Observation of quantum jumps of a single quantum dot spin using sub-microsecond single-shot optical readout

Single-shot read-out of individual qubits is typically the slowest process among the elementary single- and two-qubit operations required for quantum information processing. Here, we use resonance fluorescence from a single-electron charged quantum dot to read-out the spin-qubit state in 800 nanoseconds with a fidelity exceeding 80%. Observation of the spin evolution on longer timescales reveals quantum jumps of the spin state: we use the experimentally determined waiting-time distribution to characterize the quantum jumps. Embedding a quantum dot in a photonic nanostructure could be used to enhance the collection efficiency by a factor of 10, enabling single-shot read-out with a fidelity exceeding 97%.