Operating a bistable qubit

TL;DR

This paper presents an FPGA-based adaptive feedback protocol to stabilize a bistable superconducting qubit affected by TLS defects, significantly reducing errors and improving fidelity.

Contribution

The authors introduce a novel 1-bit feedback method that estimates the qubit's bistable frequency from a single measurement, optimizing stability and fidelity.

Findings

Successfully suppresses TLS-induced Ramsey beating.

Achieves approximately 77% error reduction in gate fidelities.

Operates with an estimation bandwidth of about 136 kHz.

Abstract

Parasitic two-level-system (TLS) defects limit the stability and performance of solid-state quantum processors. Their interaction with a qubit can cause discrete, stochastic shifts of the qubit frequency, making the qubit bistable. We experimentally demonstrate an adaptive protocol for operating a bistable qubit with high fidelity using a classical controller powered by a field-programmable gate array (FPGA). Our "1-bit feedback" protocol estimates the qubit's bistable frequency from only one single-shot measurement, reaching the information limit set by the qubit's intrinsic entropy. We validate the protocol in a superconducting qubit by suppressing TLS-induced Ramsey beating, and deploy it to stabilize gate fidelities over time with approximately 136 kHz estimation bandwidth and a 77% error reduction. Our approach provides a simple, yet fundamentally efficient strategy for mitigating dephasing errors induced by strongly coupled TLS defects, and may enable the operation of large future qubit arrays suffering from few remaining, discrete instabilities.