Deterministic Coherent Writing of a Long-Lived Semiconductor Spin Qubit Using One Ultrafast Optical Pulse

TL;DR

This paper demonstrates a method to precisely control the spin state of a long-lived semiconductor dark exciton using a single ultrafast optical pulse, enabling deterministic quantum state writing.

Contribution

It introduces a technique for deterministic, high-fidelity spin state initialization of dark excitons with a single ultrafast pulse, leveraging residual mixing with bright states.

Findings

Achieved deterministic spin state writing with high fidelity.

Demonstrated control over the Poincaré to Bloch sphere mapping.

Long coherence time of dark excitons enables potential quantum information applications.

Abstract

We use one single, few-picosecond-long, variably polarized laser pulse to deterministically write any selected spin state of a quantum dot confined dark exciton whose life and coherence time are six and five orders of magnitude longer than the laser pulse duration, respectively. The pulse is tuned to an absorption resonance of an excited dark exciton state, which acquires non-negligible oscillator strength due to residual mixing with bright exciton states. We obtain a high fidelity one-to-one mapping from any point on the Poincar\'e sphere of the pulse polarization to a corresponding point on the Bloch sphere of the spin of the deterministically photogenerated dark exciton.