Permanent variational wave functions for bosons

TL;DR

This paper demonstrates that permanent states are highly effective variational wave functions for bosonic systems, accurately approximating ground states and correlations in models like the Bose-Hubbard, with a developed greedy algorithm for optimization.

Contribution

It introduces a variational method using permanent states for bosons, including an algorithm to optimize single-particle orbitals, showing high accuracy in approximating ground states.

Findings

High overlap (≥0.96) with exact ground states in 1D Bose-Hubbard model

Permanent states accurately reproduce energy and correlation functions

The greedy algorithm effectively optimizes the variational wave function

Abstract

We study the performance of permanent states (the bosonic counterpart of the Slater determinant state) as approximating functions for bosons, with the intention to develop variational methods based upon them. For a system of $N$ identical bosons, a permanent state is constructed by taking a set of $N$ arbitrary (not necessarily orthonormal) single-particle orbitals, forming their product and then symmetrizing it. It is found that for the one-dimensional Bose-Hubbard model with the periodic boundary condition and at unit filling, the exact ground state can be very well approximated by a permanent state, in that the permanent state has high overlap (at least 0.96 even for 12 particles and 12 sites) with the exact ground state and can reproduce both the ground state energy and the single-particle correlators to high precision. For a generic model, we have devised a greedy algorithm to find the optimal set of single-particle orbitals to minimize the variational energy or maximize the overlap with a target state. It turns out that quite often the ground state of a bosonic system can be well approximated by a permanent state by all the criterions of energy, overlap, and correlation functions. And even if the error is apparent, it can often be remedied by including more configurations, i.e., by allowing the variational wave function to be a combination of multiple permanent states. The algorithm is used to study the stability of a two-particle system, with great success. All these suggest that permanent states are very effective as variational wave functions for bosonic systems, and hence deserve further studies.