We describe a frequency-coded scheme to implement the BB84 quantum key distribution protocol using spin wave (SW)-optical interactions. The interaction of SWs with optical coherent states allows TE↔TM mode conversion with simultaneous change of frequency and polarization while introducing a phase difference between the two modes. To implement the BB84 protocol, key bits are encoded as relative phases between the TE and TM modes. The proposed scheme offers a higher key rate, due to a modulation frequency as high as 25 GHz, which also relaxes the specifications on the optical filter at the receiver. In addition, SW-optical interactions yield the added security of truly single-sideband modulation. © 2011 SPIE.