*Speaker: *Aditya Venkatramani (Harvard) *Date:* Thursday, February 7th *Time:* 12:00-1:00 pm Includes Pizza. *Title: *Repulsive photons via interaction engineering in a quantum nonlinear medium *Abstract: *Photons do not typically interact with each other, however, special optical media can be engineered to achieve large non-linearities at the single photon level. One way to achieve this is to couple photons to atomic states to from a light-matter hybrid called polariton. We choose one of the atomic states to be a Rydberg state, whose strong interactions manifest in the photons because they are both part of the polariton. Using this technique, we have previously shown attraction between single photons. In this talk I will describe how we control interactions to make it single photons repel each other. I will also show results of measured two- and three-photon correlation functions which characterize these interactions. The repulsion between three photons is stronger than between two photons and shows signatures of interference and spatial ordering. These observations of repulsion between single photons and the ability to control the nature of interactions open a route to creating quantum matter composed of light such as a crystal of photons. *Location: *B-106 @ Center for Astrophysics (60 Garden Street <https://maps.google.com/?q=60+Garden+Street&entry=gmail&source=g>) *Directions: *After entering the lobby of the CfA, turn right to enter the hallway of the B building. In the hallway, turn right again, B-106 will be at the end of the hallway on the left side.