  Frequency-multiplexed quantum communication usually requires a single-shot
identification of the frequency mode of a single photon . In this paper, we
propose a scheme that can identify the frequency mode with high-resolution even
for spontaneously emitted photons whose generation time is unknown, by
combining the time-to-space and frequency-to-time mode mapping. We also
demonstrate the mapping of the frequency mode (100 MHz intervals) to the
temporal mode (435 ns intervals) for weak coherent pulses using atomic
frequency combs. This frequency interval is close to the minimum frequency mode
interval of the atomic frequency comb quantum memory with Pr3+ ion-doped Y2SiO5
crystal, and the proposed scheme has the potential to maximize the frequency
multiplexing of the quantum repeater scheme with the memory.
