One- and two-photon spectroscopy of a flux qubit coupled to a microscopic defect

TL;DR

This study investigates a superconducting flux qubit's interaction with a microscopic defect, revealing the defect's two-level nature and its dynamic coupling through one- and two-photon spectroscopy.

Contribution

The paper demonstrates the use of one- and two-photon spectroscopy to characterize a microscopic two-level defect coupled to a flux qubit, providing detailed insights into the defect's energy structure and coupling dynamics.

Findings

The defect is a two-level system with transverse coupling to the qubit.

The energy level structure of the defect was mapped using spectroscopy.

The defect's transition frequency and coupling strength varied during experiments.

Abstract

We observed the dynamics of a superconducting flux qubit coupled to an extrinsic quantum system (EQS). The presence of the EQS is revealed by an anticrossing in the spectroscopy of the qubit. The excitation of a two-photon transition to the third excited state of the qubit-EQS system allows us to extract detailed information about the energy level structure and the coupling of the EQS. We deduce that the EQS is a two-level system, with a transverse coupling to the qubit. The transition frequency and the coupling of the EQS changed during experiments, which supports the idea that the EQS is a two-level system of microscopic origin.