Enhancing the Coherence of a Spin Qubit by Operating it as a Feedback Loop That Controls its Nuclear Spin Bath

TL;DR

This paper demonstrates a feedback-based method to stabilize and narrow the nuclear spin bath in a GaAs double quantum dot, significantly enhancing the coherence time of a two-electron spin qubit and enabling faster control.

Contribution

It introduces a feedback loop technique that measures and polarizes the nuclear environment to create a stable, narrowed bath state, improving qubit coherence.

Findings

Ten-fold increase in inhomogeneous dephasing time T2*

Stable fixed point with nonzero polarization gradient

Enables fast universal qubit control

Abstract

In many realizations of electron spin qubits the dominant source of decoherence is the fluctuating nuclear spin bath of the host material. The slowness of this bath lends itself to a promising mitigation strategy where the nuclear spin bath is prepared in a narrowed state with suppressed fluctuations. Here, this approach is realized for a two-electron spin qubit in a GaAs double quantum dot and a nearly ten-fold increase in the inhomogeneous dephasing time $T_2^*$ is demonstrated. Between subsequent measurements, the bath is prepared by using the qubit as a feedback loop that first measures its nuclear environment by coherent precession, and then polarizes it depending on the final state. This procedure results in a stable fixed point at a nonzero polarization gradient between the two dots, which enables fast universal qubit control.