  Faithfully storing an unknown quantum light state is essential to advanced
quantum communication and distributed quantum computation applications. The
required quantum memory must have high fidelity to improve the performance of a
quantum network. Here we report the reversible transfer of photonic
polarization states into collective atomic excitation in a compact solid-state
device. The quantum memory is based on an atomic frequency comb (AFC) in
rare-earth ion doped crystals. We obtain up to 0.998 process fidelity for the
storage and retrieval process of single-photon-level coherent pulse. This
reliable quantum memory is a crucial step toward quantum networks based on
solid-state devices.
