Exact expression of the energy gap at first-order quantum phase transitions of a non-stoquastic Hamiltonian

TL;DR

This paper analytically derives the explicit expression for the energy gap at first-order quantum phase transitions in a non-stoquastic Hamiltonian, revealing how antiferromagnetic interactions influence quantum annealing efficiency.

Contribution

It provides a semi-classical analytical expression for the exponential decay rate of the energy gap at first-order transitions in a non-stoquastic mean-field model.

Findings

Explicit formula for the energy gap decay rate at first-order transitions.

Identification of the critical point where the first-order transition line vanishes.

Insights into how antiferromagnetic interactions modify transition properties.

Abstract

We study the energy gap between the ground state and the first excited state of a mean-field-type non-stoquastic Hamiltonian by a semi-classical analysis. The fully connected mean-field model with $p$-body ferromagnetic interactions under a transverse field has a first-order quantum phase transition for $p\ge 3$. This first-order transition is known to be reduced to second order for $p\ge 5$ by an introduction of antiferromagnetic transverse interactions, which makes the Hamiltonian non-stoquastic. This reduction of the order of transition means an exponential speedup of quantum annealing by adiabatic processes because the first-order transition is shown to have an exponentially small energy gap whereas the second order case does not. We apply a semi-classical method to analytically derive the explicit expression of the rate of the exponential decay of the energy gap at first-order transitions. The result reveals how the property of first-order transition changes as a function of the system parameters. We also derive the exact closed-form expression for the critical point where the first-order transition line disappears within the ferromagnetic phase. These results help us understand how the antiferromagnetic transverse interactions affect the performance of quantum annealing by controlling the effects of non-stoquasticity in the Hamiltonian.