Detection of time-varying noise in superconducting qubits for quantum error mitigation

In existing superconducting quantum computers, the qubits are affected by external noise, posing a critical limit on their performance. Recent studies have reported that the qubit error rates vary significantly depending on time. The instability of error rates can have a considerable impact on the control accuracy of the qubits, or the performance of error mitigation.

Here, we propose a method for quantitatively evaluating the stability of qubits and mitigating time-varying noise. We have investigated instability of a superconducting quantum computer by continuously monitoring the qubit output. We found that qubits exhibit a step-like change in the error rates. This change is repeatedly observed, and each step persists for several minutes. By analyzing the correlation between the increased errors and anomalous variance of the output, we demonstrate quantum error mitigation based on post-selection. We have reduced the errors from 5.4% to 1.6% in a Bell state measurement and from 17.5% to 12.0% in a quantum volume circuit. Numerical analysis on the proposed method was also conducted.