Quantum model for mode locking in pulsed semiconductor quantum dots

TL;DR

This paper investigates quantum mode locking in pulsed semiconductor quantum dots, showing how repeated optical pulsing enhances coherence by resonantly stabilizing oscillation frequencies, with implications for quantum computing.

Contribution

It provides a quantum mechanical analysis of mode locking using the central spin model, including coupling to nuclei and decay, and extrapolates results to realistic parameters.

Findings

Synchronization to pulsing frequency takes about 1 second.

Mode locking stabilizes oscillation frequencies against nuclear coupling.

Effects persist after pulsing is stopped.

Abstract

Quantum dots in GaAs/InGaAs structures have been proposed as a candidate system for realizing quantum computing. The short coherence time of the electronic quantum state that arises from coupling to the nuclei of the substrate is dramatically increased if the system is subjected to a magnetic field and to repeated optical pulsing. This enhancement is due to mode locking: Oscillation frequencies resonant with the pulsing frequencies are enhanced, while off-resonant oscillations eventually die out. Because the resonant frequencies are determined by the pulsing frequency only, the system becomes immune to frequency shifts caused by the nuclear coupling and by slight variations between individual quantum dots. The effects remain even after the optical pulsing is terminated. In this work, we explore the phenomenon of mode locking from a quantum mechanical perspective. We treat the dynamics using the central spin model, which includes coupling to 10-20 nuclei and incoherent decay of the excited electronic state, in a perturbative framework. Using scaling arguments, we extrapolate our results to realistic system parameters. We find that the synchronization to the pulsing frequency needs time scales in the order of 1 s.