Research Article

Quantum Superposition of Macroscopic Persistent-Current States

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Science  27 Oct 2000:
Vol. 290, Issue 5492, pp. 773-777
DOI: 10.1126/science.290.5492.773

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Microwave spectroscopy experiments have been performed on two quantum levels of a macroscopic superconducting loop with three Josephson junctions. Level 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 macroscopic states. The two classical states have persistent currents of 0.5 microampere and correspond to the center-of-mass motion of millions of Cooper pairs.

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