  We report an experimental observation of the backaction of a Single Electron
Transistor (SET) measuring the Coulomb staircase of a single electron box. As
current flows through the SET, the charge state of the SET island fluctuates.
These fluctuations capacitively couple to the box and cause changes in the
position, width, and asymmetry of the Coulomb staircase. A sequential
tunnelling model accurately recreates these effects, confirming this mechanism
of the backaction of an SET. This is a first step towards understanding the
effects of quantum measurement on solid state qubits.
