It is a truism that first impressions really count. So you would think that a paper entitled "Superfluid motion of light" that had me running off to discuss a potential experimental demonstration with colleagues would have me raving. Unfortunately, on second, third, and now fourth reading, I find myself slightly at a loss. It seems very interesting, but what is it that we are actually interested in?

The paper, published in Physical Review Letters, plays on the idea that the mathematics of some very different physical phenomena are often very similar. This means that if you find a property in one system, then you can try and figure out what the analogous property would be in the other from the mathematics. In this case, scientists have compared superfluidity with light propagation to try and figure out what superfluid motion in light might look like.

Let's start with describing what a superfluid is. A superfluid is generally a liquid that flows without dissipation. That means that when it encounters an obstacle in its path, it will flow around the obstacle without slowing or experiencing turbulence—turbulence being a source of dissipation. The explanation for this behavior lies in the quantum nature of matter, but we can boil it down to one essential ingredient: sound waves. As the atoms move and encounter obstacles, their motions set up sound waves that propagate through the liquid. In a sense, these sound waves tell the atoms where the obstacles are, so the atoms don't simply collide with them.

This tells us that a superfluid can't flow faster than the speed of sound in said fluid. And, indeed, experiments bear this out, with superfluidity shown to appear and vanish as flow speed drops below and rises above the speed of sound. So what has this got to do with light?

The interesting thing about superfluids is that the mathematics used to describe their flow are very similar to those that describe the flow of light in particular types of media, specifically, those that are nonlinear—the brightness of the light modifies the optical properties of the media. If the mathematics are the same, then it should be possible to work backwards and figure out what superfluid light would look like and how that superfluidity could be observed.

This is pretty much what Patricio Leboeuf and Simon Moulieras of France have attempted to do. So, the first question would be what would constitute a superfluid light flow. The key property of superfluid light is that it shouldn't exhibit the equivalent of dissipation. In the case of light, dissipation can be thought of as scattering.

Imagine you have a fiber optic cable, where light pulses flow down the cable. Even when the cable is bent, light remains in the cable. However, if I put a small air bubble in the center of the cable, when a light pulse encounters that, some of the light will be reflected back down the cable, but still more will scatter out of the cable entirely. Although the total amount of light remains the same, the light propagating in the direction we intended is reduced. This is dissipation and, if it's a superfluid, light must be immune to it. In the superfluid case, light would flow around the air bubble and continue down the cable.

But wait, there's more. Superfluids have a critical flow speed as well. This is where things will get a little complicated later, so suffice it to say that such a speed does exist, but it isn't the speed of light in the medium.

Now that we've got these predicted behaviors, how do we make it happen? The idea presented by the authors is to create an array of closely spaced waveguides made from material that is nonlinear in its response to light.

When light is confined in a waveguide, part of it hangs outside the waveguide. If two waveguides are closely spaced, then the part of the light outside of the first waveguide will be inside the second waveguide. This allows light to tunnel from one waveguide to another. So, in this array of waveguides, light hops back and forth across the different waveguides as it propagates along. Furthermore, because the brightness of the light modifies the optical properties of the waveguides, the rate of hopping depends on how much light is hanging around in a particular waveguide.

The end result is that light zig-zags its way along the waveguide array with its forward speed reduced. There is now a transverse speed as well—how fast the most intense part of the light pulse jumps from waveguide to waveguide. This transverse speed is determined by how intense the light pulse is: brighter pulses hop faster.

Leboeuf and Moulieras show that if an obstacle is placed in the array, then, provided the transverse speed of the light pulse is less than about one-hundredth of its speed along the waveguide, the light pulse is not scattered and continues along the array. As the light intensity increases, the transverse speed increases and the zig-zag motion becomes disrupted as light flies off in all directions. Well, actually, since this is a 2D simulation, it can only fly in two directions, but you get the picture, I hope.

This paper has me quite excited, because it probably isn't that difficult an experiment to perform. We just have to decide if we want to do it. As for what it might be good for, or mean for light transport, I really have no idea yet. It just seems interesting.

Physical Review Letters, 2010, DOI: 10.1103/PhysRevLett.105.163904