2-ELECTRON QUANTIZATION OF THE CHARGE ON A SUPERCONDUCTOR

THE theoretical understanding of superconductors is based on the notion of electron pairing into Cooper pairs1. The first direct evidence for electron pairing was the observation that the flux threading a superconducting ring is always a multiple of the flux quantum, given by the ratio of Planck's constant to the Cooper-pair charge 2e (refs 2, 3). Here we report a direct measurement of the total charge on a superconducting electrode which is free to exchange electrons with a metallic reservoir through a tunnel junction. The total charge on a non-superconducting metal electrode has been shown previously4 to increase in jumps of 1e, corresponding to the addition of single electrons. We have also observed steps of 1e, with an even-odd asymmetry, for a superconducting electrode when the charging energy exceeds the energy gap between the ground and first excited superconducting state5. Our present measurements, with the charging energy below the gap, reveal charging steps strictly quantized in units of 2e, corresponding to the simultaneous tunnelling of two electrons. The 2e steps break into 1e steps when the temperature and magnetic field are increased above threshold values, corresponding to the electrostatic breaking of a single Cooper pair. Our results indicate that Cooper pairs can be manipulated in the same way as single electrons in turnstile and pump devices4.