  A quantum memory, for storing and retrieving flying photonic quantum states,
is a key interface for realizing long-distance quantum communication and
large-scale quantum computation. While many experimental schemes of high
storage-retrieval efficiency have been performed with weak coherent light
pulses, all quantum memories for true single photons achieved so far have
efficiencies far below 50%, a threshold value for practical applications. Here,
we report the demonstration of a quantum memory for single-photon polarization
qubits with an efficiency of >85% and a fidelity of >99 %, basing on balanced
two-channel electromagnetically induced transparency in laser-cooled rubidium
atoms. For the single-channel quantum memory, the optimized efficiency for
storing and retrieving single-photon temporal waveforms can be as high as 90.6
%. Our result pushes the photonic quantum memory closer to its practical
applications in quantum information processing.
