Real-time adaptive tracking of fluctuating relaxation rates in superconducting qubits

TL;DR

This paper introduces a real-time adaptive method using FPGA-based Bayesian estimation to track rapid fluctuations in relaxation times of superconducting qubits, revealing faster dynamics than previously observed and impacting quantum device calibration.

Contribution

It presents a novel FPGA-powered adaptive protocol that significantly improves temporal resolution in tracking qubit relaxation fluctuations.

Findings

Relaxation times fluctuate nearly an order of magnitude within tens of milliseconds.

Fast fluctuations are linked to two-level systems switching at up to 10 Hz.

The method estimates relaxation times within a few milliseconds, close to decoherence timescales.

Abstract

The fidelity of operations on a solid-state quantum processor is fundamentally bounded by environmental decoherence. Characterizing environmental fluctuations is challenging because the acquisition time of nonadaptive experimental protocols limits temporal precision and can average out rapid features of the underlying dynamics. Here, we overcome this temporal-resolution limit by two orders of magnitude using a field-programmable gate-array (FPGA) powered classical controller that adaptively and continuously tracks the relaxation-time fluctuations of two fixed-frequency superconducting transmon qubits, which exhibit average relaxation times of approximately 0.17 ms and occasionally exceed 0.5 ms. We report events in which the relaxation time switches by nearly an order of magnitude over timescales of just tens of milliseconds, rather than minutes or hours as previously reported. Our real-time Bayesian estimation protocol estimates relaxation times within a few milliseconds, close to the decoherence timescale itself. Our statistical analysis further suggests that some of these fast fluctuations arise from two-level systems switching at rates up to 10 Hz, four orders of magnitude faster than earlier reports. These results redefine the timescales relevant for calibration in superconducting quantum processing units, establish a reference for rapid relaxation-rate characterization in device screening, and improve our understanding of fast relaxation dynamics.