Scalable quantum information processing critically depends on the capabilityof storage of a quantum state. In particular, a long-lived storable andretrievable quantum memory for single excitations is of crucial importance tothe atomic-ensemble-based long-distance quantum communication. Although atomicmemories for classical lights and continuous variables have been demonstratedwith milliseconds storage time, there is no equal advance in the development ofquantum memory for single excitations, where only around 10 $\mu$s storage timewas achieved. Here we report our experimental investigations on extending thestorage time of quantum memory for single excitations. We isolate and identifydistinct mechanisms for the decoherence of spin wave (SW) in atomic ensemblequantum memories. By exploiting the magnetic field insensitive state, ``clockstate", and generating a long-wavelength SW to suppress the dephasing, wesucceed in extending the storage time of the quantum memory to 1 ms. Our resultrepresents a substantial progress towards long-distance quantum communicationand enables a realistic avenue for large-scale quantum information processing.
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