Coherent states of photonic dimers

The concept of coherent states of single photons is generalized to the scenario wherein the fundamental building blocks are entangled two-photon states rather than single-photon states. To provide a concrete discussion, the case of photonic dimers (two-photon bound states) is explored. The coherent states of photonic dimers are defined. The correlation functions are calculated and expressed in terms of series expansion which is suitable for computational investigations. The quadrature properties and the squeezing effects are computed and compared with the experimental results at suitable regimes. Finally, the statistical properties of the coherent states of photonic dimers in the BEC and BCS limits are investigated. The correlation functions are shown to exhibit a BEC-BCS crossover behavior when the two-photon pairing correlations (i.e., the size of photonic dimers) are comparable to the average spacing between the dimer pairs.