Spontaneous localisation from a coarse-grained deterministic and non-unitary dynamics

Abstract: Collapse of the wave function appears to violate the quantum superposition principle as well as deterministic evolution. Objective collapse models propose a dynamical explanation for this phenomenon, by making a stochastic non-unitary and norm-preserving modification to the Schr\"odinger equation. In the present article we ask how a quantum system evolves under a {\it deterministic} and non-unitary but norm-preserving evolution? We show using a simple two-qubit model that under suitable conditions, quantum linear superposition is broken, with the system predictably driven to one or the other alternatives. If this deterministic dynamics is coarse-grained and observed over a lower time resolution, the outcomes appear random while obeying the Born probability rule. Our analysis hence throws light on the distinct roles of non-unitarity and of stochasticity in objective collapse models.