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Science 27 October 2000:
Vol. 290. no. 5492, pp. 773 - 777
DOI: 10.1126/science.290.5492.773
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Research Articles

Quantum Superposition of Macroscopic Persistent-Current States

Caspar H. van der , Wal,1* A. C. J. ter , Haar,1 F. K. Wilhelm,1 R. N. Schouten,1 C. J. P. M. Harmans,1 T. P. Orlando,2 Seth Lloyd,3 J. E. Mooij12

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.

1 Department of Applied Physics and Delft Institute for Micro Electronics and Submicron Technology (DIMES), Delft University of Technology, Post Office Box 5046, 2600 GA Delft, Netherlands.
2 Department of Electrical Engineering and Computer Science and
3 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
*   To whom correspondence should be addressed. E-mail: caspar{at}qt.tn.tudelft.nl

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