Normal forms for classical and boson systems

Abstract

The group of Bogoliubov transformations of annihilation and creation operators is a subgroup of U( n,n) where n is the number of distinct pairs of annihilation and creation operators. Here, it is established that this subgroup of U( n,n) is isomorphic to Sp(2 n, R ), which appears in classical dynamics as the group of linear canonical transformations on a 2 n-dimensional phase space. Well-known results in classical dynamics are then to used to deduce the full set of normal forms for Boson Hamiltonians. These are classified using a para-eigenvalue notation applicable to both classical and Bose field systems. A simple sufficient condition is given for the non-removability of pairs of creation operators. Explicit normal forms have not previously been given for Hamiltonians with this pathology, which may occur even when the corresponding classical Hamiltonian can be diagonalized.