General theory of feedback control of a nuclear spin ensemble in quantum dots

TL;DR

This paper develops a microscopic quantum feedback theory describing the complex dynamics of nuclear spins in quantum dots under optical pumping, revealing effects like hysteresis, resonance locking, and spin coherence modulation.

Contribution

It introduces a unified nonlinear feedback framework for nuclear spin dynamics in quantum dots, connecting and extending previous theories.

Findings

Observation of hysteresis in nuclear spin polarization

Resonance locking and avoidance phenomena

Control of electron spin coherence times

Abstract

We present a microscopic theory of the nonequilibrium nuclear spin dynamics driven by the electron and/or hole under continuous wave pumping in a quantum dot. We show the correlated dynamics of the nuclear spin ensemble and the electron and/or hole under optical excitation as a quantum feedback loop and investigate the dynamics of the many nuclear spins as a nonlinear collective motion. This gives rise to three observable effects: (i) hysteresis, (ii) locking (avoidance) of the pump absorption strength to (from) the natural resonance, and (iii) suppression (amplification) of the fluctuation of weakly polarized nuclear spins, leading to prolonged (shortened) electron spin coherence time. A single nonlinear feedback function as a "measurement" of the nuclear field operator in the quantum feedback loop is constructed which determines the different outcomes of the three effects listed above depending on the feedback being negative or positive. The general theory also helps to put in perspective the wide range of existing theories on the problem of a single electron spin in a nuclear spin bath.