Noise-specific beats in the higher-level Ramsey curves of a transmon qubit

TL;DR

This paper investigates how low-frequency charge noise affects Ramsey measurements in transmon qubits, introducing models that better estimate true dephasing times and allow extraction of noise spectral density from experimental data.

Contribution

It presents a phenomenological model for charge noise effects on Ramsey fringes and demonstrates its agreement with a Lindblad model and experimental results.

Findings

The phenomenological model accurately reproduces beating patterns in Ramsey curves.

Both models agree with experimental data on dephasing times.

The method allows inference of the noise power spectral density from Ramsey overlays.

Abstract

In the higher levels of superconducting transmon devices, and more generally charge sensitive devices, $T_2^*$ measurements made in the presence of low-frequency time-correlated $1/f$ charge noise and quasiparticle-induced parity flips can give an underestimation of the total dephasing time. The charge variations manifest as beating patterns observed in the overlay of several Ramsey fringe curves, and are reproduced with a phenomenological Ramsey curve model which accounts for the charge variations. $T_2^*$ dephasing times which more accurately represent the total dephasing time are obtained. The phenomenological model is compared with a Lindblad master equation model. Both models are found to be in agreement with one another and the experimental data. Finally, the phenomenological formulation enables a simple method in which the power spectral density (PSD) for the low-frequency noise can be inferred from the overlay of several Ramsey curves.