Fundamental limits of classical high contrast imaging and new opportunities through quantum enhanced optical processing

Coherent starlight suppression techniques are tools for enabling direct observations of faint companions near bright stars. These methods are constrained by several fundamental considerations . We discuss these limits in a unified way. One limit is set by spatial coherence. The planet and the star electric fields overlap slightly at the entrance pupil. A coronagraph suppresses the part of the star orthogonal to the planet field, but any shared component cannot be removed without suppressing the planet. This sets a floor that cannot be eliminated by coherent subtraction. Further, nulling the star at one wavelength does not null it at other wavelengths, but a broader bandwidth improves the photon statistics for field estimation. These effects define the true performance limits for any classical coronagraph combined with coherent subtraction. We describe how recent proposals including quantum enhanced optical imaging relate to these fundamental limits and provide new opportunities.