In this talk, I will give an introduction to intensity correlations for astrophysical imaging, as pionneered 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 give an overview of the past results on intensity correlations for astrophysical imaging and mention the ongoing...
Abstract: Over the past decade, significant advances in instrumentation technology have led to new astronomical capabilities and increased sensitivity in the emerging field of Stellar Intensity Interferometry (SII). The rapid emergence of SII has been made possible by the convergence of the availability of inexpensive GHz sampling digitizers, White Rabbit sub-nsec clock distribution over km...
Stellar intensity interferometry (SII) has great potential to
precisely test stellar atmosphere model predictions. Published measurements of
stars near 400 nm, e.g.VERTIAS SII observations of the A-type subgiant
$\beta$ UMa, reveal smaller uniform disk angular diameters (stronger limb
darkening) relative to Michelson interferometry measurements at longer
wavelengths. Published...
The recent resurgence of intensity interferometry through instruments like VERITAS, MAGIC, HESS, and the ASTRI array has opened new pathways for achieving beyond milli-arcsecond-scale imaging of massive and evolved stars, including OB-type, Wolf-Rayet, and pulsating stars in complex stellar systems using optical wavelengths. Among these, close binary stars continue to fascinate astronomers due...
Science cases for intensity interferometry are very diverse -- ranging in interestingness from measurements that only a small community cares about to observations that (if carried out) would create a new field -- and ranging in difficulty from already operational to dreams where even feasibility hasn't been assessed yet. This presentation will show a matrix of science cases according to...
Recently the Event Horizon Telescope measured 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,...
In this talk, I will explore the potential for long-baseline optical intensity interferometry to observe bright, active galactic nuclei (AGN) associated with rapidly accreting supermassive black holes. I will argue that that realistic telescope arrays similar in area to existing Cherenkov arrays, if equipped with modern high-precision single photon detectors, can achieve a sufficiently high...
Stellar intensity interferometry (SII) enables optical measurements at sub-milliarcsecond resolution, offering the potential to open powerful new avenues for astrophysical discovery. With its ability to probe stellar surfaces, SII may allow constraints on temperature gradients, star spots, rotation, and limb darkening at unprecedented detail. Close binary systems could become accessible for...
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We recently used the VERITAS SII (Stellar Intensity Interferometry) system to extend SII measurements from round to oblate photospheres of stars. We now aim to push SII further by measuring more complex sources such as Spica, a spectroscopic binary star with a period of approximately 4 days. Spica was first measured by Hanbury Brown using the Narrabri Interferometer in 1971 but has not been...
The VERITAS collaboration has performed SII measurements on the system Alpha Virginis (Spica) using multiple telescope pairs over several nights. We compare these preliminary data with expectations for various sources, including a round star, a single elliptical star, and a binary star system. We discuss also the challenges for presenting multiple-baseline SII measurements (including,...
The Cherenkov Telescope Array Observatory (CTAO) is a new generation of telescope arrays for gamma-ray astronomy. The telescopes are designed to collect ultraviolet to red Cherenkov photons from extensive air showers with nanosecond sampling. The total mirror area of the telescopes exceeds that of the Extremely Large Telescope at each CTAO site: Paranal in the Atacama Desert (CTAO-South) and...
In addition to their gamma-ray observations in the very high energy regime (20 GeV - 100 TeV), the two 17-m MAGIC telescopes are ideal instruments for optical stellar intensity interferometer (SII), and have been routinely performing observations in SII mode for the last six years. The calibration and validation of the setup, alongside the first measurement of the stellar angular diameter of...
Imaging Atmospheric Cherenkov Telescopes (IACTs) have recently shown great potential as optical intensity interferometers. However, due to their design there are some intrinsic limitations to improving their sensitivity, such as their low focal ratios. Implementing the 'I3T concept', in which sets of facets of the primary mirror (submirrors) are focused onto different pixels of the camera, on...
Despite the extraordinary leap forward obtained with current implementations on Imaging Air Cherenkov Telescopes (IACTs), which take advantage from the huge photon statistics, intensity interferometry on IACTs has to cope with the poor optical quality and large numerical aperture of the optics, which is a limiting factor for observations in narrow spectral bands and with sub-nanosecond...
The quantum properties of a gas of bosons were predicted by Einstein 100 years ago. The first experimental measurements of its consequences were performed by Hanbury-Brown & Twiss in 1954, obtaining the size of Sirius by correlating the arrival times of photons detected by two optical telescopes. Extremely large telescopes, 10ps resolution single photon detectors bring the key improvements to...
Optical intensity interferometry represents a powerful technique to probe astrophysical sources, such as accretion disks around compact stellar systems, at angular resolutions unattainable with conventional imaging methods. To address the stringent requirements of single-photon sensitivity and picosecond timing, we have designed a 110 nm CMOS integrated circuit combining a 528×16 SPAD array...
To achieve the angular resolutions required to resolve accretion disks using the intensity interferometry (II) technique, the QUASAR project employs multiple telescopes separated by baselines of up to several tens of kilometers. In this configuration, the conventional approach of delivering signals to a single acquisition system becomes impractical. Instead, precise clock distribution to...
We present the results of an implementation of a GPU-based correlator for timestamps. The effectiveness of the implementation was demonstrated following two data acquisition campaigns, at Skinakas and Calern observatories, where we registered zero and cross baseline data of a few bright stars. We will present the results of these observations, and the performance of the correlator compared to...
Recent developments in single-photon avalanche diodes (SPADs) have lowered detector time resolution to tens of picoseconds full width half maximum (FWHM), which represents a big enhancement compared to photon detection techniques used at the time of Hanburry Brown and Twiss. In the context of the QUASAR project, I will summarize the main enhancements and present the results obtained over the...
If we want to look into the future it is sensible to look back to the past - at least as a starting point. A first approximation is to assume that the forward trajectory of a given technique would be a continuation of the past, but with more bells and whistles. Let's see where that might lead us when we think about where the technique of Stellar Intensity Interferometry SII might go. Let us...
Superconducting nanowire single-photon detectors (SNSPD) systems have progressed rapidly over the past decade. Recent devices exhibit leading performance figures including sub-10 ps system timing jitter, 1.5 GHz detection rates, arrays up to 400,000 pixels, sub-hertz dark count rates, high efficiency, as well as intrinsic and array-based photon number resolution. In this talk I will review...
Correlations of photon pairs from entangled quantum sources offer advantages and provide additional opportunities in new sensing approaches in multiple fields. In general, strong spectro-temporal correlations inherent for entangled photons could make those sensing techniques much more precise and resource efficient. In application to astrophysics I will compare the standard techniques of...
We present our simultaneous broadband multi-wavelength HBT measurement in a spectrometer with a 25 nm range, 0.1 nm/pixel scale based on the 512-pixel SPAD sensor of the LinoSPAD2 detector. Using an LED as a broadband source of light simulating starlight, we measure the HBT effect in tens of spectral bins. Detector resolution of 40 ps rms allows us to achieve 3% contrast in under 10 hours of...
We present our recent progress in stellar intensity interferometry, highlighting how large-area single photon counting detectors can advance spatial intensity interferometry (SII) through ultrahigh temporal resolution capabilities. Our work includes two measurement campaigns: the first in April 2024 using off-the-shelf detectors, the second in August 2024 employing the new PhotonPix detector...
In addition to direct and intensity interferometry techniques, heterodyning offers several unique advantages for stellar interferometry, and benefits from mature post-processing methods developed for radio astronomy. We introduce a new lab setup which explores potential improvements to stellar interferometry using NIR heterodyning. Constructed at the Max Planck Institute for Physics (MPP), the...
Superconducting nanowire single-photon detectors (SNSPDs) have been at the forefront of many recent breakthroughs in optical science. Their ultrahigh efficiency and ultralow timing jitter make them especially promising for astronomical applications, such as intensity interferometry. However, the small active area of conventional SNSPDs and the need for large detector sizes present significant...
I will present the current status of the work done by our "Intensity Interferometry at Calern" (I2C) consortium in Nice (France) on the revival of intensity interferometry with optical telescopes.
I will first briefly summarized what we have done in the last years: We have demonstrated intensity correlations using stellar light for the first time in the photon-counting regime, using...
In recent years, intensity interferometry has seen renewed interest and successful application at Imaging Atmospheric Cherenkov Telescope arrays such as H.E.S.S., MAGIC and VERITAS. However, these measurements can only be performed during bright moon periods while the instruments' primary purpose---gamma-ray observations---cannot be fulfilled, the Mobile Intensity Interferometer for Stellar...
The Multi-Aperture Spectroscopic Telescope (MAST) is a modular array of 20 × 60 cm telescopes currently being commissioned at the Weizmann Astrophysical Observatory in the Negev desert, Israel. Designed primarily for astronomical spectroscopy, MAST employs a novel fiber-fed architecture in which each on-axis beam is collected by an optical fiber and delivered to the object plane of a dedicated...
The simultaneous record of photons with at least three telescopes not only allows traditional two-photon correlations between pairs of telescopes, but also to explore three-photon correlations in the telescope triangle. Such measurements in principle enable measurements of the (cosine of the) closure phase, the sum of the interferometric phases in the triangle, and by that retrieve at least...
Quantum mechanics has revolutionised information processes like computing and communication. However, full-scale quantum networks and quantum computing are still years away. In the short term we can expect benefits from quantum technologies in the areas of sensing and metrology. In this talk I will present the foundations of quantum imaging and metrology, and explore how we can use quantum...
I will propose a technique called the "expanding ejecta method" (EEM) to determine angular diameter distances to supernovae based purely on geometry, augmenting the calibration of the cosmic distance ladder or even enabling a direct inference of the cosmic expansion rate. I will also introduce a new variant of intensity interferometry --- "extended-path intensity correlation". EPIC enables...
The Quantum Astrometry project at Brookhaven National Laboratory (USA) is working towards a demonstration of the SNSV scheme of intensity interferometry, as first proposed in Stankus et al. (2022, DOI: astro:2010.09100). By measuring the oscillation of HBT visibility between two optically-disconnected telescope stations as the Earth rotates, the SNSV scheme determines the opening angle between...
Using intensity correlations measured by intensity interferometers one can infer the angular 2-point correlation function containing precisely half the information of corresponding amplitude interferometry measurements. If one has a phase reference, say, as provided by a bright point source in the field one can recover all the information available to amplitude interferometry. I will show...
Intensity interferometry has proven to be a powerful method for measuring the angular sizes of stellar objects and is increasingly regarded as a reliable technique for achieving high-resolution imaging. With current advancements in detector technology, the time resolution has improved to the point where we are able to to resolve the shape of the correlation peaks. Such sensitivity opens the...
