Disappearance of macroscopic superpositions in perfectly isolated systems

TL;DR

This paper demonstrates that macroscopic quantum superpositions in isolated systems are destroyed by thermalization, but can be preserved in many-body localized phases where thermalization is absent.

Contribution

It shows that even perfectly isolated quantum systems cannot sustain macroscopic superpositions due to thermalization effects, challenging previous assumptions.

Findings

Macroscopic superpositions are destroyed by thermalization in isolated systems.

Superpositions can be preserved in many-body localized phases.

Thermalization leads to the disappearance of macroscopic superpositions.

Abstract

Schr\"odinger's illustration of an imaginary cat in a box, neither alive nor dead, leads to a question of whether and how long a macroscopic quantum superposition can exist in various situations. It is well known that a macroscopic superposition is destroyed very quickly by environmental effects called decoherence. On the contrary, it is often believed that a macroscopic superposition continues to "survive" if it is ideally isolated from its environment. In this paper, using a well-established measure of macroscopic superpositions and the eigenstate thermalization hypothesis, we show that macroscopic superpositions even in ideally closed systems are destroyed by thermalization processes. We further investigate specific examples of a disordered Heisenberg spin chain varied between the thermalization phase and the many-body localized (MBL) phase. This leads to consistent results; initial macroscopic superpositions disappear under thermalization while they may be preserved in the MBL phase in which thermalization does not occur.