Decoherence benchmarking of superconducting qubits (1901.04417v2)

Abstract: We benchmark the decoherence of superconducting qubits to examine the temporal stability of energy-relaxation and dephasing. By collecting statistics during measurements spanning multiple days, we find the mean parameters $\overline{T_{1}}$ = 49 $\mu$s and $\overline{T_{2}^{*}}$ = 95 $\mu$s, 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 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 is limited by capacitance noise. Importantly, this produces a 0.8 ms limit on the pure dephasing which we also observe. Collectively, these findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.

• The paper's main contribution is identifying TLS as a key source of decoherence in superconducting qubits through extensive temporal analysis.
• It details temporal fluctuations with T1 averaging ≈49 µs and T2* ≈95 µs, necessitating frequent recalibration to maintain gate fidelity.
• The study employs rigorous 1/f noise modeling to outline noise constraints, offering critical insights for scalable fault-tolerant quantum computing.

Decoherence Benchmarking of Superconducting Qubits: An Analysis of Temporal Stability

The paper focuses on benchmarking the decoherence properties of superconducting transmon qubits, particularly investigating the temporal stability of energy relaxation, dephasing, and transition frequency. By conducting exhaustive statistical measurements over extended periods, the paper aims to elucidate the stability constraints these fluctuations impose on qubit gate fidelity, which is crucial for both fault-tolerant quantum computing and Noisy Intermediate-Scale Quantum (NISQ) systems.

Key Findings

The authors report that the mean values of the energy relaxation time, $\overline{T_{1}} = \SI{49}{\micro\second}$, and the dephasing time, $\overline{T_{2}^{*}} = \SI{95}{\micro\second}$, fluctuate significantly over time. These fluctuations necessitate frequent re-calibration in quantum computing setups. The main contribution of the paper is the identification that variations in qubit relaxation primarily stem from localized instabilities. These are attributed to the presence of near-resonant two-level systems (TLS), which switch at sub-millihertz rates, as inferred through meticulous statistical analyses.

The coupling rates of individual TLS to qubits were calculated, with coherent coupling being highlighted. Analyses reveal that a $1/f$ frequency noise spectrum accounts for the qubit’s frequency stability, which translates to a limiting $T_{\phi}$ of approximately 0.8 ms due to the presence of interacting TLS-generated $1/f$ capacitance noise.

Implications and Future Directions

The significance of these findings lies in both theoretical implications and practical applications. The consistent identification of TLS-induced fluctuations across numerous cooldowns and extended periods indicates that TLS represent a fundamental noise source, affecting the temporal stability of qubits’ coherence properties. This reinforces the necessity for techniques that extend $T_1$ lifetimes and enhance qubit robustness against such localized noise. Moreover, the research underscores the importance of comprehensive qubit metrology, incorporating long-duration measurements and statistical reproducibility to accurately report typical rather than exceptional performance metrics.

For future developments, the insights into TLS dynamics present broader questions regarding the synthesis of materials with minimized TLS participation. There is an evident need for novel techniques that can mitigate or exploit TLS-induced noise in quantum circuits. The spectrally unstable interactions characterized present new avenues for redefining noise mitigation strategies, contributing to the fundamental understanding necessary to achieve scalable, fault-tolerant quantum computing.

Conclusion

This paper provides an in-depth analysis of the decoherence parameters of superconducting qubits, attributing significant performance fluctuations to ubiquitous interactions with two-level systems. The paper's methodical approach and reliance on sustained temporal data underscores the necessity of evaluating qubit designs under realistic operating conditions. This work offers a foundational basis for further research into noise reduction techniques, a critical endeavor for enhancing the viability of quantum computing technologies.