  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.
