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
07
Okt
2026
Nonlinear Feedback in Josephson Circuit Optimization: Application to a Kerr-Reversal JTWPA
Optimizing Josephson-based nonlinear microwave devices is computationally demanding because the straightforward approach requires exploring broad circuit parameter spaces through expensive
nonlinear simulations. Josephson Circuits Optimizer addresses this problem by using harmonic balance simulations in two stages: fast linear simulations to select promising circuit configurations according to properties such as impedance and phase matching, followed by nonlinear simulations to optimize their operating conditions. However, pump-induced effects such as impedance renormalization and Kerr-induced modifications of phase matching are not captured during the initial linear stage. We therefore introduce a nonlinear feedback extension that transfers information from the pumped response back to the linear optimization. The approach is investigated using a Josephson traveling-wave parametric amplifier with a reversed-Kerr architecture, whose model is validated against experimental gain measurements. We then evaluate the difference between the linear and pumped response values of a metric based on the input reflection coefficient, while excluding configurations with insufficient third-harmonic suppression. This discrepancy is considered a candidate feedback observable, as it exhibits features similar to the gain landscape, supporting its use in subsequent optimization cycles.
06
Okt
2026
Physical Design Automation for Planar Superconducting Quantum Chips
Superconducting circuits have emerged as one of the most promising and scalable platforms for quantum com- puting, with substantial industrial adoption driving qubit counts steadily
upward. Yet the layout of the corresponding chips is still primarily performed manually, consuming multiple days or weeks of expert effort, even for moderately sized designs. Existing automation approaches sidestep this bottleneck by simplifying the underlying problem and relaxing physical constraints. Therefore, they fall short of fabrication-ready results. To overcome this scalability wall without such compromises, we first introduce a formal abstraction that translates physical device characteristics into a rigorous geometric problem formulation. This abstraction bridges physical realization and design automation. Building upon this abstraction, we then propose a three-stage design automation flow comprising geometry-aware global partitioning, ILP-based port assignment, and a hierarchical routing pipeline. Evaluations conducted by our interdisciplinary team of design au- tomation researchers and superconducting hardware experts con- firm that the resulting flow produces complete, manufacturing- ready layouts that comply with the physical design rules at the push of a button. That is, it operates fully automatically within seconds to a few minutes, and solves complex instances up to 25x faster than the state of the art. All methods are released as the fully open-source tool mqt-scpd as part of the Munich Quantum Toolkit, providing the first readily usable, end- to-end design flow for planar superconducting quantum chips that explicitly addresses the underlying physical characteristics and requirements.
04
Okt
2026
Epitaxial NbN-junction-based self-shunted superconducting flux qubit with high anharmonicity
We report a demonstration of a self-shunted superconducting flux qubit (SSFQ) as an ultracompact, strongly anharmonic, and long-lifetime qubit by using the full epitaxially grown NbN/AlN/NbN
Josephson junctions. The NbN-junction-based SSFQ achieved a high anharmonicity of 797 MHz as well as drastically reduced footprint of the qubit compared to capacitively shunted (C-shunt) flux qubits with a large shunt capacitor. The lifetime of the SSFQ reached 13 – 25 μs, which is comparable to or relatively higher than that of NbN-based C-shunt flux qubits and Al-based flux qubits with more than 10 times smaller junctions. This indicates that the large epitaxial NbN-based junction can be adopted for SSFQ as well as merged-element transmon with the relatively large junctions. The results presented in this work will offer a new choice of the scalable qubits as well as the material platform toward the realization of large-scale superconducting quantum computers.
Sidewall spacer passivated epitaxial NbN/AlN/NbN Josephson junctions for superconducting qubit applications
Epitaxial NbN/AlN/NbN Josephson junctions are promising alternatives to conventional Al/AlOx/Al junctions for superconducting qubits due to their crystalline tunnel barriers and the
high transition temperature of NbN. However, their integration into quantum circuits typically requires complex fabrication processes involving amorphous interlayer dielectrics, which can degrade qubit coherence times. To address these issues, we developed sidewall spacer passivated epitaxial NbN/AlN/NbN junctions using a fabrication process that reduces both the volume of amorphous dielectric material and fabrication complexity. The fabricated junctions exhibited a large gap voltage of approximately 5.6 mV and low subgap leakage currents. By varying the thickness of the AlN tunnel barrier, high-quality junctions were fabricated over a wide range of critical current densities Jc, from 0.3 to 104 A/cm2. In addition, the comparison of the current-voltage characteristics before and after buffered hydrofluoric acid treatment indicated that the wet etching process did not degrade the electrical characteristics of the junctions. These results establish a scalable fabrication process for epitaxial Josephson junctions and provide a promising platform for high-coherence nitride-based superconducting qubits.
Arbitrary-phase CZ gate and remote readout across spatially separated superconducting nodes
Establishing a complete set of gates across spatially separated quantum nodes is fundamental for distributed quantum computation and communication networks. One of the approaches to
do so consists of employing a superconducting device as a universal quantum node that is capable of storing and processing quantum information, and traveling microwave photons as means of communication. In this work, we realize an entangling gate across a primitive two-node superconducting circuit QED network consisting of two dilution cryostats, A and B, connected via a 30-m-long cryogenic quantum link. We perform a bidirectional photon transfer protocol, where a microwave photon emitted by the communication qubit at A acquires a conditional phase based on the state of qubit B. We vary the communication photon frequency, hence tuning the conditional phase acquired during the reflection off the remote node, effectively implementing an arbitrary-phase CZ gate. We use this entangling operation to perform single-shot readout of the qubit in node B from node A. An interesting feature of our scheme is that it does not require precise synchronization of the active node with the remote node; in particular, remote readout can be performed without any active participation by the remote node. These protocols add to the quantum communication toolbox with superconducting circuits and provide a way of realizing basic distributed quantum computing operations.
30
Sep
2026
Real-Time Adaptive Filtering and the Boxcar Limit in Superconducting Qubit Readout
Control and readout systems for superconducting quantum computing support real-time control and high-fidelity qubit-state discrimination. We examine when additional digital processing
improves readout fidelity. A common baseline for dispersive readout uses a boxcar averager to compute a uniformly weighted average over a fixed window, followed by threshold-based state assignment. In this paper, we evaluated an adaptive finite-impulse-response (FIR) filter trained in real time using the least-mean-squares (LMS) algorithm on a field-programmable gate array (FPGA) quantum controller. We also analyzed measured readout shots offline to compare the frozen filter with boxcar averaging and to evaluate per-sample weighting and sequential detection. For the fixed FIR filters tested, we show summing the filtered samples gives the boxcar sum multiplied by one complex number, apart from a small edge correction. We call this the kernel collapse. The multiplication scales the state separation and noise equally without improving state discrimination, and we call this the boxcar limit. The frozen LMS filter reduces trace noise, but same-shot offline comparisons show no resolved improvement in readout fidelity at the tested operating points. On a 2D transmon, tuning the integration window accounts for much of the observed improvement over full-window integration, highlighting the importance of an optimized boxcar baseline when evaluating per-sample weighting. We then examine how changing the decision method can reduce calibration effort and decision time. On a higher-fidelity 3D ancilla readout, model-based template estimation reaches the same fidelity target as direct averaging with about 30 rather than 300 labeled shots per state. Offline sequential replay assigns most shots before the acquisition window ends, with contrast fidelity within an absolute +/-0.5% tolerance of the boxcar on the primary…
29
Sep
2026
Time-Integrated Leakage as a Dynamical Benchmark for Multi-Mode Superconducting-Qubit Reset
Reset performance in superconducting qubits is often summarized by the population that remains at the end of the pulse. This is an important number, but it does not describe what happens
during the reset itself. A protocol may reach a small final residual while keeping the qubit in a leakage state for a comparatively long part of the trajectory. In this work, we therefore use time-integrated leakage, together with the usual endpoint population, to study a multi-mode dissipative reset model under flux-frequency control. We examine finite thermal occupation, parameter sweeps, frequency disorder, an auxiliary-chain ablation, and a literature-based rate benchmark. At the representative ablation point, adding the auxiliary chain reduces Lf from 8.56 to 0.78 ns and shifts the sustained Pf less-than- 10-2 crossing from 39.5 to 9.8 ns. Interestingly, the configuration without the chain still reaches the smaller long-time residual. The two metrics can therefore favor different reset configurations. For the same 300 ns analysis window, an effective rate model built from the f-ket and e-ket decay rates reported by Zhou et al. gives Lf is 108.0 ns, while the complete simulated configuration gives 0.783 ns. We use this only as a kinetic reference and not as a reproduction of the experiment. In the wider set of simulations, thermal occupation is the main source of degradation, while the frequency disorder considered here has a much weaker effect. The results suggest that time-integrated leakage is a useful quantity to report alongside endpoint residuals, especially when reset is part of repeated quantum-error-correction cycles.
Parametric DRAG for leakage-suppressed exchange gates in superconducting qubits
Programmable exchange interactions support quantum gates and many-body simulation in superconducting circuits. Fast excitation transfer, however, opens two leakage pathways in weakly
anharmonic transmons, degrading gates and driving simulated dynamics outside the encoded state space. We introduce parametric derivative removal by adiabatic gate (PDRAG) to suppress both pathways while preserving the target exchange. Their conjugate structure determines first- and second-derivative corrections implemented through one real frequency command. At g/2π=100 MHz, calibrated PDRAG achieves a geometric-mean leakage reduction of 5.27×105 relative to the base pulse across 12.5–35 ns in Duffing simulations. At 13 ns, endpoint leakage reaches 1.98×10−7 for the calibrated full-exchange pulse. Transferring the derivative coefficients to a charge Hamiltonian gives a mean 893-fold reduction over 12.5–22 ns after amplitude and carrier recalibration. Floquet return interference and charge-induced gap shifts explain the calibration and transfer gains, connecting compact pulse design to leakage-suppressed programmable interactions.
Directional transfer from coherence to longitudinal polarization and algebraic storage in a transmon with Markovian dephasing and a glassy environment: A non-Markovian fractional renewal model
We develop a completely positive and trace-preserving non-Markovian transmon model with longitudinal Markovian dephasing and transverse renewal events motivated by a glassy two-level-systemenvironment. A microscopic Hamiltonian identifies coupling subspaces; a normalized Prabhakar waiting-time law phenomenologically coarse grains slow defect reconfigurations. Forward chronological renewal histories yield an exact map-level Volterra equation, a noncommuting kernel–resolvent master equation, and an equivalent drift-conjugated Caputo-type representation with its required initial-value term. Without energy relaxation, Pauli-space dynamics splits into two one-dimensional subspaces and a coupled (σy,σz) subspace supporting directional transfer between coherence and longitudinal polarization; σx polarization is renewal-insensitive. For heavy-tailed waiting times with 0<α<1, branch-cut analysis gives a generic t−α storage tail in longitudinal polarization generated from initial coherence and O(t−1−α) reverse transfer; constant-rate Poisson renewal yields pole-generated exponential decay. Independent zero-temperature relaxation at finite T1 exponentially cuts off these tails when its rate is below twice the pure-dephasing rate. We compare independent inverse-Laplace reconstructions of the waiting-time density, memory kernel, and selected qubit-response components to assess numerical accuracy. For the studied parameters, Prabhakar renewal gives larger forward-transfer magnitudes at the prescribed readout than both a scale-matched constant-rate Poisson reference and an inhomogeneous Poisson reference matching the full expected-count profile, including finite-T1 tests. Both Poisson references yield larger evaluated finite-window peaks. Disorder-motivated renewal memory reshapes transmon decoherence selectively by state and direction.[/expand]
A quantum thermal machine surpassing the classical thermodynamic limit on precision
Precision in nonequilibrium processes comes at a thermodynamic cost: suppressing fluctuations generally requires increased dissipation. Thermodynamic uncertainty relations (TURs) make
this trade-off quantitative by linking current fluctuations to entropy production in classical stochastic dynamics. In the decade since its discovery, the canonical steady-state TUR and its finite-time generalizations have become a cornerstone of non-equilibrium thermodynamics, constraining the performance of molecular machines and allowing heat dissipation to be inferred from observable fluctuations. Whether the canonical TUR can be violated in a controlled quantum device remains an outstanding experimental question, in part because doing so requires resolving extremely small steady-state currents as well as their fluctuations. Here we experimentally show that steady-state quantum transport can surpass the precision permitted by the canonical TUR. We observe this violation in a superconducting quantum thermal machine coupled to a microwave waveguide acting as a cold bath and to a classical noise source providing an effective infinite-temperature bath. We observe a TUR ratio Q=1.71±0.17, in violation of the classical bound Q≥2. Our results demonstrate a fundamental distinction between classical and quantum thermodynamics, paving the way for quantum thermal devices that achieve enhanced precision at reduced energy cost.