Macroscopic Quantum Superposition in a Three-Josephson-Junction Loop

We present microwave-spectroscopy experiments on two quantum levels of a superconducting loop with three Josephson junctions. The level separation between the ground state and first excited state shows an anti-crossing where two classical persistent-current states with opposite polarity are degenerate. This is evidence for symmetric and antisymmetric quantum superpositions of two macroscopic states; the classical states have persistent currents of 0.5 µA and correspond to the center-of-mass motion of millions of Cooper pairs. A study of the thermal occupancies of the two quantum levels shows that the loop is at low temperatures in a non-equilibrium state.