Intensity interferometry at Calern – I2C

The project is divided in 4 workpackages (WPs) and 8 tasks:

WP1: Increase of the sensitivity
Task 1a: Incremental improvements of the signal-to-noise ratio
Task 1b: Intensity interferometry with superconducting nanowire single-photon detectors (SNSPDs)
Task 1c: Towards multichannel I.I.

WP 2: Improving angular resolving power
Task 2a: Long- and multi-baseline I.I. synchronization with a physical link
Task 2b: Wireless intensity interferometry

WP 3: Application to stellar physics
Task 3a: Polarization-resolved I.I.
Task 3b: Emission lines of massive early type supergiants

WP 4: Intensity interferometry at large facilities
Task 4a: I.I. at the VLTI

Task 1a – Incremental improvements
- We have replaced the polarizing filter by a polarizing beam splitter (PBS) and we now exploit simultaneously the two polarization channels, which increases the signal-to-noise ratio (SNR) by a factor v2.
- We have implemented a tip-tilt stage, which provides a much better stability of the injection into the fiber, and thus a better average injection, with much less human intervention during the data acquisition. It has been a key element allowing us to perform interferometric measurement with a mobile (less stable) 1 m telescope.
- We have implemented a scheme allowing us to simultaneously measure the zero-baseline correlation and the spatial correlation, at the cost of using twice more detectors.
All these improvements will be detailed in two forthcoming publications (de Almeida et al., MNRAS 2022, and N. Matthews et al., Proc. SPIE 2022).

Task 1b – I.I. with SNSPDs
We have engaged in fruitful discussions with a group in TU Delft, which builds SPSPDs compatible with multimode fibers. We have visited their lab and performed useful experimental tests in Delft in August 2021.

Task 2a – Long and multi-baseline synchronization
We have already performed lab tests on long-distance transport (250 m) of the signal over coaxial cables with an electronical method overcoming the detrimental effects of distortion and attenuation of the signal in the cables. This method maintained precision of time-tag events at a few picoseconds, which is excellent.

Task 3a – Polarization-resolved I.I.
The polarization-resolving setup is built and validated (see Task 1a).
Task 3b – I.I. on emission lines
- We are publishing new results of two interferometric campaigns on the Halpha emission lines of the stars P Cygni and Rigel, and we use the visibility measurements, with the spectra and a complex modelling, to infer the distance to the stars. We obtain results in good agreement with the literature and with comparable uncertainties (E. de Almeida, MNRAS 2022).
- We also have performed a measuring campaign on the Halpha line of gamma Cas, and the prelim-inary analysis shows that the results are also consistent with the known properties of the star. The results have been obtained using the laser-ranging MéO telescope and a porta-ble telescope. They will be published in a dedicated paper.

Task 4a I.I. on the VLT
We had our first mission to ESO-Paranal Observatory in March 2022 for a technical run of 6 nights on two Auxiliary Telescopes (ATs, 1.8 m). The mission has been a success with interferometric measurements of the binary star Spica. The results will be presented in the SPIE proceedings 2022.

- Ongoing: Laboratory demonstration of the increase of the signal-to-noise ratio by wavelength multiplexing using a monochromator.
- July 2022: Test of a long distance signal transport method on telecom optical fiber.
- End of 2022 or beginning of 2023: Test on the sky of SNSPDs.
- 2023: Second mission at VLTI and 3 telescopes experiment.
- 2023/24: Polarization resolved measurements.

1. I3T: Intensity Interferometry Imaging Telescope, P.-M. Gori, F. Vakili, J.-P. Rivet, W. Guerin, M. Hugbart, A. Chiavassa, A. Vakili, R. Kaiser, G. Labeyrie, MNRAS 505, 2328 (2021), arXiv:2105.07072.

2. Combined spectroscopy and intensity interferometry to determine the distances of the blue supergiants P Cygni and Rigel, E. S. G. de Almeida, M. Hugbart, A. Domiciano de Souza, J.-P. Rivet, F. Vakili, A. Siciak, G. Labeyrie, O. Garde, N. Matthews, O. Lai, D. Vernet, R. Kaiser, W. Guerin, MNRAS, in press (2022), arXiv:2204.00372.

The I2C project aims at exploiting an interdisciplinary effort in the Nice area for developing advanced photon correlation measurement techniques in space and time for astrophysics.

The idea of using photon correlations between two separated telescopes, or ‘intensity interferometry’, has been pioneered by Hanbury Brown and Twiss in the 1950s - 1970s, who used it for measuring the angular diameter of 32 stars. However, this technique was later abandoned for its lack of sensitivity, in particular compared with direct interferometry, which consists in making the light from the separated telescopes directly interfere. This latter technique is now well mastered but is also extremely demanding, and only a few such interferometers operate in the world, with telescope separations (the so-called ‘baseline’, which gives the resolving power) up to 300 m, with little prospects to go beyond.

The project I2C proposes to revive intensity interferometry and demonstrate its new potential. Indeed, photonic devices made a lot of progress since the 1970s, such that intensity interferometry should be much more efficient now. More importantly, several technologies are now available that could make intensity interferometry enter a new era. For instance, long-distance synchronization between telescopes is now possible using atomic clocks, and modern computers allow recording all the light signal and compute the correlation between any pair of telescopes afterwards. These techniques would enable intensity interferometry with kilometer-scale arrays of telescopes, leading to unprecedented resolutions. Furthermore the lack of sensitivity of intensity interferometry may eventually be compensated by wavelength multiplexing.

Capitalizing upon its recent successful revival of temporal and spatial intensity interferometry with one-meter telescopes, the Nice consortium will apply these modern technologies within the present I2C project. Technological demonstrations will be performed using the telescopes located on the Calern plateau after extensive laboratory tests and calibrations. In particular, the demonstration of multi-baseline (with three telescopes) intensity interferometry along with long-distance synchronization between telescopes without any physical link will be a milestone, since the extrapolation to arrays of telescopes spread over kilometers would then be straightforward.

Intensity interferometry will also be applied to stellar physics by measuring the angular diameters of stars as a function of the light polarization and as a function of the wavelength, in particular using emission lines. This gives information on the atmosphere and/or circumstellar envelope of the star. Polarization-resolved measurements have been proved extremely difficult with direct interferometry due to the complexity of the optical setup, and emission lines at short wavelengths are not attainable with amplitude interferometry, two limitations which are absent with intensity interferometry.

This project will demonstrate the usefulness and feasibility of a new interferometric instrument that could be installed on an existing array of large telescopes, for instance the VLTI.