title: Quantum entanglement in random physical states.
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 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, on 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.
authors.affiliation: Perimeter Institute for Theoretical Physics, 31 Caroline Street North, N2L 2Y5, Waterloo, Ontario, Canada.
authors.forename: Alioscia
authors.initials: A
authors.lastname: Hamma
authors.forename: Siddhartha
authors.initials: S
authors.lastname: Santra
authors.forename: Paolo
authors.initials: P
authors.lastname: Zanardi