Direct Measurement of the Flip-Flop Rate of Electron Spins in the Solid State

``Too good to last'' could be the motto of quantum information processing, where decoherence of qubit states is the ultimate limitation on any device's speed and fidelity. The authors use advanced electron-spin-resonance techniques to directly measure the diffusion through a solid sample of the wave function of a physically stationary spin, a phosphorus donor in silicon (think N00exV center in diamond). They observe a subtle, ubiquitous effect, ``spin flip-flop'', in which two spins swap states---a process that obviously could spoil fidelity, were readout not quick enough.