DIRECT DEMONSTRATION OF HEISENBERGS UNCERTAINTY PRINCIPLE IN A SUPERCONDUCTOR

A HEISENBERG uncertainty relation exists between any two noncommuting variables of a quantum-mechanical system. In a superconductor, two suc h variables are the number, n, of Cooper pairs and the phase, phi, of the superconducting wavefunction. Suppressing fluctuations in either v ariable should lead to enhanced fluctuations in the other(1,2). To dem onstrate this effect, we have fabricated a structure in which the quan tum-mechanical fluctuations in the phase bf a superconducting grain ca n be suppressed. We measure the supercurrent that Rows through two Jos ephson tunnel junctions of small capacitance that are connected to the grain. The capacitance of the grain is itself so small that the numbe r of Cooper pairs is well defined-charge transport through the grain i s possible only through quantum-mechanical fluctuations in n. The phas e of the grain is coupled to a large superconducting reservoir such th at the fluctuations in phi can be controllably suppressed. The enhance d fluctuations in n that result from this coupling give rise to a larg e increase in the supercurrent through the grain.