I will introduce a conceptually new astronomical technique---extended-path intensity correlation (EPIC)---and discuss its scientific applications.

EPIC is a variant of intensity interferometry wherein an optical-path modification creates a path extension in the two-photon interference amplitude. This alteration generates interference fringes for widely separated sources, allowing maximum source separations parametrically larger than the angular resolution. Augmented with advances in single-photon detectors and spectroscopic gratings, EPIC would enable ground-based astrometry at microarcsecond-level precision in a field of view as large as several arcseconds. EPIC has the potential to revolutionize astrophysical and cosmological observations requiring high-precision differential astrometry on sources of high surface brightness.

I will lay out the theory, technical requirements, and scientific potential of EPIC, and give an update on its planned experimental demonstration. Promising applications include astrometric microlensing of stars and quasar images, binary-orbit characterization, exoplanet detection, Galactic acceleration measurements, calibration of the cosmic distance ladder, all at unprecedented relative astrometric precision.