Detection of temporal fluctuation in superconducting qubits for quantum error mitigation

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.