Sensitivity of Quantum Information Measures to Local Bosonic Occupation Restrictions

Discrete lattice models play an essential role in the understanding of quantum phenomena, but their exact numerical solution is hindered by the exponentially growing size of the underlying Hilbert space. Such difficulty is more pronounced in the case of bosons due to the lack of any occupation restrictions as opposed to fermionic or even spin models. Thus, a widely adopted approximation in exact diagonalization as well as in the Density Matrix Renormalization Group is to restrict the bosonic occupation numbers to only a few bosons per lattice site. While the corresponding relative errors under this approximation in many observables including the energy or local particle number fluctuations could be negligible, we report that imposing such restrictions could have drastic effects on quantum information measures such as particle and accessible (symmetry resolved) entanglement entropies. We find that in these cases, the error scales with the system size and thus could rapidly exceed 100%, as demonstrated in the ground state of the Bose-Hubbard model.