  Scalable quantum information processing critically depends on the capability
of storage of a quantum state. In particular, a long-lived storable and
retrievable quantum memory for single excitations is of crucial importance to
the atomic-ensemble-based long-distance quantum communication. Although atomic
memories for classical lights and continuous variables have been demonstrated
with milliseconds storage time, there is no equal advance in the development of
quantum memory for single excitations, where only around 10 $\mu$s storage time
was achieved. Here we report our experimental investigations on extending the
storage time of quantum memory for single excitations. We isolate and identify
distinct mechanisms for the decoherence of spin wave (SW) in atomic ensemble
quantum memories. By exploiting the magnetic field insensitive state, ``clock
state", and generating a long-wavelength SW to suppress the dephasing, we
succeed in extending the storage time of the quantum memory to 1 ms. Our result
represents a substantial progress towards long-distance quantum communication
and enables a realistic avenue for large-scale quantum information processing.
