In conventional transmon qubits, decoherence is dominated by a large number of parasitic two-level systems (TLS) residing at the edges of the large area coplanar shunt capacitor and junction leads. Avoiding these defects by improvements in design, fabrication, and materials proved to be a significant challenge that so far led to limited progress. The merged-element transmon qubit (“mergemon”), a recently proposed paradigm shift in transmon design, attempts to address these issues by engineering the Josephson junction to act as its own shunt capacitor. With its energy mostly confined within the junctions, efforts required to improve qubit coherence can be concentrated on the junction barrier, a potentially easier to control interface compared with exposed circuit areas. Incorporating an additional aluminum deposition and oxidation into the in situ bandaged Niemeyer-Dolan technique, we were able to fabricate flux-tunable mergemon qubits achieving mean $T_1$ relaxation times of up to $130\mathrm{\mu }\mathrm{s}$ ($Q\approx 3.3\times {10}^6$). TLS spectroscopy under applied strain and electric fields, together with systematic design variations, revealed that even for mergemon qubits—despite their significantly reduced footprint and increased junction barrier volume—careful design considerations are still essential to avoid $T_1$ limitations due to surface loss.

Understanding the mechanisms that limit high-fidelity readout in circuit quantum electrodynamics is essential for its optimization. Multiphoton resonances are understood to be a limiting factor, causing population transfer from the computational states to higher-energy states under drive. This effect, known as measurement-induced state transitions, has been extensively studied for the transmon qubit. While this exploration has begun for the fluxonium qubit, a systematic study of this effect is lacking. Here, we bridge this gap by theoretically studying measurement-induced state transitions in the fluxonium qubit over a wide range of parameters, comprising essentially all experimentally explored ranges. We find that lighter fluxoniums are less susceptible to these state transitions when compared with their heavier counterparts. We attribute this effect to the combination of lower density of multiphoton resonances, a smaller requisite coupling for a given dispersive shift, and a more harmoniclike structure of the charge operator. We confirm the validity of our analysis by performing time-dependent readout simulations. Finally, we consider the impact of the superinductor’s array modes on measurement-induced state transitions over a large range of parameters.

Solid-state spin ensembles addressed via superconducting circuits are promising candidates for quantum memory applications, offering multimodal storage capability and second-long coherence times at their clock transition. Implementing practical memory schemes requires dynamic control over both the resonator frequency and bandwidth, and most crucially, deterministic access to the clock transitions. In this article, we report measurements of a superconducting resonator whose frequency can be tuned by passing a DC current through the high-kinetic-inductance thin film, and whose bandwidth can be tuned by parametric coupling to a low-Q buffer resonator. Using this resonator, we deterministically address an ensemble of bismuth donors at their clock transition, measuring a Hahn-echo coherence time of 450 ms. We demonstrate RF driving of the bismuth donor hyperfine transitions, as well as dynamic bandwidth control of the resonator.

Crosstalk refers to unwanted qubit addressing. This is particularly detrimental when scaling up quantum information systems because unintended interactions limit their overall performance. For superconducting qubits, tunable couplings and frequency tunability achieved through externally applied magnetic fluxes enable high-fidelity entangling gates; however, they also introduce crosstalk through unintended flux coupling. In this work, we investigate the impact of time-dependent external magnetic fluxes in quantized circuits on superconducting qubit couplings. We find that nontrivial cross-voltage driving emerges between capacitively linked qubits when the magnetic flux threading the superconducting loop of a qubit varies in time, in a manner analogous to Faraday’s law of induction. Crucially, we show that this effect enables fast single-qubit control through the coupler element in standard tunable-coupler architectures, potentially eliminating the need for individual microwave $XY$ control lines.

Small and device-dependent valley splittings remain a key challenge for electron spin qubits in silicon (Si), directly limiting qubit fidelity, device uniformity, and the scalability of Si-based quantum processors. In silicon-germanium (Si-Ge) heterostructures, this problem can be addressed through engineering of the epitaxial layer stack. Several heuristic strategies have been proposed to enhance the energy gap between the two nearly degenerate valley states in strained $\mathrm{Si}/\mathrm{Si}$-Ge quantum wells (QWs), e.g., sharp $\mathrm{Si}/\mathrm{Si}$-Ge interfaces, Ge spikes, or oscillating Ge concentrations within the QW. Here, we develop a systematic variational optimization approach to compute optimal Ge concentration profiles that enhance selected properties of the intervalley coupling matrix element. Our free-shape optimization framework is augmented by realistic technological constraints to ensure feasibility of the resulting epitaxial profiles and is based on an effective-mass envelope-function theory accounting for strain and compositional alloy disorder. Previously proposed heterostructure designs are recovered as special cases of the constrained optimization problem. Our main result is a novel heterostructure design, which we refer to as the modulated wiggle well, providing both a large deterministic enhancement of the valley splitting and a strong suppression of disorder-induced variability. In addition, this design enables wide electrical tunability of the valley splitting—from approximately $200\mathrm{\mu }\mathrm{eV}$ to above 1 meV—offering opportunities for engineering robust and switchable silicon qubits.

Quantum error correction (QEC) requires ancilla qubits to extract error syndromes from data qubits, which store quantum information. However, ancilla errors can propagate back to the data qubits, introducing additional errors and limiting fault tolerance. In superconducting quantum circuits, Kerr-cat qubits (KCQs), which exhibit strongly biased noise, have been proposed as ancillas to suppress this back-action and enhance QEC performance. Here, we experimentally demonstrate a beam-splitter interaction between a KCQ and a transmon, realizing an effective ${\widehat{Z}}_{\mathrm{cat}}{\widehat{X}}_q$ coupling that can be employed for parity measurements in QEC protocols. We characterize the interaction across a range of cat sizes and drive amplitudes, confirming the expected scaling of the interaction rate. These results establish a step toward hybrid architectures that combine transmons as data qubits with noise-biased bosonic ancillas, enabling hardware-efficient syndrome extraction and advancing the development of fault-tolerant quantum processors.

Spin manipulation by hopping has recently emerged as a promising strategy to control hole spins in quantum dots (QDs) using exclusively baseband control, thereby mitigating power dissipation and high-frequency management constraints in large-scale architectures. Unlike conventional approaches such as electric-dipole spin resonance, this mechanism exploits dot-to-dot variations of the spin precession axes to enable spin rotations. However, it is intrinsically disorder-dependent: in the absence of sufficient variability, the precession axes remain aligned, and spin manipulation becomes ineffective. This fundamental reliance on disorder raises concerns regarding its compatibility with the long-term evolution of spin qubit platforms toward improved material quality, cleaner interfaces, and enhanced device reproducibility. Here, we numerically assess the viability of spin manipulation by hopping as a function of disorder strength and demonstrate that its implementation is indeed increasingly constrained as disorder is reduced. To overcome this limitation, we propose an alternative strategy based on hopping between intentionally squeezed quantum dots. This approach retains the advantages of baseband control while being independent of disorder and robust against moderate variability, thereby offering improved prospects for scalable hole-spin quantum computing architectures.

High-performance numerical methods are essential not only for advancing quantum many-body physics but also for enabling integration with emerging quantum computing platforms. We present a scalable, general-purpose parallel algorithm for quantum simulations based on or-represented quantum algebra (ORQA). This framework applies to arbitrary spin systems and naturally integrates with quantum circuit simulation in the Heisenberg picture, making it particularly relevant to recent large-scale experiments on superconducting qubit processors [Y. Kim et al. Nature (London) 618, 500 (2023)]. As a benchmark, we simulate the kicked Ising model on a 127-qubit heavy-hexagon lattice, tracking the time evolution of local magnetization using up to $1\times {10}^{12}$ Pauli strings. Executed on the supercomputer Fugaku, our simulations exhibit strong scaling up to $2^{17}$ parallel processes with near-linear communication overhead. These results establish ORQA as a practical and high-performance tool for quantum many-body dynamics and highlight its potential for integration into hybrid quantum-classical computational frameworks, complementing recent advances in tensor network and surrogate simulation techniques.

Errors in superconducting qubits that are correlated in time and space can pose problems for quantum error correction codes. Radiation from cosmic and terrestrial sources can increase the quasiparticle (QP) density in a superconducting qubit device, resulting in an increased rate of QPs tunneling across proximal Josephson junctions (JJs) and causing correlated errors. Mechanical vibrations, such as those induced by the pulse tube (PT) in a dry dilution refrigerator, are also a known source of correlated errors. We measure two types of errors in the same device, linking the first to ionizing radiation and the second to PT operation. We present a method for distinguishing these two types of errors by their temporal, spatial, and frequency-domain features, enabling physically motivated error-mitigation strategies. We also present accelerometer data to study the correlation between PT-induced vibrations and the errors. We measure arrays of transmon qubits where the difference in superconducting gap across the JJ is less than the qubit energy, as well as those where the gap is greater than the qubit energy, which has been shown to mitigate radiation-induced errors. The rate of both types of errors is reduced in these latter devices, suggesting that gap engineering is also protective against PT-induced errors.

Eliminating residual $ZZ$ interactions in a two-qubit system is essential for reducing coherent errors during quantum operations. In a superconducting circuit platform, coupling two transmon qubits via a transmon coupler has been shown to effectively suppress residual $ZZ$ interactions. However, in such systems, perfect cancellation usually requires the qubit-qubit detuning to be smaller than the individual qubit anharmonicities, which exacerbates frequency crowding and microwave crosstalk. To address this limitation, we introduce transmon-fluxonium-transmon (TFT) architecture, wherein two transmon qubits are coupled via a fluxonium qubit. The coupling mediated by the fluxonium eliminates residual $ZZ$ interactions even for transmons detuned by more than their anharmonicities. We experimentally identified zero-$ZZ$ interaction points at qubit-qubit detunings of 409 and 616 MHz from two distinct TFT devices. We then implemented an adiabatic, coupler-flux-biased controlled-Z gate, achieving gate fidelities of 99.64(6)% and 99.68(8)% across the two devices.

In a recent Letter [H. Munoz-Bauza and D. Lidar, Phys. Rev. Lett. 134, 160601 (2025)], quantum annealing was reported to exhibit a scaling advantage in approximately solving quadratic unconstrained binary optimization (QUBO) problems. Here, we revisit these findings by employing the simulated bifurcation machine (SBM), a nonlinear dynamical system that exploits chaotic behavior rather than thermal fluctuations. Our approach originates from quantum dynamics and shares key operational features with quantum annealing: (i) nearly parallel evolution and (ii) a well-defined relation between the energy gap, run-time, and solution quality. We obtain comparable or superior scaling, closing the reported quantum-classical gap. We further show that the small instances studied previously are insufficient to infer asymptotic behavior. Extending the analysis to larger problems reveals robust classical performance, indicating that current quantum annealers are unlikely to exhibit a clear scaling advantage over SBM-like solvers on quantum-annealing-correction-type QUBO problems under the run-time accounting studied here. Finally, we identify sparse problem classes where future quantum devices could achieve a genuine scaling advantage once hardware overheads are mitigated.

Filtering is commonly used in quantum optics to reject noise photons, and also to enable interference between independent photons. However, filtering the joint spectrum of photon pairs can reduce the inherent coincidence probability or loss-independent heralding efficiency. Here we investigate filtering for multiphoton applications based on entanglement and interference (e.g., quantum teleportation). We multiplex C-band entangled photons and C-band classical communications into the same long-distance fibers, which enables scalable low-loss quantum networking but requires filtering of spontaneous Raman scattering noise from classical light. Using tunable-bandwidth filters, low-jitter detectors, and polarization filters, we distribute wavelength-degenerate time-bin entangled photons in the C band (1536.5 nm) over 25 km/25 km standard fibers (50 km) each carrying 10 Gbit/s C-band classical data signals. Narrow filtering enables milliwatt-level C-band power, which exceeds the power achieved by comparable studies by roughly an order of magnitude and could feasibly support terabit per second classical rates. We evaluate how performance depends on pump and filter bandwidths, multipair emission, filter shapes, loss, phase matching, and how quantum information is measured. We find a trade-off between reducing noise impact and increasing single-mode purity, and discuss mitigation methods toward optimal multiphoton applications. Importantly, these results apply to noise in free space and in quantum devices (sources, frequency converters, switches, detectors, etc.) and provide insight into filter-induced degradation of single-photon purity and rates even in noise-free environments.

Synthetic frequency dimensions offer a powerful approach in the simulation of lattice models and control photon dynamics. However, extending this concept to the quantum regime, particularly at the single-photon level, has remained challenging in photonic platforms. Here, we demonstrate quantum-state initialization and detection of single-photon evolutions within a synthetic frequency lattice by integrating a superconducting qubit with a 16 m aluminum coaxial cable. A tunable superconducting quantum interference device–based modulator is employed to synthesize lattice couplings and artificial gauge fields. We observe single-photon quantum random walks and Bloch oscillations, as well as nonadiabatic unidirectional frequency conversion under rapid temporal modulation of the lattice Hamiltonian, and present band-structure measurements. The lattice connectivity can be readily reconfigured to construct higher-dimensional lattices using a multiplicity of drive tones. Our results establish superconducting quantum circuits as a versatile platform for programmable Hamiltonians and extensible synthetic lattices with flexible single-photon control.

Fast, high-fidelity, single-shot readout of superconducting qubits in quantum processors demands quantum-limited amplifiers to preserve the optimal signal-to-noise ratio. Typically, quantum-limited amplification is achieved with parametric down-conversion of a strong pump tone, which imposes significant hardware overhead and severely limits scalability. Here, we demonstrate the first dc-powered broadband amplifier operating within 0.2 photons of the quantum limit. Our impedance-engineered inelastic Cooper-pair tunneling amplifier (ICTA)—a voltage-biased superconducting quantum interference device in which Cooper pairs tunnel inelastically by emitting signal-idler photon pairs—operates in reflection, delivering 13 dB of average gain across a 3.5-GHz bandwidth in a single stage. Semiclassical simulations accurately predict the gain and saturation power, enabling further design improvements. By eliminating the pump-tone infrastructure, the broadband ICTA promises to dramatically reduce the hardware complexity of quantum-limited amplification in superconducting quantum processors.

Quantum communication between remote chips is essential for realizing large-scale superconducting quantum computers. For such communication, itinerant microwave photons propagating through transmission lines offer a promising approach. However, demonstrations to date have relied on frequency-tunable circuit elements to compensate for fabrication-related parameter variations between sender and receiver devices, introducing control complexity and limiting scalability. In this work, we demonstrate deterministic quantum state transfer and remote entanglement generation between fixed-frequency superconducting qubits on separate chips. To compensate for the sender-receiver mismatch, we employ a frequency-tunable photon-generation technique, which enables us to adjust the photon frequency without modifying circuit parameters. To enhance the frequency tunability, we implement broadband transfer resonators composed of two coupled coplanar-waveguide resonators, achieving a bandwidth of more than 100 MHz. This broadband design enables successful quantum communication across a 30-MHz range of photon frequencies between the remote qubits. Quantum process tomography reveals state transfer fidelities of around 79% and Bell-state fidelities of around 73% across the full frequency range. Our approach avoids the complexity of the control lines and noise channels, providing a flexible pathway toward scalable quantum networks.

Electron-on-neon (eNe) charge states coupled to superconducting circuits are a promising platform for quantum computing. Control over the formation of these charge states requires techniques to track and control the growth of solid $\mathrm{Ne}$ films on the circuit surface. We demonstrate a real-time $\mathrm{Ne}$ film-growth monitor using high-transition-temperature (high-$T_c$) YBCO microwave resonators. The high $T_c$ enables tracking of the film thickness near $\mathrm{Ne}$’s triple temperature and below. Across more than 300 solidification experiments, we find that the final $\mathrm{Ne}$ thickness varies stochastically from a few nanometers to a few microns for films solidified from the liquid phase. By increasing the driving power in the resonator, we consistently reduce the final thickness to below 100 nm. These results represent an important step toward controlled formation of $\mathrm{Ne}$ films for eNe qubits and highlight the broader utility of high-$T_c$ resonators for hybrid quantum systems.

Nonlocal connectivity is a critical resource for universal logical quantum gates and low-overhead quantum error correction codes, and is unavailable on current superconducting devices with nearest-neighbor connections. To rectify the deficiency in connectivity of superconducting circuit systems, we experimentally demonstrate a convenient on-chip coupler of centimeters length and with quality factor close to $1\times {10}^6$. The entangling gate is performed between two fluxonium qubits, reaching a fidelity of 99.37% within 120 ns, while the system static $ZZ$ interaction rate remains as low as 144 Hz without active cancellation or circuit parameter targeting. This high-fidelity low-crosstalk nonlocal coupler can be the building block of a binary-tree connectivity graph, reducing the average qubit entangling distance from $O(N)$ to $O({\log }_2N)$. With the high-fidelity nonlocal entanglement, novel quantum algorithms can be implemented on the superconducting qubit system, positioning it as a strong competitor to other physics systems regarding circuit connectivity.

We report on the internal quality factor of a three-dimensional niobium quarter-wave coaxial cavity that has undergone mid-temperature annealing, exhibiting $Q_{\mathrm{int}}\gtrsim 3\times {10}^9$ at the single-photon level below 20 mK, which corresponds to an internal photon lifetime of ${\tau }_{\mathrm{int}}\sim 90\mathrm{ms}$. Moreover, $Q_{\mathrm{int}}$ of the mid-temperature-annealed cavities remains almost unchanged even after several cooldown cycles and air exposure. These results suggest that stable low-loss niobium oxides are formed by mid-temperature annealing on the surface of a three-dimensional niobium cavity. This surface treatment could be applicable to the fabrication of two-dimensional superconducting circuits and help improve the lifetime of $\mathrm{Nb}$-based superconducting qubits.

We experimentally observe the generation of magnon harmonics and subharmonics in an on-chip surface-acoustic-wave resonator incorporating a thin ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ film using microfocused Brillouin light scattering. In our devices, rotating the in-plane magnetic field allows continuous tuning of the magnon-phonon coupling from weak to strong within the same resonator. In the weak-coupling regime, we only observe a fundamental magnetoelastic wave signal at $f_0$. Conversely, in the strong-coupling regime, in addition to the fundamental magnetoelastic wave, we observe subharmonic and harmonic signals at $(3/2)f_0$, $2f_0$, and $3f_0$, which are well reproduced by our analytical model. Our results establish phonons as a means to generate and control nonlinear magnons in the strong-coupling regime, providing a route for magnonic signal processing.

We present bifrost, a first-principles model of polarization mode dispersion (PMD) in optical fibers. Unlike conventional models, bifrost employs physically motivated representations of the PMD properties of fibers, allowing users to computationally investigate real-world fibers in ways that are connected to physical parameters such as environmental temperature and external stresses. Our model, implemented in an open-source Python module, incorporates birefringence from core geometry, material properties, environmental stress, and fiber spinning. We validate our model by examining commercial fiber specifications, fiber-paddle measurements, and published PMD statistics for deployed fiber links, and we showcase bifrost’s predictive power by considering wavelength-division-multiplexed PMD compensation schemes for polarization-encoded quantum networks. bifrost’s physical grounding enables investigations into such questions as the sensitivity of fiber sensors, the evaluation of PMD mitigation strategies in quantum networks, and many more applications across fiber technologies.