Quantum superposition of macroscopic persistent-current states

Microwave spectroscopy experiments have been performed on two quantum Level s of a macroscopic superconducting Loop with three Josephson junctions. Lev el repulsion of the ground state and first excited state is found where two classical persistent-current states with opposite polarity are degenerate, indicating symmetric and antisymmetric quantum superpositions of macroscop ic states. The two classical states have persistent currents of 0.5 microam pere and correspond to the center-of-mass motion of millions of Cooper pair s.