  A dynamic blue-detuned optical dipole trap with stable $^{87}Rb$ atoms
produces a differential ac Stark shift of 18 Hz in the ground state hyperfine
transition, and it preserves the ground state hyperfine superpositions for a
long coherence time of 180 ms. The trapped atoms undergoing microwave Rabi
oscillations are sensitive to a small signal, artificially generated with a
second microwave source, phase locked to the first allow- ing a simple and
effective method for determining signal-to-noise ratio limits through
interference techniques. This provides an excellent means of calibrating
sensitivity in experiments such as our ongoing Fr parity non-conservation
measurement.
