SU(4) States of Coherent Photons and Symmetry-Breaking: A Quantum-Classical Crossover

Coherent photons possess both spin and orbital angular momentum as internal degrees of freedom. We show these states have a symmetry of Special Unitary group of degree 4, SU(4), which could be broken down to SU(2)$\times$ SU(2) states upon projection by a polariser to loose the classical entanglement.