  "Millikan oil drops" are drops of superfluid helium coated with electrons,
and levitated in a strong, inhomogeneous magnetic field. When the temperature
of the system becomes very low compared to the cyclotron gap energy, the system
remains in its quantum ground state. Two such levitated charged drops can have
their charge-to-mass ratio critically adjusted so that the forces of gravity
and electricity between the drops are in balance. Then it is predicted that the
amount of scattered electromagnetic and gravitational radiation from the drops
are equalized, along with these two kinds of forces. The cross sections for the
scattering of the two kinds of radiation can become large, hard-sphere
cross-sections at the first Mie resonance, due to the hard-wall boundary
conditions on the surfaces of the spheres for both kinds of radiations. An
efficient quantum transduction process between electromagnetic and
gravitational radiation by such a pair of drops is predicted at microwave
frequencies, and a Hertz-like experiment is proposed. A more practical
implementation of these ideas to use pairs of levitated, charged
superconducting spheres is briefly discussed.
