  Polarization-encoded qubits are particularly useful in quantum information
tasks due to the easy transportation in a single spatial and temporal mode, the
accurate qubit manipulation and the high robustness against decoherence.
Reliable storage of polarization-encoded qubits is essential for the
construction of large-scale quantum networks. Here we demonstrate a faithful
quantum memory for photonic polarization qubits using the noiseless photon echo
protocol implemented in a rare-earth-ion doped crystal (151Eu3+:Y2SiO5). Based
on a detailed spectroscopic investigation on the 151Eu3+ ions at the site 2 of
Y2SiO5 crystals, the qubit memory is implemented using a single piece of
crystal which provides a near-uniform absorption for two orthogonal
polarization states. A process fidelity of 0.919(24) is obtained for the
storage of qubits carried by single-photon-level coherent pulses, which is
beyond the maximal fidelity that can be achieved using the classical
measure-and-prepare strategy. This compact device shall provide a useful
solution for the construction of a long-lived transportable quantum memory and
the memory-based quantum networks.
