Simulating noise on a quantum processor: interactions between a qubit and resonant two-level system bath

TL;DR

This paper models the interactions between a superconducting qubit and a bath of two-level system defects to understand how material imperfections affect qubit coherence, guiding improved quantum processor design.

Contribution

The study introduces a comprehensive simulation incorporating TLS interactions and device geometry, accurately predicting qubit relaxation times based on strongly coupled TLSs.

Findings

Strongly coupled TLSs in high electric field regions significantly impact qubit relaxation.

TLSs in the Josephson junction do not always reduce relaxation time if other TLSs are more strongly coupled.

Device geometry and TLS relaxation times are crucial factors influencing qubit coherence.

Abstract

Material defects fundamentally limit the coherence times of superconducting qubits, and manufacturing completely defect-free devices is not yet possible. Therefore, understanding the interactions between defects and a qubit in a real quantum processor design is essential. We build a model that incorporates the standard tunneling model, the electric field distributions in the qubit, and open quantum system dynamics, and draws from the current understanding of two-level system (TLS) theory. Specifically, we start with one million TLSs distributed on the surface of a qubit and pick the 200 systems that are most strongly coupled to the qubit. We then perform a full Lindbladian simulation that explicitly includes the coherent coupling between the qubit and the TLS bath to model the time dependent density matrix of resonant TLS defects and the qubit. We find that the 200 most strongly coupled TLSs can accurately describe the qubit energy relaxation time. This work confirms that resonant TLSs located in areas where the electric field is strong can significantly affect the qubit relaxation time, even if they are located far from the Josephson junction. Similarly, a strongly-coupled resonant TLS located in the Josephson junction does not guarantee a reduced qubit relaxation time if a more strongly coupled TLS is far from the Josephson junction. In addition to the coupling strengths between TLSs and the qubit, the model predicts that the geometry of the device and the TLS relaxation time play a significant role in qubit dynamics. Our work can provide guidance for future quantum processor designs with improved qubit coherence times.