Earlier studies have given enough evidence in support of the BGS conjecture,
with few exceptions violating it. Here, we provide one more counterexample
using a many-body system popularly known as the model of quantum kicked top
consisting of N qubits with all-to-all interaction and kicking strength
k=NÏ€/2. We show that it is quantum integrable even though the corresponding
semiclassical phase-space is chaotic, thus violating the BGS conjecture. We
solve the cases of N=5 to 11 qubits analytically, finding its eigensystem,
the dynamics of the entanglement, and the unitary evolution operator. For the
general case of N>11 qubits, we provide numerical evidence of integrability
using degenerate spectrum, and the exact periodic nature of the time-evolved
unitary evolution operator and the entanglement dynamics.Comment: 4.5 pages (two-column) + 25 pages (one-column) + 3 figures; Comments
are welcom