Dual-rail optical gradient echo memory
  We introduce a scheme for the parallel storage of frequency separated signals in an optical memory and demonstrate that this dual-rail storage is a suitable memory for high fidelity frequency qubits. The two signals are stored simultaneously in the Zeeman-split Raman absorption lines of a cold atom ensemble using gradient echo memory techniques. Analysis of the split-Zeeman storage shows that the memory can be configured to preserve the relative amplitude and phase of the frequency separated signals. In an experimental demonstration dual-frequency pulses are recalled with 35% efficiency, 82% interference fringe visibility, and 6 degrees phase stability. The fidelity of the frequency-qubit memory is limited by frequency-dependent polarisation rotation and ambient magnetic field fluctuations, our analysis describes how these can be addressed in an alternative configuration. 
Daniel B. Higginbottom, Jiao Geng, Geoff T. Campbell, Mahdi Hosseini,   Ming Tao Cao, Ben M. Sparkes, Julian Bernu, Nick P. ...
quant-ph physics.atom-ph