Free coherent evolution of a coupled atomic spin system initialized by electron scattering

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Science  28 May 2021:
Vol. 372, Issue 6545, pp. 964-968
DOI: 10.1126/science.abg8223

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Fast spin flips for entangled atoms

The integration of pump-probe spectroscopy with scanning tunneling microscopy (STM) tools has allowed studies of spin relaxation in atoms on the nanosecond time scale. However, following the free evolution of a pair of entangled spins requires faster initial excitation than can be delivered with microwave pulses. Veldman et al. sequentially combined electron spin resonance–STM and direct-current pump-probe techniques to instantaneously flip spins while preserving spin coherence. They used this method to follow free, coherent flip-flop evolution of two coupled spin-1/2 atoms (hydrogenated titanium atom dimers) on a magnesium oxide surface.

Science, abg8223, this issue p. 964


Full insight into the dynamics of a coupled quantum system depends on the ability to follow the effect of a local excitation in real-time. Here, we trace the free coherent evolution of a pair of coupled atomic spins by means of scanning tunneling microscopy. Rather than using microwave pulses, we use a direct-current pump-probe scheme to detect the local magnetization after a current-induced excitation performed on one of the spins. By making use of magnetic interaction with the probe tip, we are able to tune the relative precession of the spins. We show that only if their Larmor frequencies match, the two spins can entangle, causing angular momentum to be swapped back and forth. These results provide insight into the locality of electron spin scattering and set the stage for controlled migration of a quantum state through an extended spin lattice.

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