Spontaneous symmetry breaking for nonautonomous Hermitian or non-Hermitian systems

Abstract: Here we first present an alternative approach to the Lewis & Riesenfeld theorem for solving the Schr\"odinger equation for time-dependent Hermitian or pseudo-Hermitian Hamiltonians. Then, we apply this framework to the problem of spontaneous breaking of time-dependent antilinear symmetries associated with those Hamiltonians. We demonstrate that if the general antilinear symmetry is unbroken, the Lewis & Riesenfeld phases are real odd functions of time, which allows up to recover the real spectra of time-independent pseudo-Hermitian Hamiltonians. For the spontaneously broken regime, imaginary components of the Lewis & Riesenfeld phases arise as even functions of time, resulting in the coalescence of the real eigenvalues of the unbroken regime. We present an illustrative example of the unbroken and broken $\mathcal{PT}$-symmetry for a time-dependent Hamiltonian modeling the non-Hermitian dynamical Casimir effect.