Observing the Spontaneous Breakdown of Unitarity

TL;DR

This paper discusses the potential spontaneous breakdown of unitarity in quantum mechanics at macroscopic scales and explores its implications for experiments with large Schrödinger cat states.

Contribution

It introduces the concept of spontaneous unitarity breaking and analyzes its possible effects on macroscopic quantum superpositions.

Findings

Spontaneous unitarity breaking could explain the emergence of classicality.

Implications for the stability of macroscopic quantum states are discussed.

Potential experimental signatures of unitarity breakdown are considered.

Abstract

During the past decade, the experimental development of being able to create ever larger and heavier quantum superpositions has brought the discussion of the connection between microscopic quantum mechanics and macroscopic classical physics back to the forefront of physical research. Under equilibrium conditions this connection is in fact well understood in terms of the mechanism of spontaneous symmetry breaking, while the emergence of classical dynamics can be described within an ensemble averaged description in terms of decoherence. The remaining realm of individual-state quantum dynamics in the thermodynamic limit was addressed in a recent paper proposing that the unitarity of quantum mechanical time evolution in macroscopic objects may be susceptible to a spontaneous breakdown. Here we will discuss the implications of this theory of spontaneous unitarity breaking for the modern experiments involving truly macroscopic Schrodinger cat states.