Correlated Charge Noise and Relaxation Errors in Superconducting Qubits

TL;DR

This paper investigates correlated charge noise in superconducting qubits, revealing that charge fluctuations and relaxation errors are spatially correlated over hundreds of micrometers, impacting quantum error correction strategies.

Contribution

It provides the first detailed characterization of spatially correlated charge noise and relaxation errors in superconducting qubits, highlighting the role of particle impacts in error correlations.

Findings

Charge fluctuations are highly correlated over 600 μm.

Discrete charge jumps cause transient suppression of relaxation time.

Correlated errors are linked to gamma rays and cosmic-ray muons.

Abstract

The central challenge in building a quantum computer is error correction. Unlike classical bits, which are susceptible to only one type of error, quantum bits ("qubits") are susceptible to two types of error, corresponding to flips of the qubit state about the $X$- and $Z$-directions. While the Heisenberg Uncertainty Principle precludes simultaneous monitoring of $X$- and $Z$-flips on a single qubit, it is possible to encode quantum information in large arrays of entangled qubits that enable accurate monitoring of all errors in the system, provided the error rate is low. Another crucial requirement is that errors cannot be correlated. Here, we characterize a superconducting multiqubit circuit and find that charge fluctuations are highly correlated on a length scale over 600~$\mu$m; moreover, discrete charge jumps are accompanied by a strong transient suppression of qubit energy relaxation time across the millimeter-scale chip. The resulting correlated errors are explained in terms of the charging event and phonon-mediated quasiparticle poisoning associated with absorption of gamma rays and cosmic-ray muons in the qubit substrate. Robust quantum error correction will require the development of mitigation strategies to protect multiqubit arrays from correlated errors due to particle impacts.