Open system entanglement and the laser quantum state

The common assertion that lasers produce light in a coherent state and recent criticism of it are discussed. Laser theories in coherent- and Fock-state bases are reviewed, and physical interpretations of each sought by considering how entanglement with the environment is resolved to reach a reduced description. Our recently proposed criterion leading to the unique assignment of coherent states [C. Noh, H.J. Carmichael, Phys. Rev. Lett. 100 (2008) 120405-1] is developed against this background.