  It has recently been shown that a non-Hermitian Hamiltonian H possessing an
unbroken PT symmetry (i) has a real spectrum that is bounded below, and (ii)
defines a unitary theory of quantum mechanics with positive norm. The proof of
unitarity requires a linear operator C, which was originally defined as a sum
over the eigenfunctions of H. However, using this definition to calculate C is
cumbersome in quantum mechanics and impossible in quantum field theory. An
alternative method is devised here for calculating C directly in terms of the
operator dynamical variables of the quantum theory. This new method is general
and applies to a variety of quantum mechanical systems having several degrees
of freedom. More importantly, this method is used to calculate the C operator
in quantum field theory. The C operator is a new time-independent observable in
PT-symmetric quantum field theory.
