  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%.
