# Quantum fluctuations beyond the Gutzwiller approximation in the Bose-Hubbard model

## TL;DR

This paper introduces a quantum many-body theory for the Bose-Hubbard model that extends the Gutzwiller mean-field approach by systematically including quantum fluctuations, providing accurate predictions across all phases.

## Contribution

It develops a systematic generalization of Bogoliubov theory based on canonical quantization of the Gutzwiller action, applicable throughout the phase diagram.

## Key findings

- Accurate correlation functions matching quantum Monte Carlo data
- Reproduction of superfluid-insulator transition universality classes
- Consistent results from weak to strong interactions

## Abstract

We develop a quantum many-body theory of the Bose-Hubbard model based on the canonical quantization of the action derived from a Gutzwiller mean-field ansatz. Our theory is a systematic generalization of the Bogoliubov theory of weakly-interacting gases. The control parameter of the theory, defined as the zero point fluctuations on top of the Gutzwiller mean-field state, remains small in all regimes. The approach provides accurate results throughout the whole phase diagram, from the weakly to the strongly interacting superfluid and into the Mott insulating phase. As specific examples of application, we study the two-point correlation functions, the superfluid stiffness, the density fluctuations, for which quantitative agreement with available quantum Monte Carlo data is found. In particular, the two different universality classes of the superfluid-insulator quantum phase transition at integer and non-integer filling are recovered.