  We demonstrate a slow-light augmented unbalanced Mach-Zehnder interferometer
(MZI) which can be used to enhance very significantly the sensitivity of
measuring the frequency shift in a laser. The degree of enhancement depends on
the group index of the slow-light medium, the degree of imbalance between the
physical lengths of the two arms of the MZI, and the spectral width of the
laser. For a laser based on a high-finesse cavity, yielding a narrow quantum
noise limited spectral width, the group index has to be larger than the finesse
in order to achieve enhancement in measurement sensitivity. For the reported
results, strong slow-light effect is produced by employing electro-magnetically
induced transparency via coherent population trapping in a buffer-gas loaded
vapor cell of Rb atoms, with a maximum group index of ~1759. The observed
enhancement in sensitivity for a range of group indices agrees well with the
theoretical model. The maximum enhancement factor observed is ~22355, and much
larger values can be obtained using cold atoms for producing the slow-light
effect, for example. The sensitivity of any sensor that relies on measuring the
frequency shift of a laser can be enhanced substantially using this technique.
These include, but are not limited to, gyroscopes and accelerometers based on a
conventional ring laser or a superluminal ring laser, and detectors for
virialized ultra-light field dark matter. We also show how similar enhancements
can be achieved in a slow-light augmented unbalanced Michelson interferometer.
