  A quantum memory is a system that enables transfer, storage, and retrieval of
optical quantum states by ON/OFF switching of the control signal in each stages
of the memory. In particular, it is known that, for perfect transfer of a
single-photon state, appropriate shaping of the input pulse is required.
However, in general, such a desirable pulse shape has a complicated form, which
would be hard to generate in practice. In this paper, for a wide class of
linear quantum memory systems, we develop a method that reduces the complexity
of the input pulse shape of a single-photon while maintaining the perfect state
transfer. The key idea is twofold; (i) the control signal is allowed to vary
continuously in time to introduce an additional degree of freedom, and then
(ii) an optimal control problem is formulated to design a simple-formed input
pulse and the corresponding control signal. Numerical simulations are conducted
for Lambda-type atomic media and a networked atomic ensembles, to show the
effectiveness of the proposed method.
