  Quantum optics potentially offers an information channel from the Universe
beyond the established ones of imaging and spectroscopy. All existing cameras
and all spectrometers measure aspects of the first-order spatial and/or
temporal coherence of light. However, light has additional degrees of freedom,
manifest in the statistics of photon arrival times, or in the amount of photon
orbital angular momentum. Such quantum-optical measures may carry information
on how the light was created at the source, and whether it reached the observer
directly or via some intermediate process. Astronomical quantum optics may help
to clarify emission processes in natural laser sources and in the environments
of compact objects, while high-speed photon-counting with digital signal
handling enables multi-element and long-baseline versions of the intensity
interferometer. Time resolutions of nanoseconds are required, as are large
photon fluxes, making photonic astronomy very timely in an era of large
telescopes.
