Two-Level System Spectroscopy from Correlated Multilevel Relaxation in Superconducting Qubits

TL;DR

This paper introduces a new spectroscopy technique for fixed-frequency transmon qubits that detects and analyzes microscopic two-level systems affecting qubit relaxation without tuning the qubit frequency.

Contribution

A novel multilevel relaxation-based spectroscopy method for fixed-frequency transmons that identifies dominant TLSs and their frequency drifts without tuning.

Findings

TLSs can influence qubit relaxation even when detuned by over 100 MHz.

The method reveals correlations in decay rates of adjacent transitions.

TLS frequency drifts can be reconstructed over time.

Abstract

Transmon qubits are a cornerstone of modern superconducting quantum computing platforms. Temporal fluctuations of energy relaxation in these qubits are widely attributed to microscopic two-level systems (TLSs) in device dielectrics and interfaces, yet isolating individual defects typically relies on tuning the qubit or the TLS into resonance. We demonstrate a novel spectroscopy method for fixed-frequency transmons based on multilevel relaxation: repeated preparation of the second excited state and simultaneous $T_1$ extraction of the first and second excited states reveals characteristic correlations in the decay rates of adjacent transitions. From these correlations we identify one or more dominant TLSs and reconstruct their frequency drift over time. Remarkably, we find that TLSs detuned by $\gtrsim 100\,\mathrm{MHz}$ from the qubit transition can still significantly influence relaxation. The proposed method provides a powerful tool for TLS spectroscopy without the need to tune the transmon frequency, either via a flux-tunable inductor or AC-Stark shifts.