Polarization-entangled photon pair

A polarization-entangled photon pair is a two-photon state whose joint electromagnetic polarization cannot be represented as a statistical mixture of independent single-photon polarization states. A standard example is $(|HV⟩+|VH⟩)/\sqrt{2}$, a Bell state in the horizontal polarization and vertical polarization basis.

Each photon alone is unpolarized in this example, while joint measurements reveal anticorrelation in the horizontal–vertical basis and phase-dependent correlations in rotated bases. A classical mixture of $HV$ and $VH$ reproduces the first observation but loses the coherent cross terms needed for the second. Entanglement tests therefore compare coincidence counts across several analyzer settings, with the detector and preparation assumptions stated. The state concerns two labeled output modes; the polarization labels do not imply that identical photons possess permanent individual identities.