Three independent teams of physicists have created the first solid-state quantum memories that store the polarization states of photons. While the three systems each use different materials, they are all based on the concept of an "atomic comb" – whereby a photon is stored in a collective excitation of the atoms within the solid.
Photons have proved to be a good choice for transmitting quantum bits (qubits) of information because they can travel for long distances without interacting with their surroundings. Their other advantage is that quantum information can be stored in a photon's polarization state, with "1" represented by horizontal polarization and "0" by vertical, for example.
Unfortunately, it is very hard to make a quantum memory that can store photon qubits, which has been bad news for anyone wishing to make a quantum computer or build "quantum repeaters" that allow quantum information to be transmitted over long distances. Physicists have been able to create solid-state memories that can store and re-emit qubits based on the temporal properties of light – "1" for the presence of a photon and "0" for its absence, for example – but not the photon's polarization itself.
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