Decoherence-protected storage of a photonic polarization qubit in a single atom

The ability to faithfully store quantum information is a key requirements for many quantum technologies. Here, we present a quantum memory based on a single $^{87}$Rb atom in a high-finesse optical resonator, capable of storing and retrieving single-photon polarization qubits with an overall efficiency of 18\% when probed with coherent laser pulses containing one photon on average. Initially the polarization of the photon is mapped onto the atom via a stimulated Raman adiabatic passage (STIRAP). Because the two atomic levels used to encode the qubit shift in opposite directions in the presence of a magnetic field the memory is susceptible to magnetic field fluctuations. This limits the coherence time to a few hundred microseconds. Using an optical Raman transfer we temporarily map the qubit to a protected subspace, thereby extending the coherence time to tens of milliseconds. It can be further increased to more than 100 milliseconds by means of a spin-echo technique. Our results are an important milestone towards the implementation of a quantum repeater allowing for long-distance quantum communication.