Experimental concept, energy level diagram, and setup. (a) The memory protocol. A horizontally (H) polarized single photon (green, 723 nm) is written into the quantum memory with a vertically (V) polarized write pulse (red, 800 nm). After a delay τ, an H-polarized read pulse recalls a V-polarized photon. (b) Energy levels in the memory. The ground state j0i and the storage state |1>correspond to the crystal ground state and an optical phonon, respectively. The signal photon and the read-write pulses are in two-photon resonance with the optical phonon (40 THz) and are far detuned from the conduction band j2i. (c) The experimental setup. The laser output is split to pump the photon source and to produce the orthogonally polarized read and write beams. The photons are produced in pairs with one (signal) at 723 nm and the other (herald) at 895 nm. The signal photon is stored in, and recalled from, the quantum memory. The herald and signal photons are detected using APDs and correlations between them are measured using a coincidence logic unit. Credit: D. G. England, K. A. G. Fisher, J-P. W. MacLean, P. J. Bustard, R. Lausten, K. J. Resch, and B. J. Sussman, Storage and Retrieval of THz-Bandwidth Single Photons Using a Room-Temperature Diamond Quantum Memory, Phys. Rev. Lett. 114, 053602 (2015).
(Phys.org)—Photonic quantum technologies – including cryptography, enhanced measurement and information processing – face a conundrum: They require single photons, but these are difficult to create, manipulate and measure. At the same time, quantum memories enable these technologies by acting as a photonic buffer. Therefore, an ideal part of the solution would be a single-photon on-demand read/write quantum memory. To date, however, development of a practical single-photon quantum memory has been stymied by (1) the need for high efficiency, (2) the read/write lasers used introducing noise that contaminates the quantum state, and (3) decoherence of the information stored in the memory.