内容：

Motion-induced dephasing is a dominant decoherence mechanism for atom-gas quantum memories.

In this paper, we develop a new coherent manipulation technique which enables arbitrary engineering

of the spin-wave momentum with neglectable noise. By zeroing the spin-wave momentum, motion-

induced dephasing can be frozen completely. We experimentally demonstrate this scheme with

laser-cooled atoms in a DLCZ con guration. By applying the freezing pulses, memory lifetime gets

extended signi cantly to the limit of atom cloud expansion and does not depend on the detection

angle anymore. The observed high cross-correlation above 20 proves that high- delity memory

operation is well preserved after coherent manipulation.