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.