Intensity interferometry with Aqueye+ and Iqueye in Asiago

Since a number of years our group is engaged in the design, construction and operation of instruments with very high time resolution in the optical band for applications to Quantum Astronomy and more conventional Astrophysics. Two instruments were built to perform photon counting with sub-nanosecond temporal accuracy. The first of the two, Aqueye+, is regularly mounted at the 1.8 m Copernicus telescope in Asiago, while the second one, Iqueye, was mounted at the ESO New Technology Telescope in Chile, and at the William Herschel Telescope and Telescopio Nazionale Galileo on the Roque (La Palma, Canary Islands). Both instruments deliver extraordinarily accurate results in optical pulsar timing. Recently, Iqueye was moved to Asiago to be mounted at the 1.2 m Galileo telescope to attempt, for the first time ever, experiments of optical intensity interferometry (a la Hanbury Brown and Twiss) on a baseline of a few kilometers, together with the Copernicus telescope. This application was one of the original goals for the development of our instrumentation. To carry out these measurements, we are experimenting a new way of coupling the instruments to the telescopes, by means of moderate-aperture, low-optical-attenuation multi-mode optical fibers with a double-clad design. Fibers are housed in dedicated optical interfaces attached to the focus of another instrument of the 1.8 m telescope (Aqueye+) or to the Nasmyth focus of the 1.2 m telescope (Iqueye). This soft-mount solution has the advantage to facilitate the mounting of the photon counters, to keep them under controlled temperature and humidity conditions (reducing potential systematics related to varying ambient conditions), and to mitigate scheduling requirements. Here we will describe the first successful implementation of the Asiago intensity interferometer and future plans for improving it.