  Yang-Mills gravity is a quantum theory of gravity with translational gauge
symmetry that is based on a flat space-time. The universal coupling of all
quantum fields to quantum Yang-Mills gravity is based on the replacement of
$\p_\mu$ by the translational gauge covariant derivative $(\p_\mu
+g\phi_\mu^\nu \p_\nu)$ in the Lagrangians of non-gravitational fields. Near
the surface of the Earth, Yang-Mills gravity causes the phase gradient $\p_k
\th$ to be altered by a factor of $h_1\approx(1- g\phi)\approx 1- 7\times
10^{-10}$. In addition, the usual gauge-invariant combination of phase
gradients and vector potentials ${\bf A}$ in Josephson junctions is modified
and is no longer gauge invariant. The voltage across a Josephson junction is
thus affected by the presence of the gravitational coupling constant $g$, and
is now given by $V_{g21}\approx Q \int_1^2 [- \mbox{\boldmath$ {\bf \nabla}$}
A_0 - h_1^{-2}{\p {\bf A}}/{\p t}]\cdot d{\bf s}$. We propose an experimental
test of Yang-Mills gravity based on this effect. If one were to compare the
voltage across a Josephson junction in a laboratory at rest on Earth with that
across a junction in free fall (e.g., in the International Space Station or in
a plane maneuvering to simulate zero-gravity such as NASA's now-retired "Vomit
Comet"), Yang-Mills gravity predicts a difference on the order of 1 part in
$10^{9}$, which should be detectable as the precision of the Josephson junction
voltage standard is on the order of a few parts in $10^{10}$.
