  The electric fields near the heterogeneous metal/dielectric surface of an
atom chip were measured using cold atoms. The atomic sensitivity to electric
fields was enhanced by exciting the atoms to Rydberg states that are 10^8 times
more polarizable than the ground state. We attribute the measured fields to
charging of the insulators between the atom chip wires. Surprisingly, it is
observed that these fields may be dramatically lowered with appropriate voltage
biasing, suggesting configurations for the future development of hybrid quantum
systems.
