently demonstrated ground-state cooling and motion squeezing of a mechanical oscillator with a quality factor of 40 million—a 100-fold improvement compared to prior works—as well as a single-photon optomechanical coupling rate of approximately 15 Hz [Youssefi et al., Nat. Phys. 19, 1697 (2023)]. Additional achievements include the realization of an optomechanical topological lattice with 24 sites [Youssefi et al., Nature 612, 666 (2022)] and the observation of quantum collective dynamics in a mechanical hexamer [Chegnizadeh et al., Science 386, 1383 (2024)]. Collectively, these results underscore the potential of vacuum-gap capacitors as a platform for coupling superconducting qubits to mechanical systems, enabling quantum storage, and probing gravitational effects in quantum mechanics.

We propose a numerical approach to design highly efficient adiabatic schedules for analog quantum computing, focusing on the maximum-independent-set problem and neutral atom platforms. On the basis of a representative dataset of small graphs, we present numerical evidence that the optimum schedules depend principally on the hardness of the problem rather than on its size. These schedules perform better than the benchmark protocols and admit a straightforward implementation in the hardware. This allows us to extrapolate the results to larger graphs and to successfully solve moderately hard problems using QuEra’s 256-qubit Aquila computer. We believe that extending our approach to hybrid algorithms could be the key to solving the hardest instances with the current technology, making yet another step toward real-world applications.

Hybrid quantum systems are highly promising platforms for addressing important challenges in quantum information science and quantum sensing. Their implementation, however, is technologically nontrivial since each component typically has unique experimental requirements. Here, we work toward a hybrid system consisting of a superconducting on-chip microwave circuit in a dilution refrigerator and optically trapped ultracold atoms. Specifically, we focus on the design optimization of a suitable superconducting chip and on the corresponding challenges and limitations. We present detailed microwave-cavity engineering strategies for maximized and tunable coupling rates to atomic Rydberg-Rydberg transitions in ${}^{87}\mathrm{Rb}$ atoms while respecting the boundary conditions due to the presence of a laser beam near the surface of the chip. Finally, we present an experimental implementation of the superconducting microwave chip and discuss the cavity characteristics as a function of temperature and applied dc voltage. Our results highlight the considerations required for realizing a flexible, tunable superconductor-atom hybrid system and lay the groundwork for realizing this exciting platform in a dilution refrigerator with vacuum Rabi frequencies approaching the strong-coupling regime.

Josephson parametric amplifiers (JPAs) typically require rf pump power that is several orders of magnitude stronger than the maximum signal power they can handle. The low power efficiency and strong pump leakage into the signal circuitry could be critical concerns in practical applications. In this work, we discuss how to optimize the pump coupling scheme for a three-wave mixing JPA by employing microwave filtering techniques, with the goal of maximizing pump power efficiency and minimizing pump leakage without sacrificing other properties of interest. We implement the corresponding filter design in a superconducting nonlinear asymmetric inductive element (SNAIL)-based JPA and demonstrate an improvement of more than 3 orders of magnitude in both power efficiency and pump-leakage suppression compared to a similar device with conventional capacitive coupling, while maintaining state-of-the-art dynamic range and near-quantum-limited noise performance. Furthermore, we show experimentally that the filter-coupled JPA is more robust against noise input from the pump port, exhibiting no significant change in added noise performance with up to 4 K of effective noise temperature at the pump port.

Lumped-element inductors are an integral component in the circuit QED toolbox. However, it is challenging to build inductors that are simultaneously compact, linear, and low-loss with standard approaches that either rely on the geometric inductance of superconducting thin films or on the kinetic inductance of Josephson-junction arrays. In this work, we overcome this challenge by utilizing the high kinetic inductance offered by superconducting granular aluminum (grAl). We demonstrate lumped-element inductors with a few nH of inductance that are up to $100$ times shorter than inductors built from pure aluminum (Al). To characterize the properties of these linear inductors, we first report on the performance of lumped-element resonators built entirely out of grAl with sheet inductances varying from $30$–$320$ pH/sq and self-Kerr nonlinearities of $0.2$–$20\mathrm{Hz}/\mathrm{photon}$. Further, we demonstrate ex situ integration of these grAl inductors into hybrid resonators with $\mathrm{Al}$ or tantalum ($\mathrm{Ta}$) capacitor electrodes without increasing total internal losses. Interestingly, the measured internal quality factors systematically decrease with increasing room-temperature resistivity of the grAl film for all devices, indicating a trade-off between compactness and internal loss. For our lowest resistivity grAl films, we measure quality factors reaching $3.5\times {10}^6$ for the all-grAl devices and $4.5\times {10}^6$ for the hybrid grAl/$\mathrm{Ta}$ devices, similar to state-of-the-art quantum circuits. Our loss analysis suggests that the surface-loss factor of grAl is similar to that of pure $\mathrm{Al}$ for our lowest resistivity films, while the increasing losses with resistivity could be explained by increasing inductor loss in the grAl film.

In this article, we propose a method for generating single microwave photons in superconducting circuits. We theoretically show that pure single microwave photons can be generated on demand and tuned over a large frequency band by making use of Landau-Zener transitions under a rapid sweep of a control parameter. We devise a protocol that enables fast control of the frequency of the emitted photon over two octaves, without requiring extensive calibration. Additionally, we make theoretical estimates of the generation efficiency, tunability, purity, and linewidth of the photons emitted using this method for both charge- and flux-qubit-based architectures. We also provide estimates of the optimal device parameters required for these architectures to realize the device.

Photon entanglement is indispensable for optical quantum technologies. Measurement-based optical quantum computing and all-optical quantum networks rely on multiphoton cluster states consisting of indistinguishable entangled photons. A promising method for creating such cluster states on demand is spin-photon entanglement using the spin of a resident charge carrier in a quantum dot, precessing in a weak external magnetic field. In this work, we show theoretically and experimentally that spin-photon entanglement is strongly affected by the hidden anisotropy of quantum dots, which can arise from mechanical stress, shape anisotropy and even specific crystal structure. In the measurements of time-resolved photoluminescence and the cross-polarized second-order photon correlation function in a magnetic field, the anisotropy manifests itself in the spin dynamics and, as a consequence, in the spin-photon concurrence. The measured time-filtered spin-photon Bell state fidelity depends strongly on the excitation polarization and reaches an extremely high value of 94% at maximum. We specify the magnetic field and excitation polarization directions that maximize spin-photon entanglement and thereby enhance the fidelity of multiphoton entangled states.

Quantum key distribution (QKD) is a promising technology for secure communication. Nevertheless, QKD is still treated with caution in certain contexts due to potential gaps between theoretical models and actual QKD implementations. A common assumption in security proofs is that the detection probability at the receiver, for a given input state, is independent of the measurement basis, which might not always be verified and could lead to security loopholes. This paper presents a security proof for QKD protocols that does not rely on the aforementioned assumption and is thus applicable in scenarios with detection probability mismatches, even when induced by the adversary. We demonstrate, through simulations, that our proof can extract positive key rates for setups vulnerable to large detection probability mismatches. This is achieved by one monitoring whether an adversary is actively exploiting such vulnerabilities, instead of considering the worst-case scenario as in previous proofs. Our work highlights the importance of accounting for basis-dependent detection probabilities and provides a concrete solution for improving the security of practical QKD systems.