# Synopsis: Optical echoes cut through the noise

The demonstration of all-optical spin-echo measurements may provide a route toward decoupling spins from noise sources.

Transverse dephasing limits quantum computing and magnetic resonance applications, so the ability to measure ${T}_{2}$ is essential. A standard method for determining ${T}_{2}$ is the spin-echo technique, in which two or more resonant pulses are applied to the spin system with a short delay between them. However, the resonant frequencies of most spin systems are in the microwave range. Noise sources that cause dephasing on faster time scales may therefore obscure the measurement of ${T}_{2}$.

In Physical Review Letters, Susan Clark at Stanford University, and collaborators at Hewlett-Packard Laboratories in the US, the National Institute of Informatics in Japan, and the University of Glasgow, UK, demonstrate a generalization of the standard spin-echo technique that uses an excited state and off-resonant pulses, allowing them to perform spin rotations with optical frequencies. By demonstrating this method in $\text{Si}$ donors in $\text{GaAs}$, they show that less noisy measures of ${T}_{2}$ in materials with fast dephasing times can be made than those obtained by using microwave spin-echo techniques. In principle, the same methods could be used to extend the decoherence time in semiconductor systems by decoupling the spins from the noise sources that cause dephasing, which is very promising for ultrafast optical dynamic decoupling of spin-based qubits. – Daniel Ucko

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