Band crossings in electronic band structures play an important role in determining the electronic, topological, and transport properties in solid-state systems, making them central to both condensed matter physics and materials science. The emergence of noisy intermediate-scale quantum (NISQ) processors has sparked great interest in developing quantum algorithms to compute band-structure properties of materials. While significant research has been reported on computing ground-state and excited-state energy bands in the presence of noise that breaks the degeneracy, identifying the symmetry at crossing points using quantum computers is still an open question. In this work, we propose a method for identifying the symmetry of bands around crossings and anticrossings in the band structure of bilayer graphene with two distinct configurations on a NISQ device. The method uses eigenstates at neighboring $\mathbf{k}$ points on either side of the touching point to recover the local symmetry by implementing a character-checking quantum circuit that uses ancilla qubit measurements for a probabilistic test, with a complexity scaling linearly with the system size. We then evaluate the performance of our method under a depolarizing noise model, using symmetry operations with four distinct matrix representations and four characters to assess its robustness. Finally, we demonstrate the reliability of our method by correctly identifying the correct band crossings of $AA$-stacked bilayer graphene around the $K$ point, using the character-checking circuit implemented on the noisy IBM quantum processor ibm_marrakesh.

Clock synchronization serves as a foundational subsystem in quantum key distribution (QKD). The recently proposed qubit-based synchronization (Qubit4Sync) presents opportunities in eliminating additional cost, noise, and potential side channels. It offers a promising alternative to dedicated synchronization hardware. However, the current frequency-recovery process in Qubit4Sync requires high data throughput and computational speed, limiting practical use. To overcome these issues, we have developed a fast frequency-recovery algorithm that increases the recovery rate by several orders of magnitude and remains robust under a bad signal-to-noise ratio (SNR). This enables Qubit4Sync to operate effectively in mainstream gated-mode QKD systems. We further establish a theoretical model for frequency recovery, showing that our algorithm is robust against disturbances such as dead time, jitter, and afterpulse. A frequency-domain SNR calculation method is also provided to guide parameter design for specific experimental conditions. This work opens the door to practical Qubit4Sync deployment in general QKD systems.

In pursuit of superconducting quantum computing, fluxonium qubits have recently garnered attention for their large anharmonicity and high coherence at the sweet spot. Towards the large-scale integration of fluxonium qubits, a major obstacle is the need for precise external magnetic flux bias: to achieve high performance at its sweet spot, each qubit requires a flux-bias line. However, such lines inductively coupled to the qubits bring in additional wiring overhead, crosstalk, heating, and decoherence, necessitating measures for mitigating the problems. In this work, we propose a flux-trapping fluxonium qubit, which, by leveraging fluxoid quantization, enables optimal phase biasing without using external magnetic flux control at the operating temperature. We introduce the design and working principle, and demonstrate the phase biasing achieved through fluxoid quantization.

We introduce an alternative method for fabricating all-aluminum Josephson junctions with highly transmitted conduction channels. Such properties are typically associated with structures requiring intricate fabrication processes, such as atomic contacts or hybrid junctions based on semiconducting nanowires and two-dimensional materials. In contrast, our approach relies solely on standard nanofabrication techniques. The resulting devices exhibit a key signature of high-transmission junctions—multiple Andreev reflections—in their current-voltage characteristics. Furthermore, we propose a straightforward superconducting circuit design based on these junctions, enabling the implementation of a parity-protected qubit.

Hybrid mechanical-superconducting systems for quantum information processing have attracted significant attention due to their potential applications. In such systems, the weak coupling regime, dominated by dissipation, has been extensively studied. The strong coupling regime, where coherent energy exchange exceeds losses, has also been widely explored. However, the transition-coupling regime, which lies between the above two and exhibits rich, unique physics, remains underexplored. In this study, we fabricate a tunable coupling device to investigate the coupling of a superconducting transmon qubit to a seven-mode surface acoustic-wave resonator (SAWR), with a particular focus on the transition-coupling regime. Through a series of phonon oscillation experiments and studies in the dispersive regime, we systematically characterize the performance of the SAWR. We then explore the complex dynamics of energy exchange between the qubit and the mechanical modes, highlighting the interplay between dissipation and coherence. Finally, we propose a protocol for qubit readout and fast reset with a multimode mechanical cavity using one mode for readout and another mode for reset. We have demonstrated in simulation that the qubit achieves both fast reset and high coherence performance when the qubit is coupled to the reset mode in the transition-coupling regime.

Spins in semiconductor quantum dots hold great promise as building blocks of quantum processors. Trapping them in $\mathrm{Si}$MOS transistorlike devices eases future industrial-scale fabrication. Among the potentially scalable readout solutions, gate-based dispersive radio-frequency reflectometry only requires the already existing transistor gates to read a quantum dot state, which alleviates the need for additional elements. In this effort toward scalability, traveling-wave superconducting parametric amplifiers significantly enhance the readout signal-to-noise ratio (SNR) by reducing the noise below that of typical cryogenic low-noise amplifiers, while offering a broad amplification band, essential to multiplex the readout of multiple resonators. In this work, we demonstrate a 3-GHz gate-based reflectometry readout of electron charge states trapped in quantum dots formed in $\mathrm{Si}$MOS multigate devices, with SNR enhanced due to a Josephson traveling-wave parametric amplifier (JTWPA). The broad tunable 2-GHz amplification bandwidth combined with more than 10-dB ON-OFF SNR improvement of the JTWPA enables frequency and time-division-multiplexed readout of interdot transitions, and noise performance consistent with JTWPA characteristics at higher frequencies. In addition, owing to a design without superconducting loops and with a metallic ground plane, the JTWPA is flux insensitive and shows stable performance up to a magnetic field of 1.1 T at the quantum dot device, compatible with standard $\mathrm{Si}$MOS spin-qubit experiments.

This work investigates amplitude-dependent nonlinear corrections to the dissipative conductivity in superconductors, using the Keldysh-Usadel theory of nonequilibrium superconductivity, which captures the nonequilibrium dynamics of both quasiparticles and the pair potential. Our rigorous formulation naturally incorporates both the direct nonlinear action of the photon field and indirect contributions mediated by nonequilibrium variations in the pair potential, namely, the Eliashberg effect and the Higgs mode. The third-harmonic current, often regarded as a hallmark of the Higgs mode in disordered superconductors, arises from both the direct photon action and the Higgs mode. Our numerical results are in excellent agreement with previous studies. In contrast, the first-harmonic current, and consequently the dissipative conductivity, receives contributions from all three mechanisms: the direct photon action, the Higgs mode, and the Eliashberg effect. It is shown that the nonlinear correction to dissipative conductivity in dirty-limit superconductors can serve as a fingerprint of the Higgs mode, appearing as a resonance peak at a frequency near the superconducting gap $\mathrm{\Delta }$. In addition, our results provide microscopic insight into amplitude-dependent dissipation at frequencies well below $\mathrm{\Delta }$, which is particularly relevant for applied superconducting devices. In particular, the long-standing issue concerning the frequency dependence of the amplitude-dependent quality factor is explained as originating from the direct nonlinear action of the photon field, rather than from contributions by the Higgs mode and the Eliashberg effect. Our practical and explicit expression for the nonlinear conductivity formula makes our results accessible to a broad range of researchers.

Nuclear spins associated with the nitrogen-vacancy (NV) center are useful resources for quantum information processing and quantum sensing, especially as quantum memories. They are also an emerging platform for constructing inertial sensors. The native nitrogen spin of the NV center can serve as a gyroscope using Ramsey interferometry protocols. The sensitivities of these nuclear-spin-based NV gyroscopes are limited by the phase coherence time of the nuclear spin, which is in turn limited by the longitudinal coherence time $(T_1)$ of the NV center’s electron spin. There is an active research effort to decouple the nuclear spin from the electron spin and other sources of decoherence to extend the phase coherence time past the NV center electron $T_1$. However, this presents an associated challenge—how to read out the nuclear spin ensemble if the electronic spins have relaxed into a thermal state. We introduce an optical repump pulse to repolarize electron spins from an artificial thermal state and investigate the effect on the readout of nuclear spins in this regime. We find that for ensemble-based sensors our readout protocol has around a sixfold improvement in sensitivity around the excited-state level anticrossing (ESLAC) at 500 G compared to low fields (200 G), despite the presence of deleterious spin flip-flop dynamics in the excited state. Our results highlight how ESLAC dynamics can benefit inertial sensing and other sensing applications involving nuclear quantum memories.

Superconducting qubits in today’s quantum processing units are typically fabricated with angle-evaporated aluminum–aluminum-oxide–aluminum Josephson junctions. However, there is a need for higher yield and tighter parameter control when scaling up the number of qubits and junctions into tens of thousands and beyond. Fabrication methods based on subtractive patterning of superconductor-insulator-superconductor trilayers, used for more classical large-scale Josephson junction circuits, could provide the solution but they in turn often suffer from lossy dielectrics incompatible with high qubit coherence. In this work, we utilize native aluminum oxide as a sidewall passivation layer for junctions based on aluminum–aluminum-oxide–niobium trilayers, and use such junctions in qubits. We design the fabrication process such that the few-nanometer-thin native oxide is not exposed to oxide removal steps that could increase its defect density or hinder its ability to prevent shorting between the leads of the junction. With these junctions, we design and fabricate transmonlike qubits and measure time-averaged coherence times up to $30\text{μ}\mathrm{s}$ at a qubit frequency of 5 GHz, corresponding to a qubit quality factor of one million. Our process uses subtractive patterning and optical lithography on wafer scale, enabling high throughput in patterning. This approach provides a scalable path toward fabrication of superconducting qubits on industry-standard platforms.

We study parametric oscillations of two capacitively coupled Kerr parametric oscillators (KPOs) with frequency-chirped two- and one-photon drives. The two-KPO system adiabatically evolves from the initial vacuum state to an oscillation state corresponding to a solution state in quantum-annealing applications. Frequency chirping dynamically changes the detuning between resonance and oscillation frequencies during parametric modulation and reduces unwanted population transfer to excited states caused by pure dephasing and photon loss. We observe that frequency chirping increases the success probability to obtain the solution state and that simulations taking into account pure dephasing reproduce experiments with and without frequency chirping. This study demonstrates the effectiveness and applicability of frequency chirping to a KPO-based quantum annealer.

In the “flopping-mode” regime of electron spin resonance, a single electron confined in a double quantum dot is electrically driven in the presence of a magnetic-field gradient. The increased dipole moment of the charge in the flopping mode significantly reduces the amount of power required to drive spin rotations; however, the susceptibility of flopping-mode spin qubits to charge noise, and consequently their overall performance, has not been examined in detail. In this work, we simulate single-qubit gate fidelities of electrically driven spin rotations in an ensemble of devices configured to operate in both the single-dot and flopping-mode regimes. Our model accounts for the valley physics of conduction-band electrons in silicon and realistic alloy disorder in the $\mathrm{Si}\text{−}\mathrm{Ge}$ barrier layers, allowing us to investigate device-to-device variability. We include charge and magnetic noise, as well as spin-relaxation processes arising from charge noise and electron-phonon coupling. We find that the two operating modes exhibit significantly different susceptibilities to the various noise sources, with valley splitting and spin-relaxation times also playing a role in their relative performance. For realistic noise strengths, we find that single-dot gate fidelities are limited by magnetic noise, while flopping-mode fidelities are primarily limited by charge noise and spin relaxation. For sufficiently long spin-relaxation times, flopping-mode spin operation is feasible with orders-of-magnitude lower drive power and gate fidelities that are on par with conventional single-dot electric dipole spin resonance.

Future quantum information processors require tunable coupling architectures that can produce high-fidelity logical gates between two or more qubits. Parametric coupling is a powerful technique for generating tunable interactions between many qubits. Here, we present a highly flexible parametric coupling scheme with superconducting qubits that provides complete removal of residual $ZZ$ coupling and the implementation of driven SWAP or SWAP-free controlled-$Z$ ($CZ$) gates. Our fully integrated two-dimensional on-chip coupler design is only weakly flux tunable, cancels static linear coupling between the qubits, avoids internal coupler dynamics or excitations, and is extensible to multiqubit circuit-QED systems. Exploring gate fidelity versus gate duration allows us to maximize two-qubit gate fidelity, while providing insights into possible error sources for these gates. Randomized benchmarking over several hours reveals that the parametric SWAP $CZ$ gate achieves an average fidelity of $99.44\pm 0.09\mathrm{\%}$ in a gate duration of 70 ns and that a dispersively driven parametric SWAP-free $CZ$ gate attains an average fidelity of $99.47\pm 0.07\mathrm{\%}$ in only 30 ns. The fidelity remained above this value for over 8 h and peaked twice with a maximum of $99.67\pm 0.14\mathrm{\%}$. Overall, our parametric approach combines versatility, precision, speed, and high performance in one compact coupler design.

We propose a digitally controlled conveyor-belt shuttling method for silicon-based quantum processors, addressing the scalability challenges of conventional analog sinusoidal implementations. By placing a switch matrix and low-pass filters in a cryogenic environment, our approach synthesizes near-sinusoidal waveforms from a limited number of dc voltage levels. Simulation results demonstrate that the proposed method achieves fidelity comparable to analog methods while significantly reducing wiring overhead and power dissipation. Moreover, the design offers robustness against device-level variations, enabling large-scale integration of high-fidelity spin shuttling for quantum error correction.

Broadband quantum-limited parametric amplifiers (PAs) are essential components in quantum information science and technology. Impedance-engineered resonator-based PAs and traveling-wave PAs are the primary approaches taken to overcome the gain-bandwidth constraint. While the former PAs are simpler to fabricate, the target characteristic impedance $Z_{\mathrm{NR}}$ of the nonlinear resonator has been restricted to be below $10\mathrm{\Omega }$, requiring large capacitance. Moreover, these PAs have only been implemented with aluminum-based Josephson junctions (JJs), hindering their operation at high temperatures or strong magnetic fields. To address these issues, we propose a three-stage impedance-transformer scheme, showcased with a 20 nm thick, 250 nm wide, high-kinetic-inductance niobium-titanium-nitride ($\mathrm{Nb}\mathrm{Ti}\mathrm{N}$) film. Our scheme enables $Z_{\mathrm{NR}}$ up to several tens of ohms—a tenfold improvement over conventional designs, achieved through an additional quarter-wavelength transmission line with a characteristic impedance of 180 $\mathrm{\Omega }$. Our kinetic-inductance impedance-engineered parametric amplifiers (KIMPAs), featuring a 330 fF shunt capacitor, demonstrate a phase-preserving amplification with a 450-MHz bandwidth at 17 dB gain, and added noise ranging from 0.5 to 1.3 quanta near the center frequency of 8.4 GHz. Owing to the high critical current of the $\mathrm{Nb}\mathrm{Ti}\mathrm{N}$ nanowire, the KIMPA also achieves an output saturation power of up to $-51\pm 3$ dBm, approximately 25 dB higher than that of JJ-based PAs. This scheme also opens new possibilities for other three-wave-mixing building blocks.

Space-division multiplexing using multimode optical fibers has been applied to quantum-level signals with time-bin and phase encoding, achieving megaqubits per second over 8 km of few-mode fiber. Time windowing is efficiently employed to eliminate modal crosstalk between quantum channels. The dead time of the single-photon detectors in each quantum channel, typically regarded as a bottleneck in quantum transmission, is synchronized with the time-window intervals, thereby optimizing the detection rates.

Spin estimation plays a crucial role in quantum metrology, particularly in systems like atomic magnetometers where atomic spin ensembles transduce magnetic-field information into measurable optical signals. However, precise spin estimation remains challenging, especially in weak polarization schemes where signals are often obscured by both classical and quantum noise. Conventional fitting methods for spin signal and variance estimation are computationally intensive, lack real-time capabilities, and exhibit sensitivity to stochastic fluctuations and data-acquisition parameters. To address these challenges, we propose and demonstrate the implementation of a Kalman filter (KF) combined with a Rauch-Tung-Striebel (RTS) smoother for enhanced spin estimation across various polarization conditions. Our comprehensive analysis of spin conditional variance estimation reveals that the KF method achieves a remarkable 4.92-dB noise suppression compared to conventional fitting methods, while the RTS smoother provides further enhancement with 6.94-dB suppression that enables quantum noise squeezing observation. These findings offer new tools for quantum noise analysis and signal recovery, offering significant potential for advancing the performance of quantum-enhanced sensors and other precision measurement technologies.

The Bennett-Brassard 1984 (BB84) protocol is one of the simplest protocols for implementing quantum key distribution (QKD). In the protocol, the sender and the receiver iteratively choose one of two complementary measurement bases. Regarding the basis choice by the receiver, a passive setup has been adopted in a number of its implementations, including satellite QKD and time-bin encoding. However, conventional theoretical techniques to prove the security of the BB84 protocol are not applicable if the receiver chooses their measurement basis passively, rather than actively, with a biased probability, followed by measurement with threshold detectors. Here we present a fully analytical security proof against coherent attacks for such a decoy-state BB84 protocol with the receiver’s passive basis choice and measurement with threshold detectors. Numerical simulations under practical situations show that the difference in secure key rate between the active and passive implementations of the protocol is negligible except for long communication distances.

Scaling superconducting quantum processors to large qubit counts faces challenges in control-signal delivery, thermal management, and hardware complexity, particularly in achieving microwave signal multiplexing and long-distance quantum information routing at millikelvin temperatures. We propose a space-time modulated Josephson-junction metasurface architecture to generate and multiplex microwave control signals directly at millikelvin temperatures. Theoretical and numerical results demonstrate the generation of multiple frequency tones with controlled parameters, enabling efficient and scalable qubit control while minimizing thermal loads and wiring overhead. We derive the nonlinear wave equation governing this system, simulate beam steering and frequency conversion, and discuss the feasibility of experimental implementation. These results lay the groundwork for a next-generation cryogenic signal-delivery paradigm that may enable scaling of superconducting quantum processors to thousands of qubits without overwhelming limited dilution-refrigerator cooling power.

Parametric modulation, valued for its versatility, is widely employed in superconducting circuits for quantum simulations and high-fidelity two-qubit gates. Conventionally, the qubit coupling strength is determined by the amplitude of the parametric flux pulse, which affects the qubit parameters dramatically. In this paper, we propose and implement a phase-modulation scheme to tune the interaction strength via adjustment of the relative phase between the parametric flux pulses applied to two coupled qubits. We characterize this modulation for sideband couplings, at both sweet and off-sweet spots, achieving a broad range of coupling strengths, as confirmed by both population dynamics and spectroscopy methods. This approach enables phase-controlled modulation of coupling strength, providing a promising candidate for parametrically driven quantum simulations and gate operations.