Non-Markovian Relaxation Spectroscopy of Fluxonium Qubits

TL;DR

This paper introduces a two-timescale relaxometry technique to analyze non-Markovian relaxation in fluxonium qubits, revealing TLS with millisecond lifetimes that impact qubit coherence.

Contribution

The study presents a novel relaxometry method capable of probing non-Markovian environmental effects in superconducting qubits, specifically applied to fluxonium devices.

Findings

Detected discrete TLS spectrum with millisecond lifetimes.

TLS distribution consistent with previous high-frequency TLS studies.

Highlighting TLS in junctions as key to improving qubit coherence.

Abstract

Recent studies have shown that parasitic two-level systems (TLS) in superconducting qubits, which are a leading source of decoherence, can have relaxation times longer than the qubits themselves. However, the standard techniques used to characterize qubit relaxation is only valid for measuring $T_1$ under Markovian assumptions and could mask such non-Markovian behavior of the environment in practice. Here, we introduce two-timescale relaxometry, a technique to probe the qubit and environment relaxation simultaneously and efficiently. We apply it to high-coherence fluxonium qubits over a frequency range of 0.1-0.4 GHz, which reveals a discrete spectrum of TLS with millisecond lifetimes. Our analysis of the spectrum is consistent with a random distribution of TLS in the aluminum oxide tunnel barrier of the Josephson junction chain of the fluxonium with an average density and electric dipole similar to previous TLS studies at much higher frequencies. Our study suggests that investigating and mitigating TLS in the junction chain is crucial to the development of various types of noise-protected qubits in circuit QED.