Lifetime reductions and read-out oscillations due to imperfect initial level preparations of atoms in a long-lived DLCZ-like quantum memory

Entanglement between a spin-wave qubit (memory qubit) and a photonic qubit is a basic building block for quantum repeaters. Duan-Lukin-Cirac-Zoller (DLCZ) scheme, which generates spin waves via spontaneous Raman scattering (SRS) of Stokes photons in atomic ensemble, provides a promising way to generate such entanglement. In a recent work [arXiv: 2006.05631, accepted by communications physics], DLCZ-like quantum memory that generates long-lived atom-photon entanglement has been experimentally demonstrated, where magnetic-field-insensitive (MFI) coherence is used to store spin waves. For realizing such MFI spin-wave storage, the atoms have to be initially prepared in a specific Zeeman sublevel, which is achieved by applying optical pumping lasers. Here, we demonstrate the memory lifetimes for the cases that the atoms are perfectly and imperfectly prepared in the specific Zeeman level, respectively. The experimental results show that the spin waves associated with magnetic-field-sensitive (MFS) and MFI coherences will be simultaneously created for the case that the atoms are imperfectly prepared in the Zeeman sublevel. Thus, the read outs will experience decay oscillations due to interferences between the two spin waves and the memory lifetime will be shorten due to dephasing of MFS coherence. A detailed theoretical analysis has been developed for explaining the experimental results. The present work will help one to understand decoherence of spin waves (SWs) and then enable one to obtain optimal lifetime of the entanglement storage in the cold atoms.