Anders Sørensen, Niels Bohr Institute

Anders Sørensen, Niels Bohr Institute - ITAMP Special Seminar
Thursday, October 29⋅1:00 – 2:30pm

Title: "Photon-photon interactions in waveguides and the physics of photon bound states"

Abstract: Photons rarely interact. In free space photons just pass through each other without any interaction. The situation is, however, dramatically different if for instance light is coupled to individual atoms. In this case the atoms can mediate an effective interaction between the photons, and this can be exploited to realise a strongly interacting photon fluid.   

I will consider the simplest possible example of strong photon-photon interactions: a waveguide coupled to a collection of two-level systems. Despite its simplicity I will show that this systems contains surprisingly rich and intersting physics. For instance I will show that in a relative simple setup with weakly coupled atoms, one can observe quantum corrections to Lambert-Beer’s law, which has been know for centuries, and these corrections may even have technological applications [1,2].

I will also discuss the physics of photon bound states, where photons bind together similar to atoms binding together in molecules. For so-called chiral systems, where the two-level systems can only decay in one direction, these bound states for any number of photons have been known for several years but their properties have not been fully explored. I will discuss the essential physics of these bound states, i.e., what their properties are, how they relate to classical solutions for large photon numbers and how one can unambiguously detect them [3].

Finally I will generalize the bound states to non-chiral interactions. Here a new type of solutions appear; not only can photons bind together, they also have scattering resonances where two photons bind together for a finite time.

[1] S. Mahmoodian, M. Cepulkovskis, S. Das, P. Lodahl, K. Hammerer, and A. S. Sørensen, Phys. Rev. Lett. 121, 143601 (2018).

[2] A. S. Prasad, J. Hinney, S. Mahmoodian, K. Hammerer, S. Rind, P. Schneeweiss, A. S. Sørensen, J. Volz and A. Rauschenbeutel, Nature Photonics (2020), https://doi.org/10.1038/s41566-020-06...

[3] S. Mahmoodian, G. Calajo, D. E. Chang, K. Hammerer, and A. S. Sørensen, Phys. Rev. X 10, 031011 (2020).