Mikhail Lukin, professor of physics at Harvard University, and Vladan Vuletić, his colleague from Serbia and professor of physics at the Massachusetts Institute of Technology, have in a unique experiment managed to make quanta of electromagnetic radiation bind together and form an entirely new state of matter, resembling a molecule.
Their work is described in the journal Nature of 25 September 2013.
Until now the possibility that such matter exists had been studied only theoretically.
A theory of this kind, in the authors' view, contradicts the basic ideas about the nature of light.
It is generally accepted that photons are particles with no mass which do not interact with one another: laser beams directed towards each other pass right through one another.
The scientists managed to construct a medium of a special type. They showed that in such a medium the quanta interact so strongly that they behave as if they had mass.
Lukin draws a comparison with the lightsabers of space fantasy. When photons interact they repel one another and are deflected to the side. What happens with the molecules reminds Lukin of the Jedi fighting with lightsabers in the film “Star Wars”.
Mikhail Lukin and his colleagues (Ofer Firstenberg and Alexey Gorshkov from Harvard, Thibault Peyronel and Qi-Yu Liang from Massachusetts) created special conditions in the medium in order to make photons, which normally have no mass, interact. For this, rubidium atoms were placed in a vacuum chamber, and then the little cloud of atoms is cooled almost to absolute zero with a laser. And finally single ultraweak laser pulses were fired into the cloud that had formed.
The basic principle is that, as a photon passes through the cloud of cold atoms, its energy brings the atoms it meets into an excited state, which leads to a considerable slowing of the quantum's motion. The photon moves through the little cloud while its energy is passed to the neighbouring atoms, and at the end it leaves the cloud together with the quantum, keeping its identity.
A similar effect is observed when light is refracted in a glass of water. The light gives up a share of its energy to the medium and is inside it both as light and as matter. But on coming out of the water it keeps its identity. In the experiment with photons we observe this same principle: the light slows down considerably and passes more energy to the surrounding medium than it does in refraction.
In the experiment the scientists found that when two quanta are fired into the cloud they come out as a single molecule. This effect is called the Rydberg blockade.
When an atom is excited, the neighbouring atoms cannot be excited to the same level. This means that when two quanta enter the cloud the first excites the nearest atom, but before the second can excite the next one, the first photon has to move forward. In other words, because the energy of the photons passes to the neighbouring atoms, they push one another through the medium. The authors conclude: the atomic interaction determines the quantum interaction, and as a result this effect makes the photons behave in the medium like a single molecule and leave it as one photon.
So what is the practical application of this effect?
At present photons are the leading means of transporting quantum information.
Creating a quantum computer calls for a system able to store and analyse quantum information, and for that one has to understand how photons interact.
The scientists have managed to prove that a quantum switch and a quantum logic operator can be made. For that the efficiency of the process they discovered has to be raised, but that is only a matter of time. For the moment this discovery is a demonstration of the basic idea of photon interaction.
Mikhail Lukin gives the example that the system created may soon be used to make three-dimensional crystal-like structures out of photons.