Photon echo protocols of optical quantum memory

The main properties of the conventional photon echo quantum memory protocols are analyzed and the results of recent experiments are discussed. The distinctive features of the effect of spectral dispersion and nonlinear interaction of light pulses with resonant atoms on the quantum storage of broadband signal pulses in the studied echo protocols are identified and discussed. Using the photon echo area theorem, closed analytical solutions for quantum memory echo protocols are obtained, describing the storage of weak and intense signal pulses, allowing us to find conditions for the implementation of high efficiency in echo protocols with strong nonlinear interaction of signal and control pulses with atoms. The key existing practical problems and ways to solve them in realistic experimental conditions are described. Finally, new optical quantum memory protocols based on the use of long-lived macroscopic quantum coherence are also proposed and their advantages are discussed.