  In this paper, we show that quantum memory for qudit states encoded in a
single photon pulsed optical field has a conceptually simple modular
realization using only passive linear optics and coherent feedback. We exploit
the idea that two decaying optical cavities can be coupled in a coherent
feedback configuration to create an internal mode of the coupled system which
is isolated and decoherence-free for the purpose of qubit storage. The qubit
memory can then be switched between writing/read-out mode and storage mode
simply by varying the routing of certain freely propagating optical fields in
the network. It is then shown that the qubit memories can be interconnected
with one another to form a qudit quantum memory. We explain each of the phase
of writing, storage, and read-out for this modular quantum memory scheme. The
results point a way towards modular architectures for complex compound quantum
memories.
