Study of Phase Method in Tantalum Superconducting Qubit T2* Measurements

In this paper, the phase method is proposed to measure the T * 2 dephasing time in a Tantalum-based superconducting qubit.In the regular Ramsey method for T * 2 measurement, a superposition state precesses on the equatorial plane of the Blochsphere due to detuning.The dephasing during the precession is recorded.In the phase method, rapid rotation is applied to rotate the superposition state on the equatorial plane through various angles (phases) to extract the dephasing during an idle time right before the rotation.While the phase method has been used in atomic physics, it has not been commonly used to characterize T * 2 in superconducting qubits.Compared to the traditional Ramsey method, the phase method does not require prior knowledge of the dephasing envelope, enabling it to capture the dynamic dephasing behavior of a qubit during the measurement more easily.It also does not rely on detuning and, thus, is insensitive to any detuning error.In this paper, a Tantalum-based superconducting qubit with a high T1 ( 220s) is used.After careful calibrations, extensive dephasing experiments were conducted using both the Ramsey and the phase methods to extract T * 2 .We show that the phase method is insensitive to detuning and works well even with a very sparse phase sampling (with a step size as large as 2 ).We also show that, statistically, it is equivalent to the Ramsey method within 10%.Therefore, if both the Ramsey method and the phase method are used together, it is expected a more accurate and reliable T * 2 extraction is possible.