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Optical echoes cut through the noise
Illustration: S. Clark et al., Phys. Rev. Lett. (2009)
Ultrafast Optical Spin Echo for Electron Spins in Semiconductors
Susan M. Clark, Kai-Mei C. Fu, Qiang Zhang, Thaddeus D. Ladd, Colin Stanley, and Yoshihisa Yamamoto
Phys. Rev. Lett. 102, 247601 (Published June 19, 2009)
A spin system loses coherence in two different ways: longitudinal decay, which describes the change in occupation of “up” and “down” spin states and occurs at a characteristic time T1, and transverse dephasing, which is the loss of phase coherence in the precession of the two spin populations, which occurs at a characteristic time T2.
Transverse dephasing limits quantum computing and magnetic resonance applications, so the ability to measure T2 is essential. A standard method for determining T2 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 T2.
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 Si donors in GaAs, they show that less noisy measures of T2 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