# Improving a solid-state qubit through an engineered mesoscopic environment

## TL;DR

This paper demonstrates how feedback control of a quantum dot nuclear environment can significantly extend qubit coherence times, advancing quantum information processing and many-body state engineering.

## Contribution

It introduces a feedback method to prepare nuclear spins in a reduced-entropy state, markedly improving electron spin coherence in a solid-state qubit.

## Key findings

- Extended electron spin dephasing time to 39 ns
- Observed emergence and decay of nuclear correlations
- Prepared nuclear environment with reduced entropy

## Abstract

A controlled quantum system can alter its environment by feedback, leading to reduced-entropy states of the environment and to improved system coherence. Here, using a quantum dot electron spin as control and probe, we prepare the quantum dot nuclei under the feedback of coherent population trapping and measure the evolution from a thermal to a reduced-entropy state, with the immediate consequence of extended qubit coherence. Via Ramsey interferometry on the electron spin, we directly access the nuclear distribution following its preparation, and measure the emergence and decay of correlations within the nuclear ensemble. Under optimal feedback, the inhomogeneous dephasing time of the electron, $T_2^*$, is extended by an order of magnitude to $39$~ns. Our results can be readily exploited in quantum information protocols utilizing spin-photon entanglement, and represent a step towards creating quantum many-body states in a mesoscopic nuclear spin ensemble.