Differential phase interferometry with the Keck telescopes

We summarize the differential phase (DP) technique as well as the planned implementation at the Keck interferometer. Multicolor phase measurements are potentially a powerful astrophysical probe - and can allow ground-based direct detection of extrasolar planets. Better than 0.1 mrad phase measurements in the infrared can allow the Keck interferometer to detect radiation from the so-called hot-Jupiter or "roaster" class of planets. At JPL, we are presently developing and testing instrumentation that enable these extremely sensitive measurements. First on-sky observations are expected to start in mid-2004. In this article we describe DP and other related techniques, provide an outline of the instrument and present results from preliminary laboratory experiments.