High-Temperature Gibbs States are Unentangled and Efficiently Preparable

TL;DR

This paper proves that high-temperature Gibbs states of local Hamiltonians are unentangled and can be efficiently approximated by shallow quantum circuits, resolving the question of entanglement presence at high temperatures.

Contribution

It establishes a temperature threshold above which thermal states are separable and provides an efficient method to sample and prepare these states.

Findings

Thermal states are separable above a certain temperature threshold.

Efficient classical sampling of high-temperature Gibbs states is possible.

States close to the Gibbs state can be prepared with shallow quantum circuits.

Abstract

We show that thermal states of local Hamiltonians are separable above a constant temperature. Specifically, for a local Hamiltonian $H$ on a graph with degree $\mathfrak{d}$, its Gibbs state at inverse temperature $\beta$, denoted by $\rho = e^{-\beta H}/ \operatorname{tr}(e^{-\beta H})$, is a classical distribution over product states for all $\beta < 1/(c\mathfrak{d})$, where $c$ is a constant. This proof of sudden death of thermal entanglement resolves the fundamental question of whether many-body systems can exhibit entanglement at high temperature. Moreover, we show that we can efficiently sample from the distribution over product states. In particular, for any $\beta < 1/( c \mathfrak{d}^2)$, we can prepare a state $\varepsilon$-close to $\rho$ in trace distance with a depth-one quantum circuit and $\operatorname{poly}(n, 1/\varepsilon)$ classical overhead.