2-ELECTRON QUANTIZATION OF THE CHARGE ON A SUPERCONDUCTOR

THE theoretical understanding of superconductors is based on the notio n of electron pairing into Cooper pairs1. The first direct evidence fo r electron pairing was the observation that the flux threading a super conducting 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 supercond ucting electrode which is free to exchange electrons with a metallic r eservoir through a tunnel junction. The total charge on a non-supercon ducting metal electrode has been shown previously4 to increase in jump s of 1e, corresponding to the addition of single electrons. We have al so observed steps of 1e, with an even-odd asymmetry, for a superconduc ting electrode when the charging energy exceeds the energy gap between the ground and first excited superconducting state5. Our present meas urements, with the charging energy below the gap, reveal charging step s strictly quantized in units of 2e, corresponding to the simultaneous tunnelling of two electrons. The 2e steps break into 1e steps when th e 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.