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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.

An atomic ensemble with many-body entanglement is desirable for precision measurement. As a type of such quantum state, the spin squeezed state has been pursued in both cold and warm atoms for applications of a quantum-enhanced atomic clock, interferometer, and magnetometer. Here, we report the numerical simulation of many-body dynamics in a Rydberg atomic array with dipole–dipole interaction, and evaluate the generation of spin squeezing. The method builds on the dissipative discrete truncated Wigner approximation, which combines the mean-field dynamics of a spin ensemble with Monte Carlo sampling. By taking into account experimental imperfections such as spin decoherence, we apply this approach to the dynamics in both strontium and rubidium Rydberg atomic arrays with the current available scale. This offers the possibility to accurately simulate the many-body dynamics of interacting quantum systems in achievable platforms for application of quantum simulation and quantum metrology.