An intensity interferometer is the name given to devices that use the
Hanbury Brown and Twiss effect.
In
astronomy, the most common use of such an
astronomical interferometer
An astronomical interferometer or telescope array is a set of separate telescopes, mirror segments, or radio telescope antenna (radio), antennas that work together as a single telescope to provide higher resolution images of astronomical objects ...
is to determine the apparent angular diameter of a radio source or star. If the distance to the object can then be determined by
parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
or some other method, the physical diameter of the star can then be inferred. An example of an optical intensity interferometer is the
Narrabri Stellar Intensity Interferometer. In
quantum optics, some devices which take advantage of
correlation
In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...
and anti-correlation effects in beams of
photons
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...
might be said to be intensity interferometers, although the term is usually reserved for
observatories.
An intensity interferometer is built from two light detectors, typically either
radio antenna or optical telescopes with
photomultiplier tubes (PMTs), separated by some distance, called the baseline. Both detectors are pointed at the same astronomical source, and intensity measurements are then transmitted to a central correlator facility. A major advantage of intensity interferometers is that only the measured intensity observed by each detector must be sent to the central correlator facility, rather than the amplitude and phase of the signal. The intensity interferometer measures interferometric visibilities like all other astronomical interferometers. These measurements can be used to calculate the diameter and limb darkening coefficients of stars, but with intensity interferometers
aperture synthesis images cannot be produced as the visibility phase information is not preserved by an intensity interferometer.
References
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Telescopes
Interferometric telescopes
Quantum optics