It was hard to miss the disturbance in the force when scientists at Harvard and MIT declared they had turned light into molecules last week, prompting a collective air punch from the kind of folk who attend sci-fi conventions, for surely a working Jedi lightsaber was now close at hand.
The story began with Mikhail Lukin, a Harvard professor, who through the medium of the double negative drew the first comparison to the Jedi’s favoured weapon. “It’s not an in-apt analogy to compare this to lightsabers,” he said of his team’s work, which appeared in the latest issue of Nature. No more was needed to send picture desks scurrying for stills of duelling Jedis.
There are reasons why few research papers on quantum nonlinear optics are covered in newspapers and one is that the name of the field is the easy part. This is a world of Rydberg blockades and EIT resonances, of Raman detuning and spatially homogeneous phase shifts. Something is needed to cut through the technical details. What better than that elegant weapon, for a more civilised age?
And it worked in spades. The analogy of the Jedi’s favoured weapon only holds so far, and that’s not very far at all, but we can come back to that later.
So what did the scientists do? This was one of the most complex experiments it is possible to carry out in a university laboratory. The room where it took place had several tables in the middle. On one sat a vacuum chamber, a metre-wide vessel hooked up to a series of pumps to remove air from inside. Another table held the optical equipment, used to control laser beams and to detect what happened when the experiment was running. On a third table – the largest at around 4 metres long – sat a variety of lasers needed for different aspects of the work.
All in, the equipment weighed about a tonne. Could you clip it to you belt? “No, no, no, no,” Ofer Firstenberg, a Harvard postdoc and first author on the paper, assures me.
To start the experiment, the scientists pumped all the air from the vacuum chamber and injected a puff of rubidium atoms. They then used lasers to cool the atoms by dampening their vibrations. This gradually brought the rubidium cloud down to half a Kelvin – that is half a degree above absolute zero, a temperature colder than you will find in outer space. Once the atoms were cold enough, they were held in place, again with a laser, in a cigar-shaped cloud one tenth of a millimetre long.
“All of this nice physics takes place within the atomic cloud right at the centre of the vacuum chamber,” says Firstenberg.
Particles of light, or photons, are massless and usually oblivious to other particles of light. Shine two torches at each other and you will see the effect in action: their beams will pass straight through one another. But in Lukin and Firstenberg’s experiment, light did not behave that way.
When they fired photons into the vacuum chamber, something odd happened when the particles hit the cloud of rubidium atoms. When one photon goes in, it dumps energy into the first rubidium atom it meets, kicking one of its electrons up to a higher energy level. This high-energy electron acts like an antenna. “It’s so high that the atom becomes about a thousand times larger than a regular atom,” says Firstenberg.
The electron doesn’t keep the energy for long, but falls back down, releasing energy back to the photon, which carries on its way. The process repeats as the photon moves through the cloud, essentially jumping from one atom to the next.
“The photon moves very slowly,” says Firstenberg. “We call it ‘slow light’.” Instead of hurtling through the vacuum at 300 million metres per second, the light slows down in the cold cloud of atoms to around 100 or 1,000 metres per second.
The strange goings-on happen when more than one photon goes into the cloud. The first photon slams into a rubidium atom and creates an antenna as before. But the presence of the first antenna affects the second: it can’t take its turn to create an antenna until the first photon has moved on. The result is a couple of photons that cosy up into what the scientists call a photonic molecule.
“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,” Lukin said in a statement.
So is a lightsaber around the corner? The photonic molecules existed for a fleeting moment in a cloud of atoms that is a fraction of a millimetre long, colder than outer space, enclosed in a metal tank, and surrounded by tables bearing a tonne of equipment. “I don’t know what to say. The lightsaber is fictitious,” says Firstenberg. “We don’t know what the physics is behind a lightsaber. I don’t know how George Lucas did it.”