Stellar Intensity Interferometry Workshop 2025

13 Oct 2025, 15:00 → 17 Oct 2025, 14:05 Europe/Berlin

Research Campus Waischenfeld (Germany) of the Fraunhofer Society

Neal Dalal (Perimeter Institute for Theoretical Physics), Stefan Funk (Friedrich-Alexander-Universität Erlangen-Nürnberg - ECAP), William Guerin (CNRS / Université de la Côte d’Azur), Robin Kaiser (CNRS / Université de la Côte d’Azur), Joachim von Zanthier (QOQI, Friedrich-Alexander-Universität Erlangen-Nürnberg)

The workshop Stellar Intensity Interferometry aims at gathering researchers involved in the modern revival of intensity interferometry, its enabling technologies (detectors, photonics…) and closely related interferometric techniques.

It combines the previous successful workshops

• „Stellar Intensity Interferometry“ (2024: Porquerolles, French Riviera) 
• „Future Prospects of Intensity Interferometry" (2024, Perimeter Institute, Canada).
• „Stellar Intensity Interferometry“ (2023: Columbus, Ohio)

The workshop will be held at Research Campus Waischenfeld, Germany from October 13-17, 2025. 
A preliminary schedule can be found here.

Topics include:

• History of Stellar Intensity Interferometry
• Science Goals
• Imaging Atmospheric Cherenkov Telescopes (IACTs)
• Optical Telescopes
• Arrays of Small Telescopes
• Amplitude Interferometry
• Detector Development
• Quantum Optics with Intensity Interferometry
• Image Reconstruction & Advanced Methods

Confirmed Invited Speakers (in alphabetical order)

• Sagi Ben-Ami (Weizmann Institute of Science)
• Jonathan Biteau (Univ. of Paris-Sud)
• Colin Carlile (Lund Univ.)
• Tarek Hassan (CIEMAT Madrid)
• David Kieda (Univ. of Utah)
• Pieter Kok (Univ. of Sheffield)
• Boris Korzh (Univ. Geneva)
• Andrei Nomerotski (Florida International University & 

  Czech Technical University in Prague)

• Prasenjit Saha (Univ. of Zurich)
• Ken Van Tilburg (New York University )
• Roland Walter (Univ. Geneva)
•  

Registration

Registration is already closed

The conference fee includes:

• full board for the entire week
• accommodation in single rooms from Monday to Friday
• bus transfer to Waischenfeld from Nuremberg via Erlangen on Monday
• bus transfer from Waischenfeld via Erlangen to Nuremberg on Friday
• excursion to the castle „Rabenstein“ and conference dinner on Wednesday

Contributions

Each participant is invited to present a poster. There are very limited spots for contributed talks available. We thus reserve the right to convert submitted talks to posters.

Scientific Organizing Committee

Neal Dalal (Perimeter Institute for Theoretical Physics)
Stefan Funk (ECAP, Friedrich-Alexander-Universität Erlangen-Nürnberg)
William Guerin (CNRS / Université de la Côte d’Azur)
Robin Kaiser (CNRS / Université de la Côte d’Azur)
Joachim von Zanthier (QOQI, Friedrich-Alexander-Universität Erlangen-Nürnberg)

Monday 13 October

16:30 → 18:00 Registration

18:00 → 20:00 BBQ Get-Together

Tuesday 14 October

09:00 → 10:30 History of Stellar Intensity Interferometry

Convener: Prof. Stefan Funk (Friedrich-Alexander-Universität Erlangen-Nürnberg - ECAP)

09:10 Intensity correlations: imaging and quantum optics in astrophysics

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 effort by our group towards resolving a white dwarf.

On the quantum optics aspects I will discuss possible applications of intensity correlations in astrophysics, beyond ultrahigh angular resolution.

Speakers: Robin Kaiser (CNRS), Robin Kaiser (CNRS / Université de la Côte d’Azur)

Waischenfeld_Kaiser.pdf

09:50 Recent Advancements in Modern Stellar Intensity Interferometry

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 baselines, massive (100TB+) high-speed disk array, and affordable digital correlators using FPGAs, multithreaded CPA, and GPU. The compactification of the size and power requirements of these technologies has enabled SII deployment on arrays of large Imaging Atmospheric Cherenkov Telescopes (IACTs), such as VERITAS, MAGIC, HESS, and CTAO-LST. This combination has created a new ultra-high angular resolution capability in astronomy at a fraction of the cost of a traditional Michelson interferometer.

The era of modern SII was heralded by the first demonstrated measurements of angular diameters of round stars in 2019, using the VERITAS Array of IACTs. It has rapidly progressed to the measurement of elongated stellar envelopes of fast-rotating stars and a recent capability to resolve aspects of binary stellar systems. In this talk, I will review the rapid emergence of the field of modern SII, including seminal discoveries and emerging techniques. I will review the historical and recent progress made by multiple international observatories and describe emerging science cases as SII technology develops. In closing, I will describe some new results from the VERITAS Northern Sky SII Survey, illustrating the rapidly emerging scientific capabilities of the field of SII.

Speaker: DAVID KIEDA (Department of Physics and Astronomy, University of Utah)

WaischenfeldWorkshopOct2025dbkV2.pdf

10:30 → 11:00 Coffee Break

11:00 → 12:30 Science goals

Convener: Andreas Zmija (ECAP)

11:00 Probing stellar atmospheres and binary stars with SII

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 simultaneous measurements of the B-type giant $\beta$ Cru at
375 nm and 470 nm with HESS SII yield the same uniform disk diameter
in the two bands, within the uncertainties. The 375 nm band however
coincides with the confluence of the hydrogen Balmer series towards
the Balmer limit, the filter capturing lines H8 to H13, while the
470 nm band lacks any strong spectral lines. To explore single stars
for which significantly different angular sizes are expected in the
two bands, I have modeled the extended atmospheres of the four
brightest A-type supergiants with angular diameters under 1 mas. I
will present our visibility predictions for the HESS bands and
sub-bands within the VERITAS 416 nm band, with and without the H$\delta$
line. I will also discuss the potential for SII measurements to turn
spectroscopic binaries into visual binaries in order to constrain the
masses of the constituent stars.

Speaker: Jason Aufdenberg (Embry-Riddle Aeronautical University)

Probing stellar atmospheres and binary stars with SII_V7.pdf

11:35 When Stars Evolve Together: Detection of Tidal Signatures in Close Binaries

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 to their dynamic interactions and complex evolutionary pathways. Tidal forces between the components of such systems play a significant role in shaping their physical evolution, influencing stellar rotation and mass transfer. In this presentation, I will focus on the tidal deformation simulation in massive close binary systems and explore how such stretching impacts the square visibility function on the observational plane. Estimation of these tidal interactions not only informs binary stellar evolution models but also sheds light on the internal dynamics and distorted surface geometries of such stars, contributing to a deeper understanding of stellar astrophysics.

Speaker: Km Nitu Rai (ARIES Nainital, India)

German.pdf

11:50 SII science cases: an xkcd-inspired view

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 interestingness and difficulty. Within the matrix we can also identify four kinds of topics that present different strategic challenges. First there are the "smooth and stationary" systems, such as stellar radii or gravity darkening, which can be modelled using a few parameters. Second, there are "smooth and periodic" systems, including not only binaries but also stellar oscillations and maybe even exoplanet transits. Third, there are "irregular but stationary" targets, like giant convective cells or (maybe one day) quasars, which require image reconstruction. Fourth and last but certainly not least, are "transients". A few new simulations will be presented: one will illustrate the usefulness of Zernike polynomials as a basis set for "smooth and stationary" systems; another is a preliminary simulation of a novel signature in quasar microlensing.

Speaker: Prasenjit Saha (University of Zurich)

pres.key

12:30 → 14:00 Lunch Break

14:00 → 15:30 Science goals

Convener: Alison Mitchell

14:00 Future Astrophysical Targets for Intensity Interferometry

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, 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 characteristics of recently 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. Another possible application is 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. A third is imaging of nearby extrasolar Jupiter-mass planets.

Speaker: Prof. Norman Murray (CITA, University of Toronto)

Interferometry SII October 2025a.pdf

14:35 Probing H0 and resolving AGN disks with ultrafast photon counters

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 signal to noise ratio not only to detect distant AGN, but also to study them in great detail. First, I will show that intensity interferometric observations of bright nearby AGN can allow detailed studies of the central accretion disks powering the AGN, allowing reconstruction of many disk properties like the radial profile. Next, I will argue that intensity interferometers can spatially resolve the broad-line regions of AGN at cosmological distances, and thereby provide a geometric determination of the angular diameter distances to those AGN when combined with reverberation mapping. Since this measurement can be performed for AGN at distances of hundreds of megaparsecs, this directly measures the Hubble expansion rate H0, with a precision adequate to resolve the recent Hubble tension.

Speaker: Marios Galanis (Stanford University)

II2025.key

II2025.pdf

14:50 Charting the High-Resolution Frontier: Science with Stellar Intensity Interferometry

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 precise orbit and mass determinations, while circumstellar disks and winds may be studied to advance our understanding of stellar evolution and mass loss. Extending to extragalactic targets, SII holds promise for constraining the environments of supernova progenitors and the compact regions of active galactic nuclei. In the following talk, I will review various science cases that illustrate this potential.

Speaker: Sagi Ben Ami (Weizmann Institute of Science)

SII with spectroscopic telescope arrays- Motivation and Science Cases.pdf

15:25 Discussion starter: ESO white paper for SII

Speaker: Alison Mitchell

ESO_whitepaper_discussion_SII.pdf

15:30 → 16:00 Coffee Break

16:00 → 18:00 SII with IACTs

Convener: Joachim von Zanthier (QOQI, Friedrich-Alexander-Universität Erlangen-Nürnberg)

16:00 State-of-the-art space-based quantum communication

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Speaker: Christoph Marquardt (Max Planck Institute for the Science of Light)

16:35 Analysis techniques for pushing the limits of stellar intensity interferometry

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 measured using SII since then. Measuring Spica's features with modern SII is an important proof of principle for the future measurements of complex systems with future observatories such as CTA. In this talk, I will describe details of the VERITAS SII observations and analysis that led to the first measurement of an oblate photosphere as well as the status and analysis techniques being developed to measure binary systems.

Speaker: John Scott (The Ohio State University)

SII on Complex Objects.pdf

16:50 Modeling preliminary VERITAS-SII measurements on Spica

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, importantly, uncertainties) of a time-evolving target, an issue that will become increasingly relevant for future observatories.

Speaker: Prof. Michael Lisa (Ohio State University)

Lisa-SII2025.pdf

Lisa-SII2025.pptx

17:25 Enabling Intensity Interferometry with the CTAO

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 Roque de Los Muchachos in the Canary Islands (CTAO-North). The dozens of CTAO telescopes, their kilometer-long baselines, and their fast photodetection capabilities make the Observatory an exceptional optical instrument. In particular, fluctuations in the intensity of optical sources can be measured to perform interferometric measurements between pairs of telescopes through the Hanbury Brown and Twiss effect.

The Observatory's scientific applications in interferometric mode include generalizing cases opened up by its precursors (HESS, MAGIC, and VERITAS), such as measurements of the radii of massive stars, limb darkening, and the flattening of fast rotators. New and ambitious applications include observing stellar flares and spots and multiband imaging of exoplanet transits. Additionally, the CTAO interferometer could opportunistically characterize the geometry of high-energy photon emitters, such as nova flares in the Milky Way and supernovae in the Local Group.

The successful implementation of intensity interferometry on the CTAO precursors has shown that only modest adjustments are needed to fulfill the key requirements. The first set of minimal modifications to the arrays entails adding dedicated optical filters and signal samplers to the cameras, a high-bandwidth fiber optic network, additional White Rabbit nodes for synchronization, and the necessary infrastructure for data computation and storage. These adjustments would enable the CTAO to extend its observation time to include full moon periods, during which Cherenkov measurements are difficult. To achieve the most ambitious scientific objectives, a second phase would involve installing new interferometry-dedicated sensors in the cameras to increase the bandwidth and the number of channels per camera through spectral multiplexing.

This contribution presents the plan to enable intensity interferometry with the CTAO. Implementing interferometry on the CTAO offers enormous scientific possibilities, including those involving other optical telescopes at the Paranal and Roque sites.

Speaker: Jonathan Biteau (Université Paris-Saclay, CNRS/IN2P3, IJCLab)

Enabling II with the CTAO - Waischenfeld-1.pdf

Wednesday 15 October

09:00 → 10:30 SII with IACTs

Convener: Pedro Ivo Silva Batista (Erlangen Centre for Astroparticle Physics)

09:00 Optical intensity interferometry observations with the MAGIC and CTAO-North LST telescopes

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 13 massive stars, were published in a performance paper in early 2024. In parallel, a similar upgrade as the one implemented for MAGIC were applied to the first Large-Sized Telescope (LST-1) of the Cherenkov Telescope Array Observatory northern hemisphere array (CTAO-North), a 23-m diameter telescope located
next to MAGIC. First SII observations using the MAGIC+LST-1 array have been successful, and are now routinely performed in 2025. Three more LSTs will be completed by 2026. This presentation will provide a summary of the MAGIC+LST1 system, current and future correlator capabilities, show prospects for its performance and briefly discuss about some of the science topics the collaboration is currently pursuing.

Speaker: Tarek Hassan Collado (CIEMAT)

Optical II observations with the MAGIC and CTAO-N LSTs-1.pdf

09:35 The Butterfly camera: expanding sensitivity and angular coverage of IACTs as stellar intensity interferometers

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 the MAGIC telescopes at La Palma together with an upgrade of the photodetection technology results in various optical configurations that can serve to overcome these design restrictions. We present three such configurations and evaluate their impact on photon arrival time distributions, filter performance, achievable angular resolution and sensitivity. Simulations show that these setups can improve sensitivity by a factor of 6, as well as significantly expanding the angular scales accessible to these telescopes. Such an upgrade enable new scientific capabilities such as milliarcsecond-scale imaging of fast-rotating stars, measuring their circumstellar disk and oblateness, and earliest stages of nova ejecta. These developments provide a practical upgrade path for existing IACT arrays, open the possibility of combining the Cherenkov telescopes with optical ones and offer a scalable framework for future facilities such as the Cherenkov Telescope Array Observatory (CTAO).

Speaker: Fernando Frías (Instituto de Astrofísica de Canarias (IAC))

Butterfly_Slides_SII_Workshop_def.pdf

09:50 Sub-nanosecond, very narrow band intensity interferometry on the ASTRI Mini-Array

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 accuracy. To improve on current Cherenkov implementations, we are realizing a new instrument, the Stellar Intensity Interferometry Instrument (SI3), to be installed on the ASTRI Mini-Array, an array of nine IACTs under construction in Tenerife, Spain. The assembly of the first instrument has been recently completed. ASTRI SI3 will lead to the first implementation of very narrow band, multi-channel intensity interferometry in photon counting on Cherenkov telescopes with a sub-nanosecond sampling time. The basic concept of SI3 is injecting light into a multimode optical fiber bundle at the telescope focal plane, working on the output beam to efficiently insert very narrow band (~1 nm) filters, and finally focussing light into modern single photon Silicon detectors fully exploiting their ~100 ps resolution. We demonstrated for the first time the feasibility of this concept with a prototype version of SI3 fully tested in the laboratory, which was capable of delivering a highly significant zero baseline narrow band measurement of the correlation from two Silicon Photon Multipliers (SiPM) detectors fed by multimode optical fibers. This result paves the way to significantly improving the boundaries and capabilities of stellar intensity interferometry with Cherenkov telescope arrays.

Speaker: Luca Zampieri (INAF-Astronomical Observatory of Padova)

ASTRI_SI3_first-instrument+lab-demonstration_20251012_red-published.pdf

10:30 → 11:00 Coffee Break

11:00 → 13:00 Detectors for SII: QUASAR

Convener: Prof. Stefan Funk (Friedrich-Alexander-Universität Erlangen-Nürnberg - ECAP)

11:00 QUASAR project status: Nano to pico degree optical resolution to resolve accretion disks

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 reach, in the optical, angular resolutions better than achieved in the radio by the Event Horizon Telescope and to obtain the first images of accretion disks around galactic compact objects, active galactic nuclei and quasars.

Speaker: Roland Walter

2025 SII Waischenfeld.pdf

11:35 A 528𝗑16 CMOS SPAD array with sub-10 ps timing for high-resolution astrophysical observations

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 with 128 Vernier TDCs. The design enables continuous photon time-stamping at 10 MHz acquisition rate, while ensuring that data can be transmitted off-chip with high reliability. Post-layout simulations confirm that the system achieves sub-10 ps timing resolution, with minimal jitter, while delivering an aggregate throughput of 48 Gb/s over 44 LVDS channels. These results demonstrate that our design provides a scalable platform for next generation astronomical instrumentation, enabling real-time photon correlation measurements at previously inaccessible angular scales.

Speaker: Ivan Cardea (EPFL)

SII_Workshop_IVAN_CARDEA.pdf

12:10 QUASAR: Multi-telescope intensity interferometry observations: picosecond-scale effects of atmosphere and time synchronisation on observed signal

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 independent detectors is required. This distributed system introduces timing jitter, both from clock distribution itself and from atmospheric effects that broaden the Hanbury Brown–Twiss (HBT) correlation peak. In this talk, I will present laboratory results on picosecond-level clock synchronization achieved with the White Rabbit system, as well as observational limits on atmosphere-induced HBT peak broadening derived from Sun observations for zero-baseline II setup.

Speaker: Vitalii Sliusar (University of Geneva, Department of Astronomy)

sliusar_ii_workshop_quasar_prel.pdf

12:25 GPU based timestamps correlation with parallax correction

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 CPU-based approaches.

Speaker: Etienne Lyard (University of Geneva)

Calern_2025_3d.mov

QUASAR_Correlator_Waischenfeld.pdf

12:40 Recent SPAD developments

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 last few years. This verification phase shows the great potential of single-photon detectors and motivates the next phases of the project.

Speaker: Gilles Koziol (University of Geneva)

Waichenfeld_2025_10_15.pdf

13:00 → 14:30 Lunch Break

14:30 → 18:45 Social Event: Excursion to Castle Rabenstein & Conference Dinner

18:30 Perspectives and Opportunities for Intensity Interferometry - A Personal View

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 reflect that the first and only purpose-built intensity interferometer was the one at Narrabri, conceived and built by Robert Hanbury Brown. In the half century that has passed since it was mothballed, a second such interferometer has not been built. Despite its undoubted success in observing stars with higher precision than before and also in overturning the understanding of photon physics, intensity interferometry sadly became a one hit wonder. Amplitude interferometry became the flavour of the month.

But now, despite heroic efforts, it is becoming clear that technological challenges are limiting what can be achieved with amplitude interferometry. It became clear some years ago that an opportunity for a revival of intensity interferometry was opening up. Perhaps surprisingly it has been the Cherenkov Telescope Array CTA project that has brought together a diverse group of astronomers wanting to explore the opportunities for SII. This resulted in the creation of the Intensity Interferometry Science Working Group under the umbrella of the CTA itself. Groups with access to IACT arrays (Imaging Atmospheric Cherenkov Telescopes) began to adapt these telescopes to carry out intensity interferometry studies. In parallel CTA managers were lobbied in a bid to include intensity interferometry as a science goal of the project. After 10 years there has been progress but it has still not happened. Whilst efforts must continue in this direction, the lesson surely is that if we are considering the longer time scale we must explore other avenues. By that I mean we must look beyond by a time equal to that of looking back to Narrabri. That would mean that we must look forward to 2075.

Therefore, if I stand back and look at the impressive progress that has been made with Intensity Interferometry up to now and attempt to divine where future progress can be made, I see a number of steps. One is to exploit the opportunity that is provided by becoming an additional scientifically productive and distinct science goal of CTA. The ultimate step in my field of view however would be to construct a second purpose-built intensity interferometer, one which builds upon the experience gained at Narrabri, VERITAS, MAGIC, HESS, ASTRI and of course CTA itself. An observatory built on a green field site (brown desert!) where the SII community itself will choose the parameters and determine its operational protocol. I will explore what the advantages of such a facility might be and what barriers need to be broached to realise it. My own vision, which is put forward to be knocked down and reimagined - a straw man - would be a 2000 telescope array with a footprint of 20 kilometers. I have been pursuing this idea for some years in a rather desultory fashion – it even has a name, SIITAR - but the current call for proposals from ESO can be a catalyst for a distinct group of intensity interferometry enthusiasts to campaign for such a facility in the longer future.

Speaker: Dr Colin Carlile (Division of Astrophysics, Lund University)

Thursday 16 October

09:00 → 10:30 Detectors for SII

Convener: Robin Kaiser (CNRS / Université de la Côte d’Azur)

09:00 Superconducting nanowire detector review and prospects for SII

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 several systems with specification requirements that overlap with those of stellar intensity interferometry, including NASA’s deep space optical communication demonstration. I will then outline future prospects for focused detector development.

Speaker: Boris Korzh (University of Geneva)

20251016_Korzh_SuperconductingNanowireProspectsForSII_final.pdf

09:35 Quantum-assisted Interferometers for astrometry

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 single-photon amplitude (Michelson) interferometry and two-photon (Hanbury Brown & Twiss) intensity interferometry, and then visit recent ideas for how they can be improved in the optical through the use of entanglement distribution. A proposed new technique of two-photon amplitude interferometry requires precise spectral binning and 10 picosecond scale time-stamping of single optical photons, and could improve the astrometric precision by orders of magnitude. I will illustrate the concepts with recent results and will discuss future directions for the technology.

Speaker: Andrei Nomerotski (Czech Technical University in Prague)

NomerotskiSII20251016.pdf

10:10 Broadband HBT in spectrometer

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 measurement. As a result, this allows for a factor of 10 improvement in astrometric uncertainty.

Speaker: Sergei Kulkov

Sergei_Kulkov_16.10.2025.pdf

10:30 → 11:00 Coffee Break

11:00 → 13:00 Detectors for SII and SII with optical telescopes

Convener: Christopher Ingenhütt (Erlangen Centre for Astroparticle Physics (ECAP), Friedrich-Alexander-Universität Erlangen-Nürnberg)

11:00 Advancements in Stellar Intensity Interferometry with PhotonPix Detectors

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 system developed by our group together with Photonscore and Photonis Inc.. The PhotonPix detector features a large detection area and is capable of handling high count rates (100 MHz) while maintaining high timing accuracy. With the two setups we measured spatial photon correlations at the Calern Observatory in Nice, France, by use of the two C2PU telescopes (15-meter baseline) providing evidence for a limb darkened model of the star Vega. We will discuss the implications of our findings and potential future applications of the PhotonPix technology in astronomical observations.

Speaker: Verena Leopold

Waischenfeld.pdf

11:35 Developing a NIR Heterodyne Interferometer at MPP

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 setup consists of entirely off-the-shelf components at 1.55 micron, a commercial SDR receiver, and a custom GNURadio data processing chain with minimal dead-time. We provide a description of the setup together with preliminary results and future plans.

Speaker: Derek Strom (Max Planck Institute for Physics)

SII-Workshop2025-STROM.pdf

11:50 Large, high performance superconducting microwire detectors for astronomy

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 challenges. While SNSPD arrays have advanced considerably, their fabrication and readout complexity remain major obstacles. In this talk, I will present our recent progress on superconducting micro-wire detectors, highlighting their potential as high-performance, single-pixel alternatives for demanding optical experiments.

Speaker: Iman Esmaeil Zadeh (TU Delft)

12:05 Intensity interferometry at Calern and beyond: towards wavelength multiplexing and the resolution of Sirius B

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 single-photon avalanche photodiodes and 1m-class telescopes at Calern Observatory. We have then dedicated some effort to demonstrate the simplicity and portability of our instrument by adapting and using it with success on different telescopes worldwide, including a 1-m portable telescope at Calern, the Auxiliary Telescopes at ESO-Paranal Observatory, and the 4-m SOAR telescope. Besides these technical demonstrations, we have also performed a few measurements of astrophysical interest, in particular on the H$\alpha$ emission line of P Cygni.
Then I will present our main current project, which is to develop a second-generation instrument working with 16 wavelength channels around 420 nm using large-area single-photon detectors. Our main science goal is the resolution of Sirius B, the brightest white dwarf ($m_B = 8.4$), using two large telescopes separated by more than 600 m in Hawaii. I will present some preliminary designs and technical tests.

Speaker: William Guerin (Université Côte d'Azur, CNRS, Institut de Physique de Nice)

WGuerin_I2C_project_2025.pptx

13:00 → 14:30 Lunch Break

14:30 → 16:30 Arrays of small telescopes and Advanced Methods

Convener: William Guerin (CNRS / Université de la Côte d’Azur)

14:30 The Mobile Intensity Interferometer for Stellar Observations---Intensity Interferometry with Fresnel Lens Telescopes

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 Observations was designed as a proof of concept for a purpose-built intensity interferometer.
Using acrylic $1\,$m diameter $1.2\,$m focal length Fresnel lenses, a compact, cheap and lightweight design was realized. The detector fixture allows for translation in the z-axis to adjust for measurements at different wavelengths (and therefore focal points) as well as easy swapping of the detector in its entirety.
Both mobility and scalability in quantity of this design allow specific targeting of projected baselines and orientations based on the target. Particularly for potential binary systems, selective coverage of a target's u-v plane is essential to probing the characteristics accurately.
A first campaign demonstrated the capability of these Fresnel lens telescopes by measuring the spatial coherence curve of Arcturus ($\alpha$ Boo). In observation time of under $11\,$h, the angular diameter was measured to milliarcsecond precision in agreement with literature values.

Speaker: Christopher Ingenhütt (Erlangen Centre for Astroparticle Physics (ECAP), Friedrich-Alexander-Universität Erlangen-Nürnberg)

MIISO-II with FLTs.pdf

15:10 Big Performance from Small Telescopes: Multiplexed SII with the MAST Array

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 spectrograph. For SII, we plan to operate a subset of the telescopes, each coupled to an independent R ~ 3,000 spectrograph and equipped with Pi Imaging’s SPAD-lambda: an 80-channel linear SPAD array. This setup enables simultaneous correlation of photon arrival times across ~80 narrow spectral channels, providing an expected √80 ≈ 9 improvement in the SNR of the second-order correlation function, g(2). Leveraging modern detectors and spectral multiplexing, we anticipate a factor of ~3.5 gain in SNR relative to the original Narrabri Interferometer, for the same observing time but with only 1/100 of the collecting area. With an initial configuration of four telescopes, MAST can simultaneously measure six unique baselines, constructing a full visibility curve six times faster than a single telescope pair. Our goal is to demonstrate that SII can be carried out with small telescopes while achieving the SNR performance of much larger apertures. Looking ahead, MAST’s modular design may also allow deployment on portable piers, enabling flexible (u,v) coverage and paving the way toward image reconstruction—an approach not easily achievable with the large, fixed reflectors commonly used for SII.

Speaker: Oren Ironi (Weizmann Institute of Science)

SII workshop.pptx

15:50 Third-order correlation measurements with H.E.S.S. - an experimental study towards closure phases in intensity interferometry

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 parts of the phase information that is inaccessible in intensity interferometry otherwise.

A major caveat is that the three-photon signal is usually significantly smaller than the two-photon signals, and necessary observation times for sensitive closure phase measurements seem yet unreasonable with the current generation of intensity interferometers. For future telescope arrays, however, the closure phase is thought to improve the image reconstruction quality.

The H.E.S.S. Intensity Interferometer performed simultaneous measurements with three telescopes in 2023. Even though the measurements are lacking sensitivity for extracting closure phase information, they are suitable for getting familiar with the concept of three-photon correlations and closure phase measurements in intensity interferometry.

In this talk we present the results of three-photon correlation measurements of two stellar systems - to our knowledge the first astrophysical three-photon correlation measurements. We introduce an analysis algorithm on how to in principle extract closure phase information from the data, and show a proof-of-concept laboratory measurement using pseudo-thermal light with large coherence times, where the cosine of the closure phase is conveniently detectable.

Speaker: Andreas Zmija (ECAP)

waischenfeld_g3.pdf

16:30 → 17:00 Coffee Break

17:00 → 19:00 Poster Session / Discussion

17:00 Intensity Interferometry with a 4-Pixel SNSPD System

Speaker: Francesco Colangelo

17:00 Spatial photon correlations using nearly dead time free ultra-high throughput single photon detection

Speaker: Iurii Datii

17:30 Custom Shack-Hartmann Sensor for Stellar Intensity Interferometry

Speaker: Aleena Nedunilath Thomas

17:30 Preliminary study of a multiplexing design for an intensity interferometry instrument at optical wavelength

Speaker: Ilian Ellafi

Poster Erlangen 2025.pdf

18:00 An experiment of intensity interferometry utilizing Fizeau interferometric telescope

Speaker: Xinyang Chen

18:00 The Large Fiber Array Spectroscopy Telescope as a Platform for Intensity Interferometry

Speaker: Chad Bender

LFAST-60-51-20251013_SIIPoster_Bender.pdf

18:30 Improved Signal to Noise Ratio Observing Sirius Using Lower Jitter Time Tagging, Fiber Optics, and Delay cables

Speaker: Tom Mozdzen

Intensity Interferometry Poster Mozdzen 2025 1.pptx

18:30 Intensity interferometer with multi channel SNSPD

Speaker: Congcong Zhang

Friday 17 October

09:00 → 10:15 Advanced methods

Convener: Joachim von Zanthier (QOQI, Friedrich-Alexander-Universität Erlangen-Nürnberg)

09:00 Quantum telescopes and quantum imaging

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 entanglement to improve telescopes. I will give a brief overview of some relevant experiments that have shown an improvement in resolution and/or noise by adopting quantum technologies, and explore some of the open questions in the field, particularly how multi-photon processes can gain improvements in resolution.

Speaker: Pieter Kok (University of Sheffield)

Quantum imaging intensity interferometry.pdf

09:40 Intensity Interferometry for Cosmology and Fundamental Physics

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 ground-based differential astrometry at microarcsecond-level precision on sources of high surface brightness, in a field of view as large as several arcseconds. The scientific applications of EPIC include measuring the astrometric lensing noise induced by the structure of dark matter on sub-parsec length scales (corresponding to halos with sub-stellar masses), an observable that is exquisitely sensitive to the microphysics of dark matter.

Speaker: Ken Van Tilburg (NYU)

20251017 - Waischenfeld - Intensity Interferometry for Cosmology and Fundamental Physics.pdf

10:15 → 10:45 Coffee Break

10:45 → 11:30 Advanced methods: Contributed Talks

Convener: Naomi Vogel

10:45 Quantum Astrometry at Brookhaven National Laboratory

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 two widely separated sources, an astrometric measurement. While this scheme recovers phase information, technically making it amplitude interferometry, it can be thought of as a generalization of the HBT measurement of a binary system to wide angles. The scheme is similar to intensity interferometry in both the challenges it faces — coupling to single-mode fibers, high time and spectral resolution, sensitivity with dim sources — and in the advantages it provides over traditional Michelson interferometry — optically-disconnected telescopes, scalability to many baselines, spectroscopic multiplexing.

After giving an overview of the SNSV scheme, I will discuss an on-the-ground simulation of a single and binary star HBT measurement with small telescopes (⌀102 mm), using multimode fibers (⌀50 μm) as simulated stars. Our source of light was a red LED filtered by an ultranarrow bandpass filter (0.1 nm FWHM). Starting with one bare fiber end emitting light 70 m from the telescopes, we determined the diameter of the fiber core by tracking the HBT visibility as the telescope baseline was varied from 20 to 80 cm. We then measured the HBT visibility of two fibers separated by 125 μm as their angular orientation relative to the baseline was varied. The visibility vs angle curve was observed to have a local minimum between two local maxima, a feature characteristic of observing an unresolved target comprised of multiple sources.

Speaker: Aaron Mueninghoff (Stony Brook University)

HBT_in_your_garage_Mueninghoff.pdf

11:00 Imaging with Intensity Interferometry

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 by example how to interpret limited 2-point correlations for a number of different source types and how limited uv coverage limits image interpretation.

Speaker: Albert Stebbins (Fermi National Accelerator Laboratory)

20251017_IntensityInterferometryImaging_ Waischenfeld.key

20251017_IntensityInterferometryImaging_ Waischenfeld.pdf

11:15 Inferring Line-of-Sight Velocity Distributions via Intensity Interferometry Correlation Functions

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 door to extracting new types of information, particularly to the internal kinematics of systems. In this work we have investigated exactly this possibility. We developed numerical models to simulate the Doppler shifts of emission and absorption lines of a decretion disk and a binary system respectively and evaluated their impact on the temporal intensity correlation function. Our simulations reveal a sensitivity of the correlation function to the internal dynamics of both systems, particularly the asymmetric shift caused by the asymmetrically red and blue shifted features. These results suggest that, with sufficiently high time resolution, intensity interferometry can be used to extract qualitative information about the internal motion within stellar system, offering a complementary perspective.

Speaker: Lucijana Stanic

251017_3DCorrWaischenfeld.pdf

11:30 → 12:00 Summary

12:00 → 13:15 Farewell Lunch