Quantum environment afterglow from broadband excitation spectroscopy in superconducting qubits

Authors:J. R. Guimarães, Y. Liu, Y. Gao, Y. Haddad, A. Galicia, D. A. Volkov, J. T. Schmieder, H. Bhardwaj, M. Neis, J. Cereijo, M. Jerger, P. A. Bushev, R. Barends

Abstract:Understanding the qubit environment is central to fault-tolerant superconducting quantum computation. Characterization typically relies on relaxation, leaving excitation largely underexplored. Here, we access this information with time-resolved broadband excitation spectroscopy. The resulting qubit excitation spectrum reveals a highly structured landscape, interspersed with cold regions. Combined with postselection, this technique enables full reconstruction of the noise power spectral density (PSD) and separates quantum from classical noise. With feed-forward, it exposes long-lived two-level-systems (TLSs), whose relaxation times span tens of microseconds to milliseconds - uncovering an intrinsic link between the qubit-TLS coupling and TLS relaxation. The data suggest that the long-lived TLSs are intrinsic to the qubit environment, and can cause excitation that lasts for many qubit operation cycles. Consequently, the environment retains a memory of prior dynamics, and properties like gate fidelity become non-Markovian and protocol-dependent. The presented approach enables identifying and bypassing the hidden roadblocks formed by long-lived TLSs in fault-tolerant quantum computation.

Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

arXiv:2609.31280 [quant-ph]

(or arXiv:2609.31280v1 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2609.31280 arXiv-issued DOI via DataCite (pending registration)

From: Jéferson Ribeiro Guimarães J. R. G. [view email] [v1] Fri, 25 Sep 2026 13:52:16 UTC (998 KB)