Quantum state transfer through time reversal of an optical channel
  Rare earth ions have exceptionally long coherence times, making them an excellent candidate for quantum information processing. A key part of this processing is quantum state transfer. We show that perfect state transfer can be achieved by time reversing the intermediate quantum channel, and suggest using a gradient echo memory (GEM) to perform this time reversal. We propose an experiment with rare earth ions to verify these predictions, where an emitter and receiver crystal are connected with an optical channel passed through a GEM. We investigate the affect experimental imperfections and collective dynamics have on the state transfer process. We demonstrate superrandiant effects can enhance coupling into the optical channel and improve the transfer fidelity. We lastly discuss how our results apply to state transfer of entangled states. 
M. R. Hush, C. D. B. Bentley, R. L. Ahlefeldt, M. R. James, M. J.   Sellars and V. Ugrinovskii
quant-ph