Observation of Resonant Tunneling between Macroscopically Distinct Quantum Levels

We report the first observation of resonant tunneling of a system between two macroscopically distinct states: energy levels in different fluxoid wells of a weakly damped superconducting quantum interference device which differ in mean current by approximately 6 μA. Near 50 mK, the tunneling rate $\gamma \left({\Phi }_x\right)$ from the metastable well vs applied flux ${\Phi }_x$ is found to exhibit a series of local maxima where the levels (spaced by $\approx 1.9$ K) in the two wells cross. The positions of these maxima agree well with the level crossings calculated using independently determined system parameters.