  This article presents a novel type of very long baseline astronomical
interferometer that uses the fluctuations, as a function of time, of the
intensity measured by a quadratic detector, which is a common type of
astronomical detector. The theory on which the technique is based is validated
by laboratory experiments. Its outstanding principal advantages comes from the
fact that the angular structure of an astronomical object is simply determined
from the visibility of the minima of the spectrum of the intensity fluctuations
measured by the detector, as a function of the frequency of the fluctuations,
while keeping the spacing between mirrors constant. This would allow a simple
setup capable of high angular resolutions because it could use an extremely
large baseline. Another major interest is that it allows for a more efficient
use of telescope time because observations at a single baseline are sufficient,
while amplitude and intensity interferometers need several observations at
different baselines. The fact that one does not have to move the telescopes
would also allow detecting faster time variations because having to move the
telescopes sets a lower limit to the time variations that can be detected. The
technique uses wave interaction effects and thus has some characteristics in
common with intensity interferometry. A disadvantage of the technique, like in
intensity interferometry, is that it needs strong sources if observing at high
frequencies (e.g. the visible). This is a minor disadvantage in the radio
region. At high frequencies, this disadvantage is mitigated by the fact that,
like in intensity interferometry, the requirements of the optical quality of
the mirrors used are far less severe than in amplitude interferometry so that
poor quality large reflectors (e.g. Cherenkov telescopes) can be used in the
optical region.
