  We report on the quantum storage and retrieval of photonic polarization
quantum bits onto and out of a solid state storage device. The qubits are
implemented with weak coherent states at the single photon level, and are
stored for 500 ns in a praseodymium doped crystal with a storage and retrieval
efficiency of 10%, using the atomic frequency comb scheme. We characterize the
storage by using quantum state tomography, and find that the average
conditional fidelity of the retrieved qubits exceeds 95% for a mean photon
number mu=0.4. This is significantly higher than a classical benchmark, taking
into account the Poissonian statistics and finite memory efficiency, which
proves that our device functions as a quantum storage device for polarization
qubits, even if tested with weak coherent states. These results extend the
storage capabilities of solid state quantum memories to polarization encoding,
which is widely used in quantum information science.
