  A long-lived and multimode quantum memory is a key component needed for the
development of quantum communication. Here we present temporally multiplexed
storage of 5 photonic polarization qubits encoded onto weak coherent states in
a rare-earth-ion doped crystal. Using spin refocusing techniques we can
preserve the qubits for more than half a millisecond. The temporal multiplexing
allows us to increase the effective rate of the experiment by a factor of 5,
which emphasizes the importance of multimode storage for quantum communication.
The fidelity upon retrieval is higher than the maximum classical fidelity
achievable with qubits encoded onto single photons and we show that the memory
fidelity is mainly limited by the memory signal-to-noise ratio. These results
show the viability and versatility of long-lived, multimode quantum memories
based on rare-earth-ion doped crystals.
