Future Prospects of Intensity Interferometry

In this talk, I will give an introduction to intensity correlations for astrophysical imaging, as pioneered by Hanbury Brown and Twiss. This triggered a wider effort for the field of quantum optics, which I will put into a larger context beyond astrophysical imaging. I will also give an overview of the past results on intensity correlations for astrophysical imaging by our group in Nice and present the ongoing effort towards resolving a white dwarf and to search for signatures of random lasing in space.

Intensity Interferometry (II) is a method that can achieve high angular resolution and was first employed in the 1960s by Robert Brown and Richard Q. Twiss (HBT). Since then, significant advancements have been made, particularly in the construction of telescopes with large light collection areas, such as Imaging Atmospheric Cherenkov Telescopes (IACTs), exemplified by instruments like H.E.S.S. , MAGIC and VERITAS. Our II setup was designed to be mounted on the lid of the Phase I H.E.S.S. telescopes in Namibia. In April 2022, our first observation campaign was conducted, during which two telescopes operated in a single wavelength band. In April-May 2023, a third telescope was added, and observations were performed in two colors simultaneously for the first time in II. In this contribution I will introduce our setup and compare the different configurations, as well as present the latest results of four southern hemisphere stars.

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.

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 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.

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.

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.

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.

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.