Probing the memory of a superconducting qubit environment

TL;DR

This paper investigates how long-lived two-level systems (TLSs) in superconducting qubit environments cause non-Markovian effects, and demonstrates methods to identify and analyze these TLSs through quantum jump traces and frequency sweeps.

Contribution

It introduces a novel approach to distinguish TLSs from standard baths by analyzing non-Poissonian quantum jumps and fitting Solomon equations to experimental data.

Findings

Long-lived TLSs cause non-Markovian dynamics in superconducting qubits.

Quantum jump analysis can identify and characterize TLSs.

Frequency sweeps reveal distinct TLS peaks, aiding microscopic understanding.

Abstract

Achieving fault tolerance with superconducting quantum processors requires qubits to operate within the regime of threshold theorems based on the Born-Markov approximation. This approximation, which models dissipation as constant energy decay into a memoryless environment, breaks down when qubits couple to long-lived two-level systems (TLSs) that become polarized during operation and retain memory of past qubit states. Here, we show that non-Poissonian quantum jump traces carry the information required to distinguish long-lived TLSs from the standard Markovian bath. By fitting the Solomon equations to measured quantum jumps dynamics arising naturally due to thermal fluctuations, we can disentangle the coupling of the qubit to the two environments. Sweeping the qubit frequency reveals distinct peaks, each associated with a TLS that outlives the qubit, providing a handle to understand their microscopic origin.