Balancing interferometers with slow-light elements

In this Letter we show that interferometers with unbalanced arm lengths can be balanced using optical elements with appropriate group delays. For matched group delays of the arms, the balanced interferometer becomes insensitive to the frequency noise of the source. For experimental illustration, a ring resonator is employed as a slow-light element to compensate the arm-length mismatch of a Mach-Zehnder interferometer. An arm-length mismatch of 9.4?m is compensated by a ring resonator with a finesse of 70 and a perimeter of 42?cm.