Quantum Key Distribution Using Spin Wave–Optical Interactions

We propose the use of spin wave-optical interactions to implement the BB84 and B92 quantum key distribution (QKD) protocols. Spin waves couple the transverse magnetic (TM) and transverse electric (TE) optical modes of a waveguide. We derive the interaction Hamiltonian to describe the coupling, and solve the resulting equations to determine the time evolution of quantized TM and TE modes. With a judicious choice of coupling coefficient, a set of four nonorthogonal states-formally equivalent to the set of polarization states of a single photon-can be generated. These states form a conjugate basis suitable as a QKD basis set. The proposed scheme can mitigate any medium-induced polarization fluctuations. The spin wave-optical interactions can be used to implement the BB84 protocol using frequency-coded coherent optical states.