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npj Quantum Information

Robust quality factor assessment of high-coherence superconducting qubits

Andrew E. Dane, Karthik Balakrishnan, Brent A. Wacaser, Li-Wen Hung, H. Jonathon Mamin, D. Rugar, Robert M. Shelby, Conal E. Murray, Kenneth P. Rodbell, Jeffrey W. Sleight

Published Feb 28, 2026• 3 citations
Publisher

Abstract

Large variations in the energy relaxation time (T 1 ) of superconducting qubits make it difficult to accurately evaluate and compare new qubit materials and fabrication processes, or to perform studies that require precise measurements of energy loss. To address this issue, we present techniques for characterizing qubit quality factors by applying electric fields to TLS in the vicinity of qubits. Introducing low-frequency (<1 Hz) AC fields allows us to stabilize the measured T 1 by averaging over the accessible TLS configurations, producing a robust estimate of T 1 that is difficult to replicate with hundreds of measurements over long periods, without applied fields. In a complementary technique, we apply a randomly selected DC field to the qubit and measure T 1 . Repeated ‘fast-random’ measurements reveal a distribution of T 1 values whose harmonic mean is consistent with that obtained through AC measurements but illustrates a wider range of qubit lifetimes induced by TLS interactions. We implement these TLS control techniques in various ways, including demonstrating a simple T 1 improvement protocol and precise measurements of T 1 vs temperature. These techniques will facilitate our understanding of how to improve qubit coherence by enabling better measurements in shorter times or with fewer devices.

DOI
10.1038/s41534-026-01199-x

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Large variations in the energy relaxation time (T 1 ) of superconducting qubits make it difficult to accurately evaluate and compare new qubit materials and fabrication processes, or to perform studies that require precise measurements of energy loss. To address this issue, we present techniques for characterizing qubit quality factors by applying electric fields to TLS in the vicinity of qubits. Introducing low-frequency (<1 Hz) AC fields allows us to stabilize the measured T 1 by averaging over the accessible TLS configurations, producing a robust estimate of T 1 that is difficult to replicate with hundreds of measurements over long periods, without applied fields. In a complementary technique, we apply a randomly selected DC field to the qubit and measure T 1 . Repeated ‘fast-random’ measurements reveal a distribution of T 1 values whose harmonic mean is consistent with that obtained through AC measurements but illustrates a wider range of qubit lifetimes induced by TLS interactions. We implement these TLS control techniques in various ways, including demonstrating a simple T 1 improvement protocol and precise measurements of T 1 vs temperature. These techniques will facilitate our understanding of how to improve qubit coherence by enabling better measurements in shorter times or with fewer devices.
Fluxonium superconducting qubits have demonstrated long coherence times and high single- and two-qubit gate fidelities, making them a favorable building block for superconducting quantum processors. We investigate the dominant limitations to fluxonium qubit energy relaxation time T1T_1T1​ using a set of eight planar, aluminum-on-silicon qubits. We find that a circuit-based model for capacitive dielectric loss best captures the frequency dependence of T1T_1T1​, which we analyze within both a two-level and a six-level energy relaxation model. We convert the measured T1T_1T1​ into an effective capacitive quality factor QCeffQ_\mathrm{C}^{\mathrm{eff}}QCeff​ to compare qubits on equal footing, accounting for independently estimated contributions from 1/f1/f1/f flux noise and radiative loss to the control and readout circuitry. We apply this methodology to compare qubits from two fabrication processes: a baseline process and one that applies a fluorine-based wet treatment prior to Josephson junction deposition. We resolve a small improvement of (13.8 ±\pm± 8.4)%)\%)% in the process mean QCeffQ_\mathrm{C}^{\mathrm{eff}}QCeff​, indicating that the fluorine treatment may have reduced loss from the metal-substrate interface, but did not address the primary source of loss in these fluxonium qubits.
Abstract Superconducting qubits are a leading candidate for quantum computing but display temporal fluctuations in their energy relaxation times T 1 . This introduces instabilities in multi-qubit device performance. Furthermore, autocorrelation in these time fluctuations introduces challenges for obtaining representative measures of T 1 for process optimization and device screening. These T 1 fluctuations are often attributed to time varying coupling of the qubit to defects, putative two level systems (TLSs). In this work, we develop a technique to probe the spectral and temporal dynamics of T 1 in single junction transmons by repeated T 1 measurements in the frequency vicinity of the bare qubit transition, via the AC-Stark effect. Across 10 qubits, we observe strong correlations between the mean T 1 averaged over approximately nine months and a snapshot of an equally weighted T 1 average over the Stark shifted frequency range. These observations are suggestive of an ergodic-like spectral diffusion of TLSs dominating T 1 , and offer a promising path to more rapid T 1 characterization for device screening and process optimization.
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A quantum computer will potentially solve far-reaching problems which are currently intractable on any classical computer. Many technological obstacles have prevented the realization of a quantum computer, the main obstacle being decoherence, which is the loss of quantum information. Decoherence arises from the undesired interaction between qubits and their environment. Isolated qubits have better coherence but are more difficult to control. Superconducting qubits are a promising platform since their macroscopic size allows for easy control and coupling to other qubits. While the coherence of superconducting qubits has substantially improved over the past two decades, further improvements in coherence are required.We have repeatedly and reliably increased the coherence times of superconducting qubits. Currently decoherence in these devices is dominated by coupling to material defects. These defects are present in the dielectrics used to fabricate these devices or introduced during fabrication. Using simpler resonators as a testbed, we individually isolate, characterize, and then improve each step of the more complicated fabrication of superconducting qubits. We increased the quality factor of resonators by a factor of four by first identifying the surfaces and interfaces as a major source of loss and then by optimizing the substrate preparation. Furthermore, we measure and subsequently mitigate additional defect loss, which is dependent on the position of ground plane holes used to limit the loss from magnetic vortices. Implementing these improvements led to an increase of our qubit coherence times by more than an order of magnitude.The progress made in coherence while maintaining a high degree of connectivity and controllability has been directly used in more complex circuits. One such device is a fully connected three qubit ring with both tunable qubit frequencies and adjustable qubit-qubit couplings. The considerable level of control allows us to generate the quantum dynamics resulting from a collision between Na and He atoms by encoding the problem into a time-dependent multi-qubit interaction. We compute a scattering cross section by measuring the probability of exciting the Na atom over a wide range of collision parameters. The scattering cross-section shows that there is a velocity where the excitation probability is maximized, in agreement with scattering experiments.
Superconducting circuits are a leading platform for quantum computing. However, their coherence times are still limited and exhibit temporal fluctuations. Those phenomena are often attributed to the coupling between qubits and material defects that can be well described as an ensemble of two-level systems (TLSs). Among them, charge fluctuators inside amorphous oxide layers contribute to both low-frequency 1/f1/f1/f charge noise and high-frequency dielectric loss, causing fast qubit dephasing and relaxation. Moreover, spectral diffusion from mutual TLS interactions varies the noise amplitude over time, fluctuating the qubit lifetime. Here, we propose to mitigate those harmful effects by engineering the relevant TLS noise spectral densities. Specifically, our protocols smooth the high-frequency noise spectrum and suppress the low-frequency noise amplitude via depolarizing and dephasing the TLSs, respectively. As a result, we predict a drastic stabilization in qubit lifetime and an increase in qubit pure dephasing time. Our detailed analysis of feasible experimental implementations shows that the improvement is not compromised by spurious coupling from the applied noise to the qubit.
We describe a superconducting qubit derived from operating a properly designed fluxonium circuit in a zero magnetic field. The qubit has a frequency of about 4 GHz and an energy relaxation quality factor Q ≈ 0.7 × 10 7 , even though the dielectric loss quality factor of the circuit components is in the low- 10 5 range. The Ramsey coherence time exceeds 100 μ s , and the average fidelity of Clifford gates is benchmarked to F > 0.999 . These figures are expected to improve with optimized fabrication and measurement procedures. Our work establishes a ready-to-use “partially protected” superconducting qubit functioning in the frequency range of conventional transmons. Published by the American Physical Society 2024