  We demonstrate a new way to extend the coherence time of separated
Bose-Einstein condensates that involves immersion into a superfluid bath. When
both the system and the bath have similar scattering lengths, immersion in a
superfluid bath cancels out inhomogeneous potentials either imposed by external
fields or inherent in density fluctuations due to atomic shot noise. This
effect, which we call superfluid shielding, allows for coherence lifetimes
beyond the projection noise limit. We probe the coherence between separated
condensates in different sites of an optical lattice by monitoring the contrast
and decay of Bloch oscillations. Our technique demonstrates a new way that
interactions can improve the performance of quantum devices.
