Neutrinos, key messengers in high-energy astrophysics, provide valuable insights into extreme environments within astrophysical objects. However, their weak interactions limit spatial imaging, requiring neutrino telescope findings to be correlated with other observational methods. In recent years, intensity interferometry has emerged as a powerful tool for observing these regions with unprecedented accuracy.
Stellar Intensity Interferometry (SII) leverages correlations in chaotic light to achieve sub- milliarcsecond resolution, with aspirations of reaching nanoarcsecond precision. This advance- ment opens pathways to imaging accretion disks around compact objects, binary systems, and stellar explosions. First demonstrated in the 1960s at the Narrabri Observatory, SII was later aban- doned due to technological constraints, such as limited mirror area and time resolution, restricting it to simple proofs of concept.
In recent years, Cherenkov telescopes like MAGIC, and soon the Large-Sized Telescopes (LSTs) for the CTAO, with their large mirror areas, have enabled SII to directly measure several stellar radii and aim to resolve smaller structures in the high-energy universe.
Additionally, the latest generation of light detectors—single-photon sensitive and operating with picoseconds timing—has made photon-counting mode SII feasible. These ultra-fast detectors measure photon arrival time differences, allowing precise determination of the second-order correlation function $𝑔^{(2)}$ and the photon bunching phenomenon.
While several initiatives are ongoing, this work focuses on the SII activities of the MAGIC telescopes, active since 2021, which have achieved stellar radius measurements with milliarcsecond precision using a uniform disk model. We also highlight the Swiss QUASAR project, which aims to develop a spectrometer based on ultra-fast detectors capable of achieving nanoarcsecond resolution using large telescopes separated by distances of up to ten kilometers.