ac Stark gradient echo memory in cold atoms

The burgeoning fields of quantum computing and quantum key distribution have created a demand for a quantum memory. The gradient echo memory scheme is a quantum memory candidate for light storage that can boast efficiencies approaching unity, as well as the flexibility to work with either two- or three-level atoms. The key to this scheme is the frequency gradient that is placed across the memory. Currently, the three-level implementation uses a Zeeman gradient and warm atoms. In this article we model an alternate gradient-creation mechanism--the ac Stark effect--to provide an improvement in the flexibility of gradient-creation and field-switching times. We propose this scheme in concert with a move to cold atoms (≅1 mK). These temperatures would increase the storage times possible, and the small ensemble volumes would enable large ac Stark shifts with reasonable laser power. We find that memory bandwidths on the order of MHz can be produced with experimentally achievable laser powers and trapping volumes, with high precision in gradient creation and switching times on the order of nanoseconds possible. By looking at the different decoherence mechanisms present in this system, we determine that coherence times on the order of tens of milliseconds are possible, as are delay-bandwidth products of approximately 50 and efficiencies over 90%.