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Accretion flows aroud black-holes, neutron stars or white dwarfs are studied since almost 60 years. Although they are ubiquitous and somewhat similar over scales reaching billions in mass and size, their study has been limited because they remain unresolved point like sources in the optical/ultraviolet and X-rays, where they emit. Two main modes of accretion have been identified in Active Galactic Nuclei. In most sources the accretion rate is low and a high pressure, low density, low collision rate, optically thin, radiatively inefficient, two temperature plasma can form (Shapiro 1976; Narayan & Yi 1994,1995). This solution is stable only for low luminosities (<1% LEDD). The Event Horizon Telescope has recently resolved such flows in Sgr A and M87, confirmed several aspects of the model and could detect particles accelerated close to the horizon of Sgr A (Wielgus, 2022) a likely signature of the Blandford-Znajek (1977) process. When the accretion rate is higher, momentum can be dissipated by viscosity and the flow proceeds via geometrically thin disk-shaped structures. These accretion disks provide feedback to their environment by accelerating winds and launching jets in their central regions. The apparent size of accretion disks are of the order of 1-40µarcsec in nearby quasars, Seyfert galaxies and galactic cataclysmic variables and of 0.1-1µarcsec in of low mass X-ray binaries in our Galaxy. Hanbury-Brown & Twiss (1954) invented intensity interferometry and measured the size of some bright stars by correlating the arrival times of photons detected by two optical telescopes. The physics has been explained as a quantum effect in the early 60s (Fano 1961) and has triggered the development of quantum optics (Glauber 1963). Its root is found in the quantum theory of statistical fluctuations in an ideal gas (Einstein 1925). The achievable signal-to-noise depends on the telescope size, the detector time resolution, and the number of spectral channels observed simultaneously. Extremely large telescope and 10ps resolution single photon detectors bring the key improvements to reach in the optical angular resolutions better than these achieved in the radio by the Event Horizon Telescope and to obtain the first images of accretion disks around galactic and extragalactic compact objects, a breakthrough. I will present the goals and the status of the QUASAR project, which started one year ago, aiming at bringing a 10ps resolution optical spectrometer on very large telescope.

Superconducting nanowire single photon detectors (SNSPDs) are of interest for intensity interferometry measurements because they have picosecond timing resolution. In addition, they work from the UV to mid-IR, with excellent eOiciency at visible and near-IR wavelengths, and are being fabricated into ever-larger detector arrays. On behalf of my colleagues in the JPL SNSPD group, I will present on the Deep Space Optical Communication (DSOC) demonstration, in which an SNSPD array was coupled to the 5 mHale telescope at Palomar, and received data at 267 Mbps from the Psyche spacecraft, the first optical communication between Earth and interplanetary space. The DSOC infrastructure at Palomar is suitable for intensity interferometry, as demonstrated by g(2) correlation (photon bunching) measurements of the stars Rigel and Procyon. I will also describe our current work on SNSPD array readout schemes, extending detector sensitivity into the mid-IR, and improving the system timing jitter of SNSPD arrays.

In this talk we want to promote a new kind of single photon detector able to record high count rates with the prospect of making stellar intensity interferometry (SII) measurements more effective. This micro-channel plate based photomultiplier tube from Photonscore (LINPix) is nearly dead time free and offers an active area of 8mm diameter. By choosing a matching photocathode, the quantum efficiency (QE) can take values greater than 30% at the desired wavelength. Using a Hi-QE Blue photocathode in a testbench featuring a fs-pulsed laser we were able to measure the timing resolution of the LINPix at different count rates from 190kHz up to 95 MHz. We find that the timing resolution of the detector only increases marginally when increasing the laser power and stays well below 100ps. Hence, we conclude that together with the LINTag, a suitable time to digital converter from Photonscore able to process the high throughput, this system can contribute significantly to the further development of SII.

Interferometry is the use of wave interference to measure the properties of a source observed by two or more detectors. For example, the Event Horizon Telescope measures the phase and amplitude of 1.3 mm wavelength radiation at telescopes up to ten thousand kilometers apart to reveal event horizon scale images of supermassive black holes. Measuring wave phases in the optical has been demonstrated for baselines no longer than hundreds of meters. Intensity interferometry dispenses with the need to measure phases, allowing much larger baselines, and hence much higher spatial resolution. The technique has been in use for seven decades, but recent advances in detector technology have reinvigorated interest in the method. I will discuss the basics of intensity interferometry, the characteristics of the new detectors, and possible applications of broad astrophysical and cosmological interest. The latter include estimates of the Hubble constant from observations of the disks of active galactic nuclei (AGN), with possible impact on the Hubble tension. The same observations will provide detailed information on the AGN disk and line emission regions; the latter may be crucial for estimating the mass loss rates in AGN winds, which are believed to impact their host galaxies. Other possible applications include spatially resolved measurements of stellar oscillations, which, by analogy with helioseismology, would provide constraints on the run of temperature in stellar interiors, as well as the interior differential rotation.

We present the design and initial results of a stellar intensity interferometer using small 0.25 m Newtonian-style telescopes in an urban backyard setting. The primary purpose of the interferometer is to measure the angular diameters of stars. Recent advances in low jitter time-tagging equipment and Single Photon Avalanche Detectors have made the detection of second-order correlation signals, necessary for Intensity Interferometry as demonstrated by Hanbury Brown and Twiss in 1956, feasible with small telescopes. Using Sirius as a target star, we observe a strong second-order correlation spike with an integrated signal to noise ratio (SNR) ∼7 after 13.55 h of integration over a three-night period using a 3.3 m baseline. The measured signal agrees with the theoretical estimates of both coherence time, 𝜏coh = 0.74 ± 0.26 ps and SNR. We discuss the future expansion of this technique with multiple wavelengths simultaneously via a prism grating and multiple detectors.

The Multi-Aperture Spectroscopic Telescope is an array of 20x60cm prime-focus telescopes with single F/3 parabolic mirrors. The telescope array is being commissioned in the Negev Desert, with 10 telescopes expected to see first light by the end of the year. In the following talk, I will present the array, its various properties, including unique fiber coupling and imaging units, and its potential as an intensity interferometry facility.

The VERITAS Imaging Atmospheric Cherenkov Telescope array was augmented in 2019 with high-speed focal plane electronics to allow VERITAS for Stellar Intensity Interferometry (VSII) observations. Since December 2019, VSII has been used to measure angular diameters of bright (OBA) stars at an effective wavelength of 416 nm. VSII observations have also served as a testbed to explore hardware and analysis improvements to advance the instrument's sensitivity. VSII has performed more than 730 hours of moonlit observations on 56 bright stars and binary systems ($ -1.46 < m_V < 4.22$). This talk will describe the VSII observatory, highlight selected observations made by the VSII observatory, and describe ongoing improvements in detector instrumentation and analysis.

The renaissance in stellar intensity interferometry has resulted in two main types of telescope arrays: those using large "light bucket" telescopes and photomultiplier tubes, such as CTA, VERITAS, MAGIC, and others, and those that instead use smaller, more traditional astronomical telescopes with high-grade optics, such as the systems at the Cote d'Azur and Asiago Observatories. To detect and timestamp photons, these latter systems have used single-photon avalanche diode (SPAD) detectors. This talk will focus on the latter type of instrument, which is also being pursued at Southern Connecticut State University. The current status of our instrument, the Southern Connecticut Stellar Interferometer (SCSI), will be reviewed, and prospects for improved sensitivity will be discussed. Principal among these is the use of SPAD arrays, which are increasingly available, to record different wavelengths simultaneously. If a sufficient number of channels can be employed, this type of intensity interferometer can reach much fainter magnitudes than currently possible. The talk will also briefly discuss work toward wireless intensity interferometry with SCSI, which will make larger baselines easier to set up and use, and ideas for quantum-assisted intensity interferometry that might be employed with SCSI in the future.