Schmidt-mode analysis of quadrature entanglement in superpositions of two-mode multiphoton states

Schmidt-decomposition formalism is proposed to be used for evaluation of the degree of quadrature entanglement in two-mode multiphoton states. A series of examples are considered, including analysis of the quadrature entanglement of the two-mode squeezed vacuum. Single-mode and two-mode coherent states with stochastic phases are analyzed by the same method though in this case features of quadrature states are interpreted in terms of decoherence rather than entanglement.