  Spin-based quantum computing and magnetic resonance techniques rely on the
ability to measure the coherence time, T2, of a spin system. We report on the
experimental implementation of all-optical spin echo to determine the T2 time
of a semiconductor electron-spin system. We use three ultrafast optical pulses
to rotate spins an arbitrary angle and measure an echo signal as the time
between pulses is lengthened. Unlike previous spin-echo techniques using
microwaves, ultrafast optical pulses allow clean T2 measurements of systems
with dephasing times T2* fast in comparison to the timescale for microwave
control. This demonstration provides a step toward ultrafast optical dynamic
decoupling of spin-based qubits.
