Wavelength convertible quantum memory satisfying ultralong photon storage and near perfect retrieval efficiency

Quantum coherence control is presented for wavelength convertible quantum memory in a double-lambda-type solid ensemble whose spin states are inhomogeneously broadened. Unlike typical atomic media whose spin decay is homogeneous, a spin inhomogeneously broadened solid ensemble requires a counter-intuitive access in the quantum coherence control to avoid spontaneous emission-caused quantum noises. Such quantum coherence control in a solid ensemble results in a near perfect retrieval efficiency and is applicable to ultralong photon storage up to the spin phase relaxation time. Here, the basic physics of the counter-intuitive quantum coherence control is presented not only for two-photon (Raman) coherent transients, but also for a detailed coherence transfer mechanism resulting in frequency up-/down-conversion. This work sheds light on potential applications of quantum optical memories satisfying noise free, near perfect, ultralong, and multimode photon storage, where quantum repeaters, scalable entangled qubits, and magnetometry would be imminent beneficiaries.