Let's start this blog with a story: one of my favorite writers of fiction is Gabriel Garcia Marquez, and one of my favorite short stories by him is Light is Like Water. Long after I read this story I began noticing a number of papers in the physics literature which investigated the flow of light as a liquid. Of course, light, being a wave, shows some of the behaviors we commonly associate with water, such as diffraction, interference and even the formation of vortices. But what I found interesting about these papers was that they were reporting the behavior of light as a quantum fluid. Quantum fluids, as opposed to classical fluids, can show superfluid behavior. For example, they can flow around an obstacle without being scattered, that is, without rippling.
Now we know that photons, which make up light, are massless. Also, photons pretty much do not interact with each other. In order for light to behave like a quantum fluid, photons have to somehow 'acquire' mass, and also begin interacting with each other. Interestingly, a number of experiments have been devised where both these steps have been implemented.
One way to endow photons with mass is to confine them between two highly reflecting mirrors. Roughly, speaking, because the photons bounce back and forth between the mirrors many times before they leak out, light takes longer to travel through the distance between the two mirrors than it would if they mirrors were absent. Thus, each photon seems to travel at a speed lower than the speed of light, which implies that it has acquired mass. (As I said, this is a rough argument; technically we say that 'the dispersion relation between the photon frequency and wavenumber is rendered particle-like by the optical cavity').
In addition, these photons can be made to interact with each other by including a material medium (between the two mirrors) which interacts strongly with the photons, leading to an effective interaction between the photons themselves. This medium could be made of atoms or molecules, for instance.
Using these two 'tricks', several phenomena, well known from other superfluid systems (such as liquid Helium and atomic Bose-Einstein Condensates), have been observed for light. These include flow of light without loss around a defect for low light speeds, and vortices and solitons. While these phenomena have been observed earlier in other systems, the fact that they can be seen using light implies new possibilities and applications for transporting light (and therefore information) without the usual scattering.
For those who have journal access, more details can be found in I. Carusotto and C. Ciuti, Reviews of Modern Physics 85, 299 (2013). A recent review is available here for free.
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