  In the noisy intermediate-scale quantum era, variational algorithms have
become a standard approach to solving quantum many-body problems. Here, we
present variational quantum eigensolver (VQE) results of selected oxygen
isotopes within the shell model description. The aim of the present work is to
locate the neutron drip line of the oxygen chain using unitary coupled cluster
(UCC) type ansatze with different microscopic interactions (DJ16, JISP16, and
N3LO), in addition to a phenomenological USDB interaction. While initially
infeasible to execute on contemporary quantum hardware, the size of the problem
is reduced significantly using qubit tapering techniques in conjunction with
custom circuit design and optimization. The optimal values of ansatz parameters
from classical simulation are taken for the DJ16 interaction, and the tapered
circuits are run on IonQ's Aria, a trapped-ion quantum computer. After applying
gate error mitigation for three isotopes, we reproduced exact ground state
energies within a few percent error. The post-processed results from hardware
also clearly show $^{24}$O as the drip line nucleus of the oxygen chain. Future
improvements in quantum hardware could make it possible to locate drip lines of
heavier nuclei.
