Quantum entanglement in random physical states

TL;DR

This paper investigates how random local quantum circuits generate entanglement in physical states, showing that short circuits produce area-law entanglement and longer circuits lead to volume-law entanglement, with states approaching maximal mixedness.

Contribution

It introduces a physically motivated ensemble of states generated by local random circuits and analyzes their typical entanglement properties, revealing saturation of area law and volume law behaviors.

Findings

Short circuits produce area-law entanglement.

Longer circuits lead to volume-law entanglement.

States become nearly maximally mixed for large circuit depth.

Abstract

Most states in the Hilbert space are maximally entangled. This fact has proven useful to investigate - among other things - the foundations of statistical mechanics. Unfortunately, most states in the Hilbert space of a quantum many body system are not physically accessible. We define physical ensembles of states by acting on random factorized states by a circuit of length k of random and independent unitaries with local support. We study the typicality of entanglement by means of the purity of the reduced state. We find that for a time k=O(1) the typical purity obeys the area law. Thus, the upper bounds for area law are actually saturated {\em in average}, with a variance that goes to zero for large systems. Similarly, we prove that by means of local evolution a subsystem of linear dimensions $L$ is typically entangled with a volume law when the time scales with the size of the subsystem. Moreover, we show that for large values of k the reduced state becomes very close to the completely mixed state.