Do single-shot projective readouts necessarily estimate the $T_1$ lifetime ?

TL;DR

This paper investigates why single-shot projective readouts in multilevel qubit systems often do not match theoretical $T_1$ lifetime estimates, identifying extrinsic population dynamics as a key factor and proposing a revised measurement protocol.

Contribution

It introduces an integrated theory that accounts for extrinsic effects, improving the accuracy of $T_1$ lifetime estimates in multilevel qubits, especially in spin-valley states in bilayer graphene.

Findings

Extrinsic population dynamics cause discrepancies in $T_1$ estimates.

Intrinsic noise sources include phonon and Johnson noise.

A revised readout protocol better estimates the valley qubit $T_1$.

Abstract

When single-shot qubit readout protocols are adapted for multilevel systems, theoretical $T_1$ lifetime calculations often fall short of capturing the experimental lifetime trends. We identify extrinsic population dynamics as the fundamental origin of this disparity, establishing that the lifetime estimates can, in certain operating regions, be distinct from the intrinsic $T_1$ time. We clarify these aspects with an integrated theory to address recent measurements [Nat. Nano, 20, 494, (2025)] on spin-valley states in bilayer graphene. While confirming that phonon and Johnson noise are the dominant intrinsic sources, we show that the inclusion of extrinsic factors provide the critical match to the experimental estimates. The extrinsic factors also effectuate violations of generalized Mathiessen's rules. With an improved handle on the design space, a revised readout protocol to estimate the $T_1$ lifetime of the valley qubit is proposed.