# Accessing the dark exciton spin in deterministic quantum-dot microlenses

## TL;DR

This paper demonstrates optical control and readout of the dark exciton spin in quantum dots using microlenses, revealing its coherent precession and enabling scalable quantum information applications.

## Contribution

It introduces a method to access and manipulate the dark exciton spin in quantum dots with deterministic microlenses, advancing solid-state quantum information technologies.

## Key findings

- Dark exciton spin precession period of 0.82 ns
- Fine-structure splitting of 5.0 μeV between eigenstates
- Successful optical preparation and readout of dark exciton spin

## Abstract

The dark exciton state in semiconductor quantum dots constitutes a long-lived solid-state qubit which has the potential to play an important role in implementations of solid-state based quantum information architectures. In this work, we exploit deterministically fabricated QD microlenses with enhanced photon extraction, to optically prepare and readout the dark exciton spin and observe its coherent precession. The optical access to the dark exciton is provided via spin-blockaded metastable biexciton states acting as heralding state, which are identified deploying polarization-sensitive spectroscopy as well as time-resolved photon cross-correlation experiments. Our experiments reveal a spin-precession period of the dark exciton of $(0.82\pm0.01)\,$ns corresponding to a fine-structure splitting of $(5.0\pm0.7)\,\mu$eV between its eigenstates $\left|\uparrow\Uparrow\pm\downarrow\Downarrow\right\rangle$. By exploiting microlenses deterministically fabricated above pre-selected QDs, our work demonstrates the possibility to scale up implementations of quantum information processing schemes using the QD-confined dark exciton spin qubit, such as the generation of photonic cluster states or the realization of a solid-state-based quantum memory.