Criteria for the absence of quantum fluctuations after spontaneous symmetry breaking

Abstract

The lowest-energy state of a macroscopic in which symmetry is spontaneously broken, is a very stable wavepacket centered around a spontaneously chosen, classical direction in symmetry space. However, for a Heisenberg ferromagnet the quantum groundstate is exactly the classical groundstate. This coincides with other exceptional properties of the ferromagnet, including spontaneous time-reversal symmetry breaking, a reduced number of Nambu-Goldstone modes and the absence of a thin spectrum (Anderson tower of states). Recent discoveries of other non-relativistic systems with fewer Nambu-Goldstone modes suggest these specialties apply there as well. I establish precise criteria for the absence of quantum fluctuations and all the other features. In particular, it is not sufficient that the order parameter commute with the Hamiltonian. It leads to a measurably larger coherence time of superpositions in small but macroscopic systems.