Deterministic coherent writing of a long-lived semiconductor spin qubit using one ultrafast optical pulse

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 nonnegligible oscillator strength due to residual mixing with bright exciton states. We obtain a high-fidelity one-to-one mapping from any point on the Poincare sphere of the pulse polarization to a corresponding point on the Bloch sphere of the spin of the deterministically photogenerated dark exciton.