Photon polarization

Photon polarization is the quantum mechanical property of a photon that describes the orientation of its electric field oscillation. In the context of quantum communication, polarization is the most commonly used degree of freedom for encoding quantum bits because it is relatively easy to manipulate and measure with standard optical components.

A photon can be prepared in states of linear polarization (horizontal, vertical, or diagonal) or circular polarization. The non-commutativity of different polarization bases — rectilinear versus diagonal, for instance — is the operational basis of the BB84 protocol and other quantum key distribution schemes. Any attempt to measure a photon in a basis different from its preparation basis destroys the original state information, a manifestation of the uncertainty principle at the level of single quanta.

The fragility of polarization states in optical fiber — due to birefringence and environmental perturbations — is one of the principal engineering challenges in implementing quantum communication over long distances.