Entanglement and separability in continuum Rokhsar-Kivelson states

TL;DR

This paper investigates continuum Rokhsar-Kivelson states, proving their subsystems are unentangled, analyzing deformations leading to gapped states, and exploring their entanglement properties and relations to quantum spin chains.

Contribution

It establishes the separability of reduced density matrices in continuum RK states and analyzes their entanglement and deformation properties, including connections to quantum spin chains.

Findings

Reduced density matrix of disconnected subsystems is separable.

Deformations can induce a gap and restore cluster decomposition.

The entanglement entropy's c-function decreases along RG flow.

Abstract

We study a vast family of continuum Rokhsar-Kivelson (RK) states, which have their groundstate encoded by a local quantum field theory. These describe certain quantum magnets, and are also important in quantum information. We prove the separability of the reduced density matrix of two disconnected subsystems, implying the absence of entanglement between the two subsystems -- a stronger statement than the vanishing of logarithmic negativity. As a particular instance, we investigate the case where the groundstate is described by a relativistic boson, which is relevant for certain magnets or Lifshitz critical points with dynamical exponent $z=2$, and we propose nontrivial deformations that preserve their RK structure. Specializing to 1D systems, we study a deformation that maps the groundstate to the quantum harmonic oscillator, leading to a gap for the boson. We study the resulting correlation functions, and find that cluster decomposition is restored. We analytically compute the $c$-function for the entanglement entropy along a renormalization group flow for the wavefunction, which is found to be strictly decreasing as in CFTs. Finally, we comment on the relations to certain stoquastic quantum spin chains. We show that the Motzkin and Fredkin chains possess unusual entanglement properties not properly captured by previous studies.