We present a laboratory demonstration of the Kramers-Kronig relation between the resonant
absorption and refractive index in rubidium gas. Our experiment uses a rubidium vapor cell
in one arm of a simple Mach-Zehnder interferometer. As the laser frequency is scanned over
an atomic resonance, the interferometer output is affected by variations of both the
absorption and refractive index of the gas with frequency, all of which can be calculated in a
straightforward manner. Changing the vapor density and interferometer phase produces a …