  We have observed the stationary interference oscillations of a
triple-entangled neutron state in an interferometric experiment. Time-dependent
interaction with two radio-frequency (rf) fields enables coherent manipulation
of an energy degree of freedom in a single neutron. The system is characterized
by a multiply entangled state governed by a Jaynes-Cummings Hamiltonian. The
experimental results confirm coherence of the manipulation as well as the
validity of the description.
