Quantum memory is a quantum interaction between an ensemble of multi-particle clusters of atoms and the light that is mapped onto them.This process can be implemented on materials such as cold atoms, ions and crystals with photons usually being the information carrier.Another strategy to store and retrieve quantum information is derived from photon echo, where information is mapped over spectral components of inhomogeneously broadened absorption bands.The use of photonic quantum information allows quantum memory to be equivalent to the memory found in a classical computer.
The proposed advantage of quantum computation hinges on the difference between classical computation by maintaining a non-classical superposition of quantum states. However, when the quantum state is measured, the superposition is lost, making the information classical. In 2007, the concept of an off-resonant atomic quantum memory centered on the idea of a Raman interaction was suggested, and in 2009 was demonstrated by using Caesium vapor kept at room- temperature. The demonstration was capable of storing pulses at 0.3 billionths of a second duration.
The idea of quantum memory becoming a part of long-distance quantum communications or linear optics quantum computation has been experimented with as the use of quantum memory would fit for these applications.
As of 2018, Researchers at Laboratoire Kastler Brossel (LKB) produced 68% efficiency on memory retrieval rates using polarized qubits with 99% conditional fidelity. Qubits were encoded using weak coherent states on a single photon.
