Photons, the fundamental constituents of light, are well known to be non-interacting particles in vacuum, and as such, they are free to pass through one another unimpeded. This distinguishing property of light is pivotal to the success of optical-communication technologies, allowing for long-distance transmission of information. Yet, there are many other emerging technologies that would instead benefit from the ability to make two photons interact with one, whereby one photon can strongly influence the propagation or state of another. This is nonlinear optics at its most extreme, at the level of individual light quanta.

One way to make photons interact with one another, at least artificially, is to convert them into socalled polariton excitations in materials that feature strong interactions between their microscopic constituents. Under these circumstances, photons can effectively borrow the interactions of the medium. Based on this philosophy, the two researchers present a new mechanism for engineering controlled photon-photon interactions. Specifically, a situation can be engineered where the conditional presence of one polariton can cause another to be converted between two different ‘flavors’ that have drastically different propagation characteristics. The two aarhus based scientists show how this polariton switching type of interaction can cause photons to effectively behave like mirrors, and bounce off one another like billiard balls.

The papter, titeled Coherent Photon Manipulation in Interacting Atomic Ensembles is available here.