I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
27
Okt
2026
A Unified Protocol for Arbitrary Z Rotations of a Class of Bosonic cQED Qubits
Bosonic-qubit encodings provide diverse approaches to storing and processing quantum information, with logical operations often implemented through encoding-specific physical strategies.
Here, we identify a class of bosonic Fock-based qubits in circuit QED for which distinct implementations of arbitrary Z rotations share a common structure. We formulate this structure as a six-step protocol that provides a unified procedure for implementing arbitrary Z rotations across this class of bosonic cQED qubits. The applicability of the protocol is determined by a specific condition on the encoding, providing a criterion for identifying bosonic Fock-based cQED qubits that admit the common implementation.
26
Okt
2026
Suppressing microwave crosstalk in superconducting quantum processors by designing parasitic drive capacitances
As superconducting quantum processors scale to large qubit counts, unwanted microwave drive crosstalk becomes an important source of coherent errors. Here, we investigate the role of
parasitic coupling paths in the suppression of drive crosstalk in a processor with two fixed-frequency transmon qubits and a tunable coupler. To account for the many different coupling paths, we develop a detailed circuit model to capture the dependence of crosstalk on the coupler frequency. We experimentally demonstrate suppression of crosstalk, finding that the suppression does not originate solely from cancellation of the effective qubit-qubit coupling, but also is a result of interference between other coupling paths. Building on these insights, we show that drive crosstalk can be suppressed with the right choice of parasitic capacitances, which otherwise often are treated as unwanted design imperfections. Our results provide practical guidance for low-crosstalk device design of superconducting quantum processors and point toward a broader strategy in which parasitic coupling paths are engineered rather than simply minimized.
A Control Line Multiplexing Framework for Scalable Quantum Computing
Universal fault-tolerant superconducting quantum computers are expected to require millions of physical qubits; yet extending the prevailing dedicated-control-line paradigm to this
scale would exceed the wiring density and cooling-power budget of dilution refrigerators. Microwave frequency-division multiplexing (FDM) revolutionized classical communications by enabling many independent channels to share common transmission infrastructure. Translating this principle to superconducting quantum control, however, requires an all-microwave architecture and experimental validation of frequency-multiplexed gate control. Recent work in quantum error correction has demonstrated microwave-based readout, single-qubit gates and leakage suppression, whereas two-qubit gates remain predominantly driven by unipolar pulses and thus constitute the missing element of an all-microwave architecture. Here, we propose a periodically extensible hybrid multiplexing framework in which all gate operations are frequency-division multiplexed using polychromatic microwave pulses and band-pass filters, thereby reducing the required control-line count by one to two orders of magnitude. In a 1:4 demonstration, we achieve average errors of (6.4±0.2)×10−3 for parallel FDM CZ gates and (10.6±0.2)×10−4 for parallel FDM XY(π/2) gates, without statistically significant multiplexing-induced degradation. Together with on-chip DC sources, this FDM-based gate-control scheme completes the last missing module in a scalable multiplexed-control architecture toward universal fault-tolerant quantum computing.
Growth Phase Map of α and \b{eta} Tantalum as a Function of Growth Temperature and Oxygen Partial Pressure
Tantalum thin films are used in low-loss superconducting quantum circuits, yet the growth conditions governing selection of α-Ta(110), α-Ta(111), and metastable β-Ta on c-plane sapphire
remain uncertain. We deposited 100 nm Ta films by dc magnetron sputtering on Al2O3(0001) at 300-900 ∘C while varying the O2 partial pressure from a 5.5×10−11 Torr ultrahigh-vacuum baseline to 2.23×10−4 Torr. Absorption-corrected X-ray diffraction texture coefficients were used to construct a ternary phase-orientation growth map in terms of temperature and the oxygen-to-Ta impingement flux ratio (2ΦO2/ΦTa≤4.8×10−6 to 19). Without added oxygen, α-Ta(110) dominated at every temperature. At an O2 pressure of only 2.5×10−9 Torr (2ΦO2/ΦTa=2.2×10−4), α-Ta(111) accounted for 98-99% of the normalized texture weight from 300 to 700 ∘C, decreasing to 65% at 900 ∘C. Substantial β-Ta formed only at the highest oxygen dose, dominating at 300 ∘C and disappearing at 900 ∘C. The apparent β(001)/α(110) selection energy of 1.07±0.16 eV is comparable to the activation energy for oxygen diffusion in Ta, consistent with oxygen transport influencing phase selection. Oxygen also produced orientation-dependent changes in crystallite size and microstrain. These findings establish oxygen-controlled processing windows for phase- and orientation-selective growth of Ta films for superconducting devices.
25
Okt
2026
Real-Time Correlation of Relaxation, Dephasing, and Two-level System Dynamics in Transmon Qubits
Materials improvements in transmon qubits have yielded remarkable advances in qubit lifetimes (T1)and coherence times (T2). However, state-of-the-art superconducting qubits experience
temporal fluctuations in their performance, which limit gate fidelities and quantum processor performance. We observe correlated increases in qubit relaxation and dephasing rates that last for minutes to hours, which we call T1 and T2 excursions. Using interleaved lifetime and coherence time measurements, noise spectroscopy, and two-level-system (TLS) spectroscopy, we find that these excursions are caused by single or few strongly-coupled TLS defects that spectrally diffuse into resonance with the qubit frequency. Our measurement protocol provides a powerful tool for understanding the noise sources that cause temporal fluctuations in superconducting qubit performance.
23
Okt
2026
Trace-distance characterization of recurrent pairwise synchronization in cavity-coupled superconducting qubits
We theoretically investigate recurrent pairwise synchronization in three super?conducting qubits coupled to a common single-mode microwave cavity. The qubits interact indirectly through
the cavity field and are driven by a coher?ent microwave field. We obtain the coherent dynamics in the dressed-state basis and quantify pairwise synchronization by the trace distance between reduced single-qubit density matrices. The trace-distance dynamics exhibit oscillatory wave-packet structures, with low-trace-distance regions identifying finite syn?chronization windows. An envelope-threshold procedure is introduced to extract the representative duration of these windows. By varying the middle-qubit coupling strength and the relative coupling ratio of the outer qubits, we con?struct a parameter-dependent map of the synchronization-window duration. The duration decreases rapidly with increasing relative outer-qubit coupling and becomes weakly dependent on the middle-qubit coupling in the stronger-coupling regime. These results provide a reduced-state-based characterization of coherent collective dynamics in multiqubit circuit-QED systems.
22
Okt
2026
Optimal Control of a Fluxonium Circuit with the Q-PRONTO Numerical Method
Optimizing the control of large quantum systems is a resource-intensive task, where the convergence rate of the underlying iterative schemes is crucial. The recently developed Quantum
Projection Operator-Based Newton Method for Trajectory Optimization (Q-PRONTO) offers a unique quadratic convergence for controlling quantum states, while most model-based solvers guarantee only an approximately linear convergence rate. As a proof of concept, here, we design and benchmark single-qubit gates optimized by Q-PRONTO based on the circuit Hamiltonian of a fluxonium superconducting qubit. To ensure that we can apply a long string of arbitrary pulses to the circuit, we modify the firmware associated with the Quantum Instrumentation Control Kit (QICK) by altering its memory allocation. We characterize the performance of the optimal solutions using standard randomized benchmarking and achieve a total error per gate of 6.6×10−4.
High-fidelity and robust controlled-Z gates for superconducting transmon qubits. II. System-parameter uncertainties and overall errors
In the companion paper (Part I), we developed an optimal-control framework to suppress intrinsic and decoherence-induced errors in controlled-Z (CZ) gates between directly coupled,
frequency-tunable transmons. Here, we extend this framework to incorporate uncertainties in coupling strength, anharmonicities, and zero-flux qubit frequencies, enabling the design of high-fidelity gates robust against noise and parameter uncertainties. We classify these uncertainties by whether they perturb the off-diagonal coupling strengths or the diagonal detunings of the Hamiltonian, and we identify distinct suppression mechanisms for each class. Coupling errors are mitigated through dynamical decoupling induced by strong detuning modulation. For a 30-ns gate, this strategy reduces the coupling error to 6.21×10−7 and tolerates coupling-strength uncertainties up to 2π×4 MHz while keeping total gate infidelity below 10−3. Detuning errors are minimized near a symmetric-detuning operating point that balances the two dominant leakage-mediated interactions. A 24-ns gate operated near this point tolerates individual uncertainties of up to 2π×5 MHz in each relevant diagonal parameter under the same infidelity threshold. We further optimize gate performance under realistic conditions where intrinsic errors, decoherence, and parameter uncertainties coexist across six representative stages of progressively improving hardware performance. The resulting stage-specific controls achieve lower infidelity and greater robustness. Together with the companion study (Part I), this framework offers a practical approach to balance speed, fidelity, and robustness while reducing the need for exhaustive characterization and frequent recalibration in large-scale superconducting quantum processors.
21
Okt
2026
Superconducting parametric amplifiers based on arrays of unequal Josephson junction elements
Impedance matching is a crucial feature of an amplifier used to increase its instantaneous bandwidth and is typically achieved using static LC resonators. Here we show an approach to
impedance matching in parametric amplifiers using arrays of unequal Josephson-junction-based elements (rf/dc SQUIDs, SNAILs, etc). The elements act either as small-footprint flux-tunable inductors forming an impedance-matching circuit, or ‚active‘ amplifier elements which result in amplifier gain. We develop an analytical theory describing these systems, predicting a quasi-non-periodic amplifier frequency and gain modulation with applied flux and the broadening of gain bandwidths. We demonstrate this in devices using arrays of flux-focused SNAILs where flux biasing varies across the array. We find instantaneous bandwidths of 200 MHz with >17 dB of gain centred at ∼5.4 GHz in a device without a static impedance matcher. This broadening of amplifier bandwidth is stable with >100 MHz bandwidth at 17 dB gain across >1.2 GHz tunable bandwidth. Our analytical model captures flux dependence and qualitatively reproduces gain-bandwidths giving confidence in our understanding of these systems and allowing operational figures of merit to be optimised in future devices.
Meter-scale quantum interconnects between large quantum processors
Modular quantum architecture requires long-distance quantum interconnects to stitch together individual processor modules. Such interconnects, long-distance couplers or L-couplers,
must support coherent quantum operation over meter-scale distances and scale to multiple connections between any two modules. Here we demonstrate four L-couplers operating between two 120-qubit IBM Nighthawk R1 superconducting qubit processors. Each L-coupler joins two Nighthawk lattice qubits through two frequency-tunable L-qubits, two RF-SQUID based „i2m“ couplers, and 80 cm of superconducting aluminum coax. Each cable installs perpendicular to the processor surface, to enable denser connections, and is joined to the chip by an indium-to-indium room temperature pressure bond made after both modules were mounted. L-coupler modes near 5 GHz, with internal quality factors of 0.86 to 1.09e6, carry relay-based state transfer at a mean network benchmarking fidelity of 99.00%, with a best link of 99.29 +/- 0.07%. We then realize distant CZ gates between lattice qubits in separate modules at a mean fidelity of 94.75%, with a best link of 96.42 +/- 0.28%, and use them to generate entangled states across the two processors, including a demonstration of simultaneous Bell pair generation using two adjacent L-couplers. These four high-fidelity L-couplers establish a practical path to scalable quantum interconnects for modular quantum computing.