  A minimal coupling rule for the coupling of the electron spin to curved
spacetime in general relativity suggests the possibility of a coupling between
electromagnetic and gravitational radiation mediated by means of a quantum
fluid. Thus quantum transducers between these two kinds of radiation fields
might exist. We report here on the first attempt at a Hertz-type experiment, in
which a high-$\rm{T_c}$ superconductor (YBCO) was the sample material used as a
possible quantum transducer to convert EM into GR microwaves, and a second
piece of YBCO in a separate apparatus was used to back-convert GR into EM
microwaves. An upper limit on the conversion efficiency of YBCO was measured to
be $1.6\times10^{-5}$ at liquid nitrogen temperature.
