Spin echo of electron spins in semiconductors using ultrafast, small-angle, optical pulses

Spin-based quantum computing and magnetic resonance techniques rely on the ability to measure the coherence time, T 2 , of a spin system. We report on the experimental implementation of all-optical spin echo to determine the T 2 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 T 2 measurements of systems with dephasing times (T * 2 ) fast in comparison to the timescale for microwave control. We measure a 7 μs coherence time, which is similar to previous measurements in quantum dots and indicates that nuclear spin diffusion is the primary mechanism for decoherence. This demonstration is a critical step towards optical, dynamic decoupling, which can eliminate fast decoherence and can be integrated into quantum computer architectures based on opticallycontrolled spin qubits.