Fluxoniums, as partially protected superconducting qubits, are promising to be employed in building high-performance large-scale quantum processor. The recently proposed “integer fluxonium” operates at zero external flux bias, with a frequency of approximately 3 GHz. Single-qubit gate fidelity has been demonstrated to exceed $99.9\mathrm{\%}$ [Mencia et al., PRX Quantum 5, 040318 (2024)], whereas two-qubit gate schemes and scalable architectures remain underexplored. In this work, we investigate a fluxonium-transmon-fluxonium (FTF) coupling architecture using integer fluxoniums. We first confirm suppression of $ZZ$ interaction in the FTF system and then propose two high-fidelity controlled-$Z$ (CZ) gate schemes utilizing the coupler control: a flux-activated adiabatic gate scheme and a microwave-activated nonadiabatic gate scheme. Both schemes are capable of achieving low coherent error of the order of $1\times {10}^{-6}$ within gate durations of several tens of nanoseconds. In addition, we discuss a hybrid circuit system in which an integer fluxonium is coupled to a conventional fluxonium through a transmon coupler. Our proposal provides insights for future implementations of large-scale quantum circuits based on integer fluxonium devices.

There have been impressive breakthroughs in semiconductor quantum dots in recent years, with single- and two-qubit gate fidelities matching other leading platforms and scalability still remaining a relative strength; however, due to qubit wiring considerations, mobile electron architectures have been proposed to facilitate upward scaling, and this opens the possibility of enlarging the motional amplitude for electric-dipole spin resonance (EDSR)-based qubit manipulation. In this work, we examine and demonstrate the possibility of significantly outperforming static-EDSR-type single-qubit pulsing by taking advantage of greater spatial mobility to achieve higher Rabi frequencies and reduce the effect of charge noise. Our theoretical results indicate that fidelities are ultimately bottlenecked by spin-valley physics, which can be suppressed through the use of quantum optimal control. We demonstrate that, across different potential regimes and competing physical models, shuttling-based single-qubit gates retain significant advantages over existing alternatives.

Multiphoton processes deteriorate the quantum nondemolition (QND) character of the dispersive readout in circuit QED, causing the readout to lag behind single- and two-qubit gates in both speed and fidelity. Alternative methods such as longitudinal readout have been proposed; however, it is unknown to what extent multiphoton processes hinder this approach. Here, we investigate the QND character of the longitudinal readout of the transmon qubit. We show that the deleterious effects that arise due to multiphoton transitions can be heavily suppressed with detuning because the longitudinal interaction strength is independent of the transmon-resonator detuning. We consider the effect of circuit disorder, the selection rules that act on the transmon, and the description of longitudinal readout in the classical limit of the transmon to qualitatively demonstrate that longitudinal readout is robust. We show that fast, high-fidelity QND readout of transmon qubits is possible with longitudinal coupling.

Quantum computation holds great potential in revolutionizing information processing, with high-fidelity quantum gates serving as the cornerstone for realizing fault-tolerant quantum architectures. Here, we present a robust protocol for implementing high-fidelity entangling gates in Rydberg atomic systems via unconventional geometric quantum control strategies. By removing the stringent zero-dynamical-phase constraint inherent to conventional nonadiabatic geometric quantum computation, our scheme achieves high flexibility in designing driving fields while simultaneously enhancing gate fidelity and resilience against various errors. Notably, our protocol enables direct extension to multiqubit controlled-phase gate construction, with the gate duration remaining independent of the number of qubits involved. The results establish a practical and scalable framework for error-resilient quantum operations in Rydberg platforms, with broad implications for both fault-tolerant quantum computing and the quantum simulation of complex many-body systems.

We compare methods for signal classification applied to voltage traces from transition edge sensors (TES) which are photon-number resolving detectors fundamental for accessing quantum advantages in information processing, communication, and metrology. We quantify the effect of numerical analysis on the distinction of such signals. Furthermore, we explore dimensionality reduction techniques to create interpretable and precise photon-number embeddings. We demonstrate that the preservation of local data structures of some nonlinear methods is an accurate way to achieve unsupervised classification of TES traces. We do so by considering a confidence metric that quantifies the overlap of the photon-number clusters inside a latent space. Furthermore, we demonstrate that for our dataset previous methods such as the signal’s area and principal component analysis can resolve up to 16 photons with confidence above 90% whereas nonlinear techniques can resolve up to 21 photons with the same confidence threshold. In addition, we showcase implementations of neural networks to leverage information within local structures, aiming to increase confidence in assigning photon numbers. Finally, we demonstrate the advantage of some nonlinear methods to detect and remove outlier signals.

Hyperentangled states are fascinating resources in quantum information processing as they can significantly increase the channel capacity and enhance noise resistance. We explore a hyperfusion mechanism to fuse one $n$-photon hyper-$W$ state and one $m$-photon hyper-$W$ state into a large-scale $(n+m-2)$-photon hyper-$W$ state. Another mechanism to fuse one $n$-photon hyper-$W$ state, one $m$-photon hyper-$W$ state, and one $t$-photon hyper-$W$ state into an $(n+m+t-3)$-photon hyper-$W$ state is also proposed. These two hyperfusion mechanisms are constructed employing only polarizing beam splitters, balanced beam splitters, half-wave plates, single-photon detectors, and cross-Kerr nonlinearities. Conditional quantum gates, path couplers, and ancillary photons are not required in our constructions. Moreover, our fused $W$ states are hyperentangled in the polarization and spatial degrees of freedom of single-photon systems. The presence of only one garbage output state demonstrates that high efficiency can be achieved in our schemes.

Ultracold atoms trapped in optical lattices have emerged as a scalable and promising platform for quantum simulation and computation; however, gate speeds remain a significant limitation for practical applications. In this work, we employ quantum optimal control to design fast, collision-based two-qubit gates within a superlattice based on a Fermi-Hubbard description, reaching errors in the range ${10}^{-3}$ for realistic parameters. Numerically optimizing the lattice depths and the scattering length, we effectively manipulate hopping and interaction strengths intrinsic to the Fermi-Hubbard model. Our results provide five times shorter gate durations by allowing for higher energy bands in the optimization, suggesting that standard modeling with a two-band Fermi-Hubbard model is insufficient for describing the dynamics of fast gates, and we find that four to six bands are required. Additionally, we achieve nonadiabatic gates by employing time-dependent lattice depths rather than using only fixed depths. The optimized control pulses not only maintain high efficacy in the presence of laser-intensity and phase noise but also result in negligible interwell couplings.

We present an extensive study of 4H-$\mathrm{Si}\mathrm{C}$ nanomechanical resonators electrochemically etched out of a monocrystalline wafer. Combining piezo-driven interferometric determination of the mechanical spectra with scanning laser Doppler vibrometry, an unambiguous assignment of resonance peaks to flexural and torsional modes is achieved. The investigation of multiple harmonic eigenmodes of singly and doubly clamped resonators with varying geometry allows for a comprehensive characterization. Excellent intrinsic mechanical quality factors up to $2\times {10}^5$ are found at room temperature, approaching the thermoelastic limit at eigenfrequencies exceeding 10 MHz. Mechanical stress is essentially absent. Young’s modulus is in agreement with the literature. These findings are robust under postprocessing treatments, in particular atomic layer etching and high-temperature thermal annealing. The resulting on-chip high-quality mechanical resonators represent a valuable technological element for a broad range of applications. In particular, the monolithic architecture meets the requirements of spin-based photonic quantum technologies on the upcoming $\mathrm{Si}\mathrm{C}$ platform.

In the pursuit of scalable superconducting quantum computing, tunable couplers have emerged as a pivotal component, offering the flexibility required for complex quantum operations of high performance. In most current architectures of superconducting quantum chips, such couplers are not equipped with dedicated readout circuits to reduce complexity in both design and operation. However, this strategy poses challenges in precise characterization, calibration, and control of the couplers. In this work, we develop a hardware-efficient and robust technique based on adiabatic control to address this issue. The critical ingredient of this technique is adiabatic swap operation between a tunable coupler and nearby qubits. Using this technique, we have characterized and calibrated tunable couplers in our chips and achieved straightforward and precise control over these couplers. For example, we have demonstrated the calibration and correction of the flux distortion of couplers. In addition, we have also expanded this technique to tune the dispersive shift between a frequency-fixed qubit and its readout resonator over a wide range.

Temporal fluctuations in the superconducting qubit lifetime, $T_1$, present additional challenges in the pursuit of fault-tolerant quantum computing. Although the exact mechanisms remain unclear, $T_1$ fluctuations are generally attributed to strong coupling between the qubit and a few near-resonant two-level systems (TLSs), which can exchange energy with an ensemble of thermally fluctuating two-level fluctuators (TLFs) at low frequencies. Here, we report $T_1$ measurements of qubits with varying geometrical footprints and surface dielectrics as a function of temperature. By analyzing the noise spectrum of the qubit depolarization rate, ${\mathrm{\Gamma }}_1=1/T_1$, we disentangle the contributions of TLSs, nonequilibrium quasiparticles (QPs), and equilibrium (thermally excited) QPs to the variance in ${\mathrm{\Gamma }}_1$. We find that the ${\mathrm{\Gamma }}_1$ variance in qubits with smaller footprints is more susceptible to QP and TLS fluctuations than that in larger-footprint qubits. Furthermore, the QP-induced variances in all qubits align with the theoretical framework of QP diffusion and fluctuation. These findings offer valuable insights for future qubit design and engineering optimization.

Training-free quantum architecture search (QAS) has shown substantial promise in automating quantum circuit design for variational quantum algorithms (VQAs) with minimal computational expense. However, various VQA tasks and search spaces often favor specific training-free proxies. Furthermore, circuit performance is influenced by multiple factors, such as expressibility, trainability, and circuit topology, making it challenging for a single proxy to assess circuit performance accurately. To address these limitations, we propose an adaptive QAS framework that dynamically integrates multiple training-free proxies, combining their strengths and mitigating individual biases. This framework uses a mixture-of-experts (MoE) model that adapts to different VQA tasks and search spaces. Specifically, we develop an expert model for each training-free proxy, where each expert focuses on a distinct feature, such as expressibility, trainability, or topology. A gating network then assigns weights to each expert based on the specific VQA task and search space. Our proposed QAS framework consistently outperforms state-of-the-art QAS algorithms across diverse VQAs, including variational quantum eigensolvers (VQEs) and quantum neural networks (QNNs), as well as multiple search spaces, such as gatewise, layerwise, and blockwise constructions. Moreover, our approach demonstrates transferability across different VQA tasks and search spaces.

Quantum key distribution (QKD) offers a theoretically secure method to share secret keys, yet practical implementations face challenges due to noise and loss over long-distance channels. Traditional QKD protocols require extensive noise compensation, hindering their industrial scalability and lowering the achievable key rates. Alternative protocols encode logical qubits in noise-resilient states but at the cost of using many physical qubits, increasing susceptibility to loss and limiting transmission distance. In this work, we introduce a logical-qubit encoding that uses antisymmetric Bell states in the continuous photonic degrees of freedom, frequency and time. By leveraging the continuous space, we overcome this noise-loss robustness trade-off by minimizing the number of photons per logical qubit while optimizing the encoding resilience over noise fluctuations. We analyze the security of our encoding and demonstrate its robustness compared to existing state-of-the-art protocols. This approach provides a path toward scalable, efficient QKD implementations under realistic noise conditions.

We study the optimization of the transport and transfer of neutral atoms between optical tweezers, both critical steps in the implementation of quantum computers and simulators. We analyze four experimentally relevant pulse shapes (piecewise linear, piecewise quadratic, minimum jerk, and a combination of linear and minimum jerk), and we also develop a protocol using shortcuts-to-adiabaticity (STA) methods to crucially incorporate the time-dependent effects of static traps. By computing a measure of the final transport error and two measures of the heating during transport, we show that our proposed STA protocol comprehensively outperforms all the experimentally inspired pulses. After further optimizing the pulse shapes, we find a lower bound on the protocol duration, compatible with the time at which the vibrational excitations exceed half of the states hosted by the moving tweezer. This lower bound is at least eight times faster than the one reported in recent experiments, which highlights the importance of including and optimizing the transfer from and to static traps, which may be the largest bottleneck to speed. Finally, our STA results demonstrate that a modulation in the depth of the moving tweezer designed to time-dependently counteract the effect of the static traps is key to reducing errors and reducing the pulse duration. To motivate the implementation of our STA pulses in future experiments, we provide a simple analytical approximation for the moving-tweezer position and depth controls.

When using unitary gate sequences, the growth in depth of many quantum circuits with output size poses significant obstacles to practical quantum computation. The quantum fan-out operation, which reduces the circuit depth of quantum algorithms such as the quantum Fourier transform and Shor’s algorithm, is an example that can be realized in constant depth independent of the output size. Here, we demonstrate a quantum fan-out gate with real-time feedforward on up to four output qubits using a superconducting quantum processor. By performing quantum state tomography on the output states, we benchmark our gate with input states spanning the entire Bloch sphere. We decompose the output-state error into a set of independently characterized error contributions. We extrapolate our constant-depth circuit to offer a scaling advantage compared to the unitary fan-out sequence beyond 25 output qubits with feedforward control, or beyond 17 output qubits if the classical feedforward latency is negligible. Our work highlights the potential of midcircuit measurements combined with real-time conditional operations to improve the efficiency of complex quantum algorithms.

Superconducting inductors with impedance surpassing the resistance quantum, i.e., superinductors, are important for quantum technologies because they enable the development of protected qubits, enhance coupling to systems with small electric dipole moments, and facilitate the study of phase-slip physics. We demonstrate superinductors with densely packed meandered traces of granular aluminum ($\mathrm{grAl}$) with inductances up to 4 µH, achieving impedances exceeding 100 kΩ in the 4–8 GHz range. Ring resonators made with $\mathrm{grAl}$ meandered superinductors exhibit quality factors on the order of ${10}^5$ in the single-photon regime, and low nonlinearity on the order of tens of hertz. Depending on the $\mathrm{grAl}$ resistivity, at 10 Hz, we measure frequency-noise spectral densities in the range of ${10}^2$–${10}^3\mathrm{Hz}/\sqrt{\mathrm{Hz}}$. In some devices, in the single-photon regime, we observe a positive Kerr coefficient of unknown origin. Using more complex fabrication, the devices could be released from the substrate, either freestanding or suspended on a membrane, thereby further improving their impedance by a factor of 3.

Quantum communication theory studies how quantum information is transmitted through quantum channels, with the transmission rate quantified by the quantum channel capacity. Quantum channels often exhibit rich and complex structures—such as nonadditivity—that make evaluating their capacities particularly challenging. In this work, we show that a type of quantum channel known as the antidegradable one-mode Gaussian channel—whose capacity is believed to be zero—can be “activated” to transmit quantum information with the assistance of quantum entanglement. Although the channel’s output alone cannot be used to retrieve the input signal, combining it with additional entanglement makes this possible. Beyond its theoretical implications, it can also be realized in practical systems. For example, in electro-optic systems used for quantum transduction in the two-mode squeezing interaction regime, the transduction channel is antidegradable. We demonstrate that this system can transmit microwave-optical quantum information with the assistance of entanglement with an ancillary mode. This results in a type of quantum transducer that exhibits positive quantum capacity over a wide parameter space.

We present and characterize a modular, open-source system to perform feedback control experiments on configurations of atoms and molecules in arrays of optical tweezers. The system features a modular computer architecture with a motherboard and peripheral cards, enabling continuous upgrades and flexible development while serving multiple applications. It supports efficient data transfer to and from graphics processing units (GPUs) using remote direct memory access, leveraging GPU efficiency in matrix multiplication and parallelism, while enabling direct data transfer between devices without involving the CPU. We first describe the architecture and workflow of the system, detailing its hardware components and software modules. We then evaluate the computational run time for preparing defect-free chains and grids of atoms using efficient implementations of atom-reconfiguration algorithms. Finally, we discuss timing bottlenecks and strategies to reduce latency. Beyond solving reconfiguration problems, the system can readily be used to implement adaptive and feedforward protocols, as well as digital quantum algorithms relying on particle displacement. Our results lay the groundwork for developing low-latency feedback control systems, benchmarking their performance, and advancing scalable quantum hardware.

Quantum networks and distributed quantum computing rely on entanglement generation between photons and long-lived stationary qubits. For large-scale architectures, one of the most crucial parameters is the efficiency at which entanglement can be created and detected. Here, we maximize the efficiency for the detection of hybrid entanglement between a nuclear spin qubit in diamond with a photonic time-bin qubit. Our approach relies on the optimal implementation of a photonic qubit analyzer, for which we use a high-speed electro-optical deflector to direct photons deterministically along the optimal interferometer paths. This way, we eliminate all cases in which photons are randomly lost due to propagation in the wrong interferometer path. We use nitrogen-vacancy centers in diamond, for which we demonstrate nuclear spin–photon entanglement with a fidelity of 0.6 reducing the experimental acquisition time by a factor of 2. An extension to other spin–photon entanglement systems and higher-dimensional entanglement schemes is outlined.