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Science 26 February 1988:
Vol. 239. no. 4843, pp. 992 - 997
DOI: 10.1126/science.239.4843.992
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Articles

Quantum Mechanics of a Macroscopic Variable: The Phase Difference of a Josephson Junction

JOHN CLARKE 1, ANDREW N. CLELAND 1, MICHEL H. DEVORET 2, DANIEL ESTEVE 2, and JOHN M. MARTINIS 2

1 Department of Physics, University of California, and the Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720.
2 Service de Physique, Centre d'Études Nucléaires de Saclay, 91191 Gif-sur-Yvette Cedex, France.

Experiments to investigate the quantum behavior of a macroscopic degree of freedom, namely the phase difference across a Josephson tunnel junction, are described. The experiments involve measurements of the escape rate of the junction from its zero voltage state. Low temperature measurements of the escape rate for junctions that are either nearly undamped or moderately damped agree very closely with predictions for macroscopic quantum tunneling, with no adjustable parameters. Microwave spectroscopy reveals quantized energy levels in the potential well of the junction in excellent agreement with quantum-mechanical calculations. The system can be regarded as a "macroscopic nucleus with wires."


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