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arXiv:2610.34485

Real-Time Correlation of Relaxation, Dephasing, and Two-level System Dynamics in Transmon Qubits

Ambrose Yang, Alexander M. Young, Paal H. Prestegaard, Atharv Joshi, Matthew S. Ai, Q. Rumman Rahman, Faranak Bahrami, Matthew P. Bland, Jui-Lin Niu, Ourania-Maria Glezakou-Elbert, Ray D. Chang, Jocelyn Liu, Zhengyi Lu, Robert J. Cava, Nathalie P. de Leon, Andrew A. Houck

quant-phcond-mat.mtrl-sciSubmitted Oct 25, 2026• Announced Oct 27, 2026• 0 citations
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Abstract

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.

AI Summary

The paper investigates what causes minutes-to-hours-long reductions (“excursions”) in relaxation and coherence times of fixed-frequency tantalum-on-silicon transmons with near-millisecond coherence. Nine datasets across five qubits combine shot-to-shot interleaved $T_1$, Hahn-echo, CPMG noise spectroscopy, and AC-Stark-shift TLS spectroscopy. Interleaving aligns the measurements’ sampling in time without improving their temporal resolution, typically minutes. Excursions show correlated increases in relaxation and pure-dephasing rates. In a 54-hour dataset, the Spearman correlation is 0.51 after outlier removal. A piecewise-linear fit finds approximately constant dephasing for $T_1>683\,\mu\mathrm{s}$ and correlated rates below that threshold. CPMG measurements reveal excess noise during excursions, including a Lorentzian spectrum with a $365\pm18$ kHz roll-off; other excursions have different roll-offs. TLS spectroscopy directly tracks narrow loss features drifting or jumping into resonance with the qubit. A reconstruction separating resolvable TLSs, time-varying broadband loss, and static background agrees with independently measured relaxation rates with concordance coefficient 0.831. The TLS contribution correlates with measured relaxation at 0.902 and has 10.8 times the variance of the broadband contribution. These observations support individual spectrally diffusing TLSs as the dominant source of excursions. Their proposed near-junction location is inferred from coupling strengths and electric-field concentration, not directly mapped. Stated limitations include the roughly 10–20 MHz spectroscopy bandwidth, coherence- and pulse-fidelity-limited noise-spectroscopy bandwidth, and non-Gaussian or nonstationary noise during some excursions, which violates assumptions used to extract noise spectra and produces anomalous reconstructed spectra.

Little sign of AI-generated textIncrementalStrong assumptions

Error found: The quantitative TLS reconstruction establishes the relaxation contribution, while the attribution of excess dephasing to those same defects remains indirect rather than quantitatively reconstructed. Some excursion spectra are extracted under Gaussianity and stationarity assumptions that the paper itself reports are violated, so those particular spectra cannot be interpreted as physical noise spectral densities. The written CPMG spectral conversion mixes angular and ordinary frequency and handles filter normalization inconsistently, leaving the numerical spectral calibration insufficiently justified. The Fig. 2(f) caption reverses the correlated and uncorrelated T1 regimes relative to the main text.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

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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.
The paper investigates what causes minutes-to-hours-long reductions (“excursions”) in relaxation and coherence times of fixed-frequency tantalum-on-silicon transmons with near-millisecond coherence. Nine datasets across five qubits combine shot-to-shot interleaved T1T_1T1​, Hahn-echo, CPMG noise spectroscopy, and AC-Stark-shift TLS spectroscopy. Interleaving aligns the measurements’ sampling in time without improving their temporal resolution, typically minutes.
Excursions show correlated increases in relaxation and pure-dephasing rates. In a 54-hour dataset, the Spearman correlation is 0.51 after outlier removal. A piecewise-linear fit finds approximately constant dephasing for T1>683 μsT_1>683\,\mu\mathrm{s}T1​>683μs and correlated rates below that threshold. CPMG measurements reveal excess noise during excursions, including a Lorentzian spectrum with a 365±18365\pm18365±18 kHz roll-off; other excursions have different roll-offs. TLS spectroscopy directly tracks narrow loss features drifting or jumping into resonance with the qubit. A reconstruction separating resolvable TLSs, time-varying broadband loss, and static background agrees with independently measured relaxation rates with concordance coefficient 0.831. The TLS contribution correlates with measured relaxation at 0.902 and has 10.8 times the variance of the broadband contribution.
These observations support individual spectrally diffusing TLSs as the dominant source of excursions. Their proposed near-junction location is inferred from coupling strengths and electric-field concentration, not directly mapped. Stated limitations include the roughly 10–20 MHz spectroscopy bandwidth, coherence- and pulse-fidelity-limited noise-spectroscopy bandwidth, and non-Gaussian or nonstationary noise during some excursions, which violates assumptions used to extract noise spectra and produces anomalous reconstructed spectra.
Abstract We benchmark the decoherence of superconducting transmon qubits to examine the temporal stability of energy relaxation, dephasing, and qubit transition frequency. By collecting statistics during measurements spanning multiple days, we find the mean parameters T1‾\overline {T_1}T1​​ T 1 ¯ = 49 μs and T2∗‾\overline {T_2^ \ast }T2∗​​ T 2 * ¯ = 95 μs; however, both of these quantities fluctuate, explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine sub-millihertz switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit’s frequency stability produces a 0.8 ms limit on the pure dephasing which we also observe. These findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.
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.
Transmon qubits are a cornerstone of modern superconducting quantum computing platforms. Temporal fluctuations of energy relaxation in these qubits are widely attributed to microscopic two-level systems (TLSs) in device dielectrics and interfaces, yet isolating individual defects typically relies on tuning the qubit or the TLS into resonance. We demonstrate a novel spectroscopy method for fixed-frequency transmons based on multilevel relaxation: repeated preparation of the second excited state and simultaneous T1T_1T1​ extraction of the first and second excited states reveals characteristic correlations in the decay rates of adjacent transitions. From these correlations we identify one or more dominant TLSs and reconstruct their frequency drift over time. Remarkably, we find that TLSs detuned by ≳100 MHz\gtrsim 100\,\mathrm{MHz}≳100MHz from the qubit transition can still significantly influence relaxation. The proposed method provides a powerful tool for TLS spectroscopy without the need to tune the transmon frequency, either via a flux-tunable inductor or AC-Stark shifts.
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.
Developing fault-tolerant quantum processors with error correction demands large arrays of physical qubits whose key performance metrics (coherence times, control fidelities) must remain within specifications over both short and long timescales. Here we investigated the temporal stability of subtractively fabricated CMOS-compatible superconducting transmon qubits. During a single cooldown and over a period of 95 hours, we monitored several parameters for 8 qubits, including coherence times T1T_1T1​ and T2∗T_2^*T2∗​, which exhibit fluctuations originating primarily from the interaction between two-level system (TLS) defects and the host qubit. We also demonstrate that subtractively-fabricated superconducting quantum devices align with the theoretical predictions that higher mean lifetimes T1T_1T1​ correspond to larger fluctuations. To assess long-term stability, we tracked two representative qubits over 10 cooldown cycles spanning more than one year. We observed an average total downward shift in both qubit transition frequencies of approximately 61 MHz within the thermal cycles considered. In contrast, readout resonator frequencies decreased only marginally. Meanwhile, T1T_1T1​ exhibits fluctuations from cycle to cycle, but maintains a stable baseline value.
We report on long-term measurements of a highly coherent, nontunable superconducting transmon qubit, revealing low-frequency burst noise in coherence times and qubit transition frequency. We achieve this through a simultaneous measurement of the qubit's relaxation and dephasing rate as well as its resonance frequency. The analysis of correlations between these parameters yields information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits. Our results are consistent with a small number of microscopic two-level systems located at the edges of the superconducting film, which is further confirmed by a spectral noise analysis.