Unitarity through PT symmetry in quantum quadratic gravity

Abstract

Theories described by non-Hermitian Hamiltonians are known to possess strictly positive energy eigenvalues and exhibit unitary time evolution if the Hamiltonian is symmetric under discrete parity and time (PT) transformation. In this work, we demonstrate how quantum quadratic gravity (QG), a theory that generally violates unitarity when viewed as a Hermitian quantum field theory, can be complex-deformed into such a PT-symmetric theory with an action that consists of a ghost-less Hermitian free part and non-Hermitian interactions. Paying special attention to the gauge symmetry present in the theory, we quantize in the covariant operator formalism after suggesting how the framework might be extended to the pseudo-Hermitian picture. We find compelling evidence that the resulting quantum theory possesses a unitary inner product and a sensible interpretation of quantum probability, thus avoiding the ghost problem present in the Hermitian formulation of QG.