Stopping Light, Photonic Molecules & Bose-Einstein Condensates
Have you ever wondered if it’s possible to build a real-life lightsaber? And if so, would it be possible to make it out of light (as opposed to using plasma instead)? I mean, after all, we want to make a lightsaber not a plasma-saber… right? Most people think that – if we are to ever build a lightsaber – we’ll most likely need to use super-hot high-energy plasma. Some say that it would be impossible to build a lightsaber made out of actual light, since we can’t make light stop at a specific length, and since a lightsaber made of light would be useless because light has no charge and no mass and hence it cannot interact with other lightsabers.Ok, perhaps that’s the case. But… what if it’s not? What if the limits of what is possible are only in our imagination? Let’s explore the science that is already out there… And why not speculate a little bit along the way?! Thinking about how to build a lightsaber is going to be the perfect excuse to learn about new exotic states of light and matter at the cutting edge of science and technology, so… without further ado, let’s dive in!
In my previous video, we used a laser beam to demonstrate how Heisenberg’s Uncertainty Principle explains the phenomenon of diffraction in the single-slit experiment. We also saw how the Uncertainty Principle explains certain properties of Bose-Einstein condensates. Lasers, quantum mechanics and Bose-Einstein condensates… It really got me thinking. Could the fascinating quantum mechanical properties of Bose-Einstein condensates help us get a step closer to building a real lightsaber? How much can we currently manipulate light, and what exotic states of light and matter with unexpected amazing new properties are awaiting on the horizon? Let’s find out!
Technical Problem Number 1: The Lightsaber’s Length. Can We Stop Light?
We know that light travels at a speed of approximately 300 hundred thousand km/s in the vacuum; this is a fundamental speed limit (nothing can travel faster than that). We also know that light is slowed down as it propagates through different materials. This manifests in very familiar physical effects such as the phenomenon of refraction, for instance. However, is there a limit to how slow light can travel? How much can we manipulate light’s speed and how much can we actually contain light within the confines of a particular length or a particular volume of space?
Here’s something very interesting. In 1999 Danish physicist Lene Hau and her team at Harvard University managed to slow down a beam of light to an unprecedented low speed by firing a laser through a Bose-Einstein condensate. A Bose-Einstein condensate is a state of matter in which the separate atoms of a gas which has been cooled down to near-absolute zero temperatures have become a single quantum entity, an entity which shows quantum mechanical properties on a macroscopic scale. Some scientists like to call this entity a super-atom.
Ok, so what Lene Hau did was fire light through a cigar-shaped Bose-Einstein condensate and she discovered that – while in that medium – light could be slowed down to 17 miles per hour (which is kind of the average speed of a bicycle) without any loss of information. That is, in the condensate, light travelled about 20 million times more slowly than in the vacuum. But wait, that’s not all; not only did she slow down light to an extremely low speed, but when Hau did the experiment again in 2001 she actually managed to completely stop light for a small fraction of a second. She brought light to a complete halt without any loss of information. That’s incredible!
Here’s some more research. in 2013 scientists at the University of Darmstadt in Germany stopped light entirely for a full minute, by trapping it inside an opaque crystal. And more recently, in 2016, researchers at the Australian National University in Canberra managed to replicate Lena Hau’s original light stopping experiment, also using Bose-Einstein condensates. They actually improved the original results by creating a self-correcting arrangement and they were also able to image the condensate side-on and show the light exchange in action.
Now, time for some speculation. Here’s something to think about… Could all this research prove useful one day so we can create a laser blade made of light which kind of stands still and hence doesn’t go on indefinitely but that is instead of a particular length… something like, say, 3 feet long? It certainly seems like a crazy idea right now, but… who knows what the future holds.
Going back to reality, one of the problems is that we are still using matter in order to manipulate light. As we have seen, the light photons need to be combined with matter, for instance a Bose-Einstein condensate – which is made of gas atoms – in order for us to be able to slow down and trap light within a particular area. Not to mention the fact that in order to create a Bose-Einstein condensate made of matter we need temperatures near absolute zero! Having said that, the current work by scientists like Lene Hau on transferring quantum bits of light into matter and then back to light using condensates, without any loss of information, has incredible potential applications in the field of quantum computation. The storage and retrieval of light information from matter is vital if we want to build light-based quantum computers, so this research is really great news!
Technical Problem number 2: Light beams that Repel Each Other. Can we Make Light Behave a bit Like Charged Matter?
Ok, so here’s another big issue we’ll definitely encounter when attempting to build a lightsaber made out of light. How can we possibly replicate the matter-like properties of a lightsaber, such as the fact that two lightsabers repel one another? We know that light is massless and that it has no charge, so in ordinary conditions two beams of light would pass straight through each other. Surely there’s not much fun in that! So the big question is: is it possible to make a light beam interact strongly with another light beam? How could we make this happen?
In 2013 scientists at the Centre of Ultracold Atoms – a joint Harvard University and MIT venture – used the Interaction between a Bose-Einstein condensate and light to create something quite remarkable: photonic molecules, a new state of quantum matter. At the time, the press was filled with sensationalist headlines (“Star Wars lightsabers finally invented”, “MIT and Harvard just made a real lightsaber” or “Lightsaber molecule created by scientists”). But hold on… you may wonder, did they actually create a lightsaber or anything like it? Well, no, of course they didn’t create a lightsaber… although, to be fair, they did make light behave in ways that are definitely reminiscent of certain properties that lightsabers have! Lightsabers aside though, the slowing down of light in a Bose-Einstein condensate combined with the creation of photonic molecules is a pretty cool achievement indeed!
So… what did the 2013 experiment involve? Well, it involved firing pairs of photons through a Bose-Einstein condensate. Yep, again, an experiment involving light and Bose-Einstein condensates. However, rather than a whole beam of light, we are talking about just two photons at a time here. As the photons were fired through the condensate, the researchers found that a new kind of interaction caused the photons to sort of stick together and become entangled, traveling together through the condensate in a bound state, interacting strongly with each other as if they were a molecule with mass, while strangely, at the same time, maintaining the properties of light. These photonic molecules were considered to be a completely new state of matter. They behaved less like traditional lasers and more like something you might find in a science fiction movie: yep, you guessed it… lightsabers!
Now, what’s even more fascinating… The photons’ strange bond appeared to be maintained even as they emerged out of the condensate! Here’s what Mikhail Lukin, the research team leader, had to say:
“Most of the properties of light we know about originate from the fact that photons are massless, and that they do not interact with each other. What we have done is create a special type of medium in which photons interact with each other so strongly that they begin to act as though they have mass, and they bind together to form molecules. This type of photonic bound state has been discussed theoretically for quite a while, but until now it hadn't been observed.”
“It's not an in-apt analogy to compare this to lightsabers; when these photons interact with each other, they're pushing against and deflect each other. The physics of what's happening in these molecules is similar to what we see in the movies.”
As Lukin pointed out, these photonic molecules jostle against each other, not unlike the way two lightsaber blades clash in Star Wars. And this is precisely what we want, isn’t it? Light that interacts with light! In this way, two lightsaber blades that were made of photonic molecules wouldn’t go through each other, but instead would repel each other as if they were made of matter carrying the same charge! Well, this is total speculation of course, but who knows what lies in the horizon? As Lukin suggests, this type of system might one day be used to create complex three-dimensional structures wholly out of light. "What it will be useful for we don't know yet, but it's a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules' properties," he said.
And that’s not all. In 2015 new research by scientists at the National Institute of Standards and Technology took the creation of photonic molecules one step further. By tweaking a few parameters, the new study shows that the photons can move in coordinated tandem, travelling side by side at a specific distance from each other. In Alexey Gorshkov words: "We're learning how to build complex states of light that, in turn, can be built into more complex objects. This is the first time anyone has shown how to bind two photons a finite distance apart."
Well, I find all this research fascinating. We are currently doing things with light that were unimaginable only a few decades ago. And I think that right now we can’t even imagine the complex objects we’ll be able to build with light in just a few years; I mean, we are dealing with entirely new states of matter and light and that’s really exciting! In the meantime, however, as I mentioned earlier, the most immediate technological applications of this type of research are quite likely going to be in the field of quantum computation. These are very exciting times indeed!
Technical Problem Number 3: a Working Handle. Do we Really Have to use Near-Absolute-Zero Temperatures?
Ok, so we now need to figure out how to re-create the extreme low-temperature conditions needed to make the condensates and hence the right environment to slow light and bind photons together, all within the confines of a tiny sword’s handle. Actually… not just within the confines of the handle, because the gas cloud would probably need to surround the actual blade area if we are to create a lightsaber of a fixed length… Hmmm… Big problem ahead because this means we would need some sort of rod to act as a container… But surely that would defy the whole point of making a real lightsaber! I mean, can you imagine a jedi using a lightsaber with a supporting rod? They’d be the laughing stock of the whole galaxy! There has to be another way…
Hold on… Here’s an idea. How about condensing photons directly, without needing to use a condensate made of matter? Has this been done before? Could we create a Bose-Einstein condensate made out of actual light? And what properties would this new exotic state of light have? I mean, if we want a feasible lightsaber, it sort of would help if we didn’t need to use a rod to contain the gas AND it would also be incredibly convenient if we could make everything happen at room temperature and pressure. Wouldn’t that be nice?
Well, turns out there is some light at the end of the tunnel. In 2010 a team of physicists in Germany led by Martin Weitz successfully created a Bose-Einstein condensate entirely made of photons, which was previously thought to be impossible, since photons have no mass. What’s more, this was done at room temperature. What? A photonic condensate? And at room temperature? Yep, they did it, and the condensate was aptly-called a super-photon, to describe the fact that the particles in a Bose-Einstein condensate – in this case the particles are photons – act as if they are one single entity. What’s more, the creation of a photonic Bose-Einstein condensate was successfully replicated in 2014 at Imperial College London.
In addition, over the past few years, PhD student Alex Kruchkov at the Swiss Federal Institute of Technology has developed a mathematical model for condensing light in a three-dimensional space under realistic conditions. Quoting from his webpage: “Alex’s theoretical model of photon condensation corresponded to experimental measurements and demonstrated how light energy can be accumulated in a Bose-Einstein condensate state. The new model offers a more complete theory of photon Bose-Einstein condensates which is a new and exciting area of physics. In addition, this phenomenon was observed at room-temperature, which makes it much more accessible to technological implementation than the ultra-low temperatures required for BEC of helium-4 (superfluidity) or laser-cooled atoms. Practically implemented, photon BEC could be applied to develop a new type of lasers.”
"In some sense, the BEC of light is a bridge between light and matter – a bridge that was unknown before” said physicist Sergiy Katrych, who was not involved in the study.
Think about it: A bridge between light and matter – that’s quite a poetic way to describe it, isn’t it? I like that….
As exciting as these new photonic condensates might be, turns out that two reflective surfaces – in other words, mirrors – are required to bounce the light back and forth between them in order to create the photon condensate… which of course is rather inconvenient for our lightsaber-building plan. We may have overcome the need to use a rod, only to go back to square one by having to stick a tiny mirror at the end of our lightsaber. Can you imagine a jedi using one of these? Yeah… I thought so; not that glamorous.
But… let’s look at the bright side. If we have learned anything from the past history of scientific discovery and technological progress is that, right now, we can’t even imagine what we’ll be able to create in a few decades… What seems like an obstacle at the moment is likely not to be an issue at all in the future, as we learn new ways in which these exotic states of light and matter can be manipulated. And again, lightsabers aside… Photonic BEC could be used to create a myriad of new exciting things, such as building sources of coherent light that don’t involve a conventional laser and new applications in quantum computing. The new quantum revolution is only at its infancy; what seems impossible now needs only a leap of our imagination in order to become a reality one day. I truly belief that’s all it takes. As Arthur C. Clark said “The only way to discover the limits of the possible is to venture beyond them into the impossible”.
Thanks for watching and see you very soon!
For further info:
Using BEC to Slow Down Light:
Physics for the 21st Century – Manipulating Light:
What is Slow Light?
Stopped light means go for quantum computers (eventually):
Scientists create never-before-seen form of matter (2013):
Physicists create 'molecules' of light (2013):
NIST Physicists Show 'Molecules' Made of Light May Be Possible (2015):
Physicists create new source of light: Bose-Einstein condensate 'super-photons' (2010):
Trapping photons: a model for containing light (Alex Kruchkov – 2014):
Bose-Einstein Condensation of Photons (Imperial College London):
New State of Light Revealed With Photon-Trapping Method: