We investigated the use of an unbalanced Mach-Zehnder interferometer augmented with slow-light-effect induced via electromagnetically induced transparency for enhanced-sensitivity measurement of frequency shift, and demonstrated conditions corresponding to an enhancement factor greater than ten thousand.
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 factor 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 with a quantum noise limited spectral width, the group index has to be larger than the finesse of the laser cavity in order to achieve enhancement in measurement sensitivity. For the reported results, slow-light effect is produced by employing electro-magnetically induced transparency via coherent population trapping in a paraffin coated vapor cell of Rb atoms, with a maximum group index of ~2170. The maximum enhancement factor realized is ~70. This differs from the theoretically expected value of ~183 by a factor of ~2.6. This discrepancy can be attributed to effects of unidentified excess noise, and the fact that the test laser spectral width may not the quantum noise limited. Much larger values can potentially be obtained by modifying the apparatus, and using cold atoms for producing the slow-light effect. 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.
Recently, it has been shown that a slow-light augmented unbalanced Mach-Zehnder interferometer (SLAUMZI) can be used to enhance significantly the sensitivity of measuring the frequency shift of a laser, compared to the heterodyne technique. Here, we show that a similar enhancement can be realized using a slow-light augmented Fabry-Perot Cavity (SLAFPC), due to the fact that an FPC is inherently unbalanced, since different bounces of the field traverse different path lengths before interfering with the other bounces. We show how the degree of enhancement in sensitivity depends on the spectral width of the laser and the finesse of the FPC. For potentially realizable conditions, we show that a sensitivity enhancement factor ~2.8*10^6 can be achieved using a SLAFPC.
We explore the properties of electromagnetically induced transparency (EIT) and its applications as a frequency filter in the field of gravitational wave interferometry. Through modeling and simulation, we determine parameters for atom-light configurations of multi- state atoms which will theoretically allow for transmission frequencies and intensities of squeezed light in a range suitable for increasing sensitiviy levels in gravitational wave interferometers. This corresponds to contrasts greater than 50% and linewidths of 100 Hz or less. We produce EIT experimentally and characterize the distributions by fitting them to a generalized Lorentzian. The largest contrast observed is 3.9% with a linewidth of 657 Hz. The smallest linewidth observed is 202 Hz with a contrast of 0.84%.