Study on the cavity photon induced dephasing of a transmon qubit in circuit QED due to the measurement microwave leakage

Qubit coherence is one of key features in quantum computing. In circuit QED architecture, it is known that the cavity photon induces dephasing of the qubit even in the dispersive regime [1, 2]. In this study, we studied in detail the dephasing of a 3D transmon qubit generated by the measurement microwave tone. First, we observed that the intense measurement microwave tone decreases the dephasing time. In order to avoid the dephasing, we controlled the mixer leakage by applying dc-offset voltage. We observed clear dependence of the measurement microwave leakage and dephasing, so that we can optimize our measurement setup to achieve maximum T₂^*. This calibration method is one of the standard features in our measurement system to preserve the qubit coherence.

[1] D. I. Schuster et al., Phys. Rev. Lett. 94, 123602 (2005).
[2] J. Gambetta et al., Phys. Rev. A 74, 042318 (2006).