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
02
Dez
2025
The Pound-Drever-Hall Method for Superconducting-Qubit Readout
Scaling quantum computers to large sizes requires the implementation of many parallel qubit readouts. Here we present an ultrastable superconducting-qubit readout method using the multi-tone
self-phase-referenced Pound-Drever-Hall (PDH) technique, originally developed for use with optical cavities. In this work, we benchmark PDH readout of a single transmon qubit, using room-temperature heterodyne detection of all tones to reconstruct the PDH signal. We demonstrate that PDH qubit readout is insensitive to microwave phase drift, displaying 0.73∘ phase stability over 2 hours, and capable of single-shot readout in the presence of phase errors exceeding the phase shift induced by the qubit state. We show that the PDH sideband tones do not cause unwanted measurement-induced state transitions for a transmon qubit, leading to a potential signal enhancement of at least 14~dB over traditional heterodyne readout.
01
Dez
2025
Microwave Circulation in an Extended Josephson Junction Ring
Circulators are nonreciprocal devices that enable directional signal routing. Nonreciprocity, which requires time-reversal symmetry breaking, can be produced in waveguides in which
the propagation medium moves relative to the waveguide at a moderate fraction of the wave speed. Motivated by this effect, here we propose a design for nonreciprocal microwave transmission based on an extended, annular Josephson junction, in which the propagation medium consists of a train of moving fluxons. We show how to harness this to build a high-quality resonant microwave circulator, and we theoretically evaluate the anticipated performance of such a device.
Fabrication and Properties of NbN/NbNx/NbN and Nb/NbNx/Nb Josephson Junctions
Increasing integration scale of superconductor electronics (SCE) requires employing kinetic inductors and self-shunted Josephson junctions (JJs) for miniaturizing inductors and JJs.
We have been developing a ten-superconductor-layer planarized fabrication process with NbN kinetic inductors and searching for suitable self-shunted JJs to potentially replace high Josephson critical current density, Jc, Nb/Al-AlOx/Nb junctions. We report on the fabrication and electrical properties of NbN/NbNx/NbN junctions produced by reactive sputtering in Ar+N2 mixture on 200-mm wafers at 200 oC and incorporated into a planarized process with two Nb ground planes and Nb wiring layer. Here NbN is a stoichiometric nitride with superconducting critical temperature Tc =15 K and NbNx is a high resistivity, nonsuperconducting nitride deposited using a higher nitrogen partial pressure than for the NbN electrodes. For comparison, we co-fabricated Nb/NbNx/Nb JJs using the same NbNx barriers deposited at 20 oC. We varied the NbNx barrier thickness from 5 nm to 20 nm, resulting in the range of Jc from about 1 mA/um^2 down to ~10 uA/um^2, and extracted coherence length of 3 nm and 4 nm in NbNx deposited, respectively at 20 oC and 200 oC. Both types of JJs are well described by resistively and capacitively shunted junction model without any excess current. We found the Jc of NbN/NbNx/NbN JJs to be somewhat lower than of Nb/NbNx/Nb JJs with the same barrier thickness, despite a much higher Tc and energy gap of NbN than of Nb electrodes. IcRn products up to ~ 0.5 mV were obtained for JJs with Jc~ 0.6 mA/um^2. Jc(T) dependences have been measured.
30
Nov
2025
Observation of individual vortex penetration in a coplanar superconducting resonator
We demonstrate the detection and control of individual Abrikosov vortices in superconducting microwave resonators. λ/4 resonators with a narrowed region near the grounded end acting
as a vortex trap were fabricated and studied using microwave transmission spectroscopy at millikelvin temperatures. Sharp stepwise drops in resonance frequency are detected as a function of increasing external magnetic field, attributed to the entry of individual Abrikosov vortices in the narrow region. This interpretation is confirmed by NV center magnetometry revealing discrete vortex entry events on increasing field. Our results establish a method to investigate and manipulate the states of Abrikosov vortices with microwaves.
29
Nov
2025
Four-body interactions in Kerr parametric oscillator circuits
We theoretically present new unit circuits of Kerr parametric oscillators (KPOs) with four-body interactions, which enable the scalable embedding of all-to-all connected logical Ising
spins using the Lechner-Hauke-Zoller (LHZ) scheme. These unit circuits enable four-body interactions using linear couplers, making the circuit fabrication and characterization much simpler than those of conventional unit circuits with nonlinear couplers. Numerical calculations indicate that the magnitudes of the coupling constants can be comparable to those in conventional circuits. On the basis of this theory, we designed a four-KPO circuit and experimentally confirmed the four-body correlation by measuring the pump-phase dependence of the parity of the four-KPO states. We show that the choice of the pump frequencies are important not only to enable the four-body interaction, but to cancel the effects of other unwanted interactions. Using the circuit, we demonstrated the quantum annealing based on the LHZ scheme, where the strength of the interaction between the logical Ising spins is mapped to the local field and controlled by a coherent drive applied to each KPO.
28
Nov
2025
Evidence for unexpectedly low quasiparticle generation rates across Josephson junctions of driven superconducting qubits
Microwave drives applied to superconducting qubits (SCQs) are central to high-fidelity control and fast readout. However, recent studies find that even drives far below the superconducting
gap frequency may cause drive-induced quasiparticle generation (QPG) across Josephson junctions (JJs), posing a serious concern for fault-tolerant superconducting quantum computing. Here, we find experimental evidence that the actual QPG rates in strongly driven SCQs are remarkably lower than expected. We apply intense drive fields through readout resonators, reaching effective qubit drive amplitudes up to 300 GHz. The nonlinear response of the resonators enables quantification of the energy loss from SCQs into their environments, including the contribution from QPG. Even when conservatively attributing all measured dissipation to QPG, the observed energy dissipation rates are far lower than expected from the ideal QPG model. Meanwhile, calculations incorporating high-frequency cutoffs (HFCs) near 17-20 GHz in the QPG conductance can explain the experiments. These HFCs yield QPG rates a few orders of magnitude smaller than those without HFCs, providing evidence that the QPG rates are lower than predicted by the ideal model. Our findings prevent overestimation of drive-induced QPG and provide crucial guidance for operating superconducting quantum processors. Identifying the microscopic origin of the discrepancy opens new material and device opportunities to further mitigate QPG.
27
Nov
2025
Ultrafast Single Qubit Gates through Multi-Photon Transition Removal
One of the main enablers in quantum computing is having qubit control that is precise and fast. However, qubits typically have multilevel structures making them prone to unwanted transitions
from fast gates. This leakage out of the computational subspace is especially detrimental to algorithms as it has been observed to cause long-lived errors, such as in quantum error correction. This forces a choice between either achieving fast gates or having low leakage. Previous works focus on suppressing leakage by mitigating the first to second excited state transition, overlooking multi-photon transitions, and achieving faster gates with further reductions in leakage has remained elusive. Here, we demonstrate single qubit gates with a total leakage error consistently below 2.0×10−5, and obtain fidelities above 99.98% for pulse durations down to 6.8 ns for both X and X/2 gates. This is achieved by removing direct transitions beyond nearest-neighbor levels using a double recursive implementation of the Derivative Removal by Adiabatic Gate (DRAG) method, which we name the R2D method. Moreover, we find that at such short gate durations and strong driving strengths the main error source is from these higher order transitions. This is all shown in the widely-used superconducting transmon qubit, which has a weakly anharmonic level structure and suffers from higher order transitions significantly. We also introduce an approach for amplifying leakage error that can precisely quantify leakage rates below 10−6. The presented approach can be readily applied to other qubit types as well.
Quantum-Enhanced Picostrain Sensing with Superconducting Qubits
We propose a quantum-enhanced picostrain sensor that achieves Heisenberg-limited strain sensing using superconducting qubits. A strain-sensitive qubit s Hamiltonian is coupled to the
momentum quadrature of a microwave resonator, transducing mechanical strain ϵ into amplified spatial displacements of the resonator s phase space. Using homodyne detection of the resonator field and multipartite entanglement of N qubits, the protocol achieves a strain sensitivity Δϵ∼pϵ (picostrain), two orders of magnitude better than classical sensors. The scheme integrates natively with superconducting processors, enabling in-situ diagnostic and nanoscale material characterization.
Superconducting Qubit Gates Robust to Parameter Fluctuations
State-of-the-art single-qubit gates on superconducting transmon qubits can achieve the fidelities required for error-corrected computations. However, parameter fluctuations due to qubit
instabilities, environmental changes, and control inaccuracies make it difficult to maintain this performance. To mitigate the effects of these parameter variations, we numerically derive gates robust to amplitude and frequency errors using gradient ascent pulse engineering (GRAPE). We analyze how fluctuations in qubit frequency, drive amplitude, and coherence affect gate performance over time. The robust pulses suppress coherent errors from drive amplitude drifts over 15 times more than a Gaussian pulse with derivative removal by adiabatic gate (DRAG) corrections. Furthermore, the robust gates, originally designed to compensate for quasi-static errors, also demonstrate resilience to stochastic, time-dependent noise, which is reflected in the dephasing time. They suppress added errors during increases in dephasing by up to 1.7 times more than DRAG.
Raising the Cavity Frequency in cQED
The basic element of circuit quantum electrodynamics (cQED) is a cavity resonator strongly coupled to a superconducting qubit. Since the inception of the field, the choice of the cavity
frequency was, with a few exceptions, been limited to a narrow range around 7 GHz due to a variety of fundamental and practical considerations. Here we report the first cQED implementation, where the qubit remains a regular transmon at about 5 GHz frequency, but the cavity’s fundamental mode raises to 21 GHz. We demonstrate that (i) the dispersive shift remains in the conventional MHz range despite the large qubit-cavity detuning, (ii) the quantum efficiency of the qubit readout reaches 8%, (iii) the qubit’s energy relaxation quality factor exceeds 107, (iv) the qubit coherence time reproducibly exceeds 100 μs and can reach above 300 μs with a single echoing π-pulse correction. The readout error is currently limited by an accidental resonant excitation of a non-computational state, the elimination of which requires minor adjustments to the device parameters. Nevertheless, we were able to initialize the qubit in a repeated measurement by post-selection with 2×10−3 error and achieve 4×10−3 state assignment error. These results encourage in-depth explorations of potentially transformative advantages of high-frequency cavities without compromising existing qubit functionality.