Absence of Quantum Time Crystals

TL;DR

This paper defines 'time crystals' using time-dependent correlation functions and proves a no-go theorem showing they cannot exist in ground states or canonical ensembles of typical Hamiltonians with limited-range interactions.

Contribution

It provides a rigorous definition of time crystals and establishes a no-go theorem ruling out their existence under common physical conditions.

Findings

Time crystals cannot exist in ground states.

Time crystals cannot exist in canonical ensembles.

The no-go theorem applies to Hamiltonians with not-too-long-range interactions.

Abstract

In analogy with crystalline solids around us, Wilczek recently proposed the idea of "time crystals" as phases that spontaneously break the continuous time translation into a discrete subgroup. The proposal stimulated further studies and vigorous debates whether it can be realized in a physical system. However, a precise definition of the time crystal is needed to resolve the issue. Here we first present a definition of time crystals based on the time-dependent correlation functions of the order parameter. We then prove a no-go theorem that rules out the possibility of time crystals defined as such, in the ground state or in the canonical ensemble of a general Hamiltonian, which consists of not-too-long-range interactions.