Colóquio Field and intensity correlations: from atoms to stars - dia 27 de Fevereiro de 2024, às 16 horas

The knowledge of these coherence properties provides many information : about the light source itself, such as its angular intensity profile if one measures the spatial coherence, but also on the underlying light matter interaction processes when, for example, one measures the temporal coherence of light emitted or scattered by a medium. Thanks to the work of Glauber, it is well known that to have a full description of the temporal coherence properties, one has to know the correlation functions at allorders. In this presentation, I will focus on the first order correlation function, corresponding to electric field correlations, and second order correlation function or intensity correlations. In addition, for chaotic light sources, a relation exists between field and intensity temporal correlations, this relation being known as the Siegert relation [1,2]. I will show how this relation can be verified and used in two different domains: for light scattered by atoms that can be considered as quantum emitters, and in astronomy.

I will first discuss the Siegert relation for the light scattered by cold atoms. To validate the Siegert relation, one needs to verify different assumptions. One of this assumption is that the emitted or scattered phases should be random and uncorrelated. While the process of phase randomization is obvious in stars with thermal radiation, it is a bit more complex for light scattered by quantum scatterers which depends on the scattering regime. We will show how we can probe the transition from the classical regime, where the loss of coherence is due the atomic thermal motion [3], to the quantum regime dominated by spontaneous emission [4, 5].

Finally, I will discuss the Siegert relation in astronomy, for light coming from stars. We can show that this relation can be used to extract astrophysical information, such as the coherence time due to emission lines. Combined to spatial interferometry, this can be used
to determine the angular diameter of the source. We will show that it is especially interesting to perform such measurements on emission lines, allowing characterizing the extended atmosphere of the star, but also giving access to other fundamental parameters such as its distance [3], giving the opportunity to propose a new method for astrophysical distance calibration.

[1] D. Ferreira et al., "Connecting field and intensity correlations:
The Siegert relation and how to test it", AJP 88, 831 (2020)
[2] P. Lassègues et al., "Field and intensity correlations: the
Siegert relation from stars to quantum emitters", EPJD 76, 246 (2022)
[3] A. Eloy et al., "Diffusing-wave spectroscopy of cold atoms  in
ballistic motion", Phys. Rev. A 97, 013810 (2018)
[4] L. Ortiz et al., "Mollow triplet in cold atoms" New Journal of
Physics 21, 093019 (2019) [5] P. Lassègues et al., "From classical to
quantum loss of light coherence", arXiv:2210.01003 (2022)
[6] E.S.G. de Almeida et al., "Combined spectroscopy and intensity
interferometry to determine the distances of the blue supergiants P
Cygni and Rigel", MNRAS 515, 1 (2022)