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
03
Nov
2026
Autonomous cooling of Kerr-cat qubits to minutes-long bit-flip times
Quantum error correction becomes more hardware efficient when physical noise is strongly biased towards one error channel. Cat qubits realize such a bias by encoding information in
two opposite coherent states of a superconducting oscillator, so that bit flips are exponentially suppressed with photon number while phase-flip rates increase only linearly. In Kerr-cat qubits, a driven double-well potential confines these states and enables fast coherent control, but excitations can induce interwell switching, limiting bit-flip times to milliseconds. Here we show that the nonlinear interaction generating the confinement can also cool these excitations. When an intra-well transition becomes resonant with a lossy readout mode, excitations are transferred out of the qubit and dissipated, as established by spectroscopy and relaxation measurements. We observe the predicted exponential scaling of Kerr-cat bit-flip times with photon number, reaching 450 s, over five orders of magnitude beyond previous demonstrations. Phase-flip rates remain consistent with intrinsic photon loss at operating points with seconds-long bit-flip times. We further demonstrate fast Z gates without measurable degradation of bit-flip protection and single-shot readout with a bit-flip probability below 10−6 per measurement. Autonomous cooling thus brings minutes-long bit-flip protection to Kerr-cat qubits while retaining fast coherent control, without additional cooling drives or circuit elements.
02
Nov
2026
Analog quantum simulation of paraparticle statistics
Bosonic and fermionic quantum statistics describe all known elementary and emergent particles in three dimensions, although many attempts have been made to discover other valid quantum
statistical frameworks. One recent formulation proposes „R-paraparticles,“ indistinguishable particles with nonlocal internal degrees of freedom governed by generalized exchange rules [Wang and Hazzard, Nature 637, 314 (2025)]. Here, we emulate a species of paraparticle that exhibits a generalized Pauli exclusion principle using a superconducting qubit lattice. We confirm its unique quantum statistics by measuring a 180∘ phase shift under paraparticle exchange, observing antibunching independent of internal degrees of freedom under two-particle interference at a beamsplitter, and finding a thermal occupation inconsistent with both Bose-Einstein and Fermi-Dirac statistics. These results demonstrate that parastatistics are observable on quantum simulation platforms, further motivating their use to explore theorized new phases of matter.
30
Okt
2026
Topologically Protected Edge States in a Single Flux Qubit
We show that topological edge states can be realized in a simple superconducting circuit composed of an inductively shunted flux qubit. The Josephson potential of that qubit forms a
honeycomb lattice similar to the case of graphene. The additional shunting element produces a quartic potential along one flux direction, which effectively creates a boundary in flux space. We map analytically the low-energy dynamics of the circuit to an extended Su–Schrieffer–Heeger (SSH) chain, in which a quartic potential is added to the usual SSH Hamiltonian. This mapping is confirmed by numerical simulations of the full superconducting circuit. We argue that chiral symmetry still holds for the edge states of the extended model around a chiral point in parameter space. Close to this point, we numerically find that edge states in our superconducting circuit are topologically protected against perturbations that obey chiral symmetry, as is the case for charge noise. Our results show that topological effects previously thought to occur in multi-qubit systems, such as protection against decoherence, can also be found in much simpler single superconducting qubits.
Symmetries and model reduction in a cos2φ transmon qubit
Engineered symmetries in superconducting circuits can protect quantum information; much like selection rules in atomic physics, they forbid transitions between the qubit states. Protected
circuits, however, combine several loops and islands and host multiple nonlinearly coupled modes, calling for reduced low-energy models that inherit the circuit symmetries and make the protection manifest. We develop such a reduction for the cos2φ transmon qubit, recently realized experimentally with a two-Cooper-pair interferometer, whose circuit hosts three coupled modes. Exploiting the separation of energy scales between the heavy transmon mode and the light internal modes of the interferometer, we derive an effective single-band Born-Oppenheimer model, including its geometric correction, and extend it to several bands. We identify the symmetries of the full and reduced Hamiltonians and derive exact selection rules. Finally, we identify a parameter regime beyond the single-band description that suppresses flux-noise dephasing without degrading charge-noise protection.
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.