Polarization Modulated Vertical-Cavity Surface-Emitting Lasers in Quantum Key Distribution

Vertical-cavity surface-emitting lasers (VCSELs) have multiple beneficial properties in quantum key distribution (QKD). However, polarization switching (PS), which happens between two orthogonally polarized eigenmodes, is characteristic of these lasers. PS is extremely problematic if it is unwanted in all polarization-sensitive applications. The origin and properties of PS is discussed, along with potential error scenarios introduced to QKD protocols BB84 and B92. We propose a new transmitter design for the BB84 protocol using only two VCSELs—both corresponding to one of the two bases in which polarized photons are sent—, which are modulated in polarization, purposely generating switches between two orthogonally polarized modes. The methods of polarization modulation are described, along with advantages and design difficulties of the new design. We also consider the possibility of a spectral attack performed by an eavesdropper, originating from the frequency splitting between the polarization eigenmodes of a single VCSEL, and offer a theoretical solution that can protect the key from eavesdroppers utilizing this kind of attack.