Direct Measurement of the Destruction of Charge Quantization in a Single Electron Box

We report here direct measurements of the destruction of charge quantization in a single electron box, the first over the full range of box-to-lead conductance values from G ~ 0 to the conductance quantum G_q = 2e^2/h, using a sensitive single-electron transistor (SET) electrometer. The sensitivity of the electrometer is measured to be dq ~ 6 x 10^-5 e/sqrt{Hz} and its superiority to conductance measurements of charge fluctuations is clearly demonstrated. As the rate of quantum mechanical tunneling from the box to its lead is increased, the quantization of charge is destroyed, disappearing entirely at G = G_q in agreement with theory.