Though they’re careful to say it’s not a light saber, physicists at Harvard and MIT have apparently created a new form of matter, in which photons react to each other as particles, rather than the non-interacting, massless things we’ve come to know and see.

The article, entitled “Attractive photons in a quantum nonlinear medium,” appears in the 25 September 2013 issue of Nature, bylined to Ofer Firstenberg, Thibault Peyronel, Qi-Yu Liang, Alexey V. Gorshkov, Mikhail D. Lukin, and Vladan Vuletić. Nature‘s rather dry synopsis of the article (hidden behind a paywall) is: “The fundamental properties of light derive from its constituent particles—massless quanta (photons) that do not interact with one another. However, it has long been known that the realization of coherent interactions between individual photons, akin to those associated with conventional massive particles, could enable a wide variety of novel scientific and engineering applications. Here we demonstrate a quantum nonlinear medium inside which individual photons travel as massive particles with strong mutual attraction, such that the propagation of photon pairs is dominated by a two-photon bound state. We achieve this through dispersive coupling of light to strongly interacting atoms in highly excited Rydberg states. We measure the dynamical evolution of the two-photon wavefunction using time-resolved quantum state tomography, and demonstrate a conditional phase shift exceeding one radian, resulting in polarization-entangled photon pairs. Particular applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light.”

Phys.org loosens up a bit for its explanation, writing in part that the scientists “have managed to coax photons into binding together to form molecules – a state of matter that, until recently, had been purely theoretical.

“The discovery, Lukin said, runs contrary to decades of accepted wisdom about the nature of light. Photons have long been described as massless particles which don’t interact with each other – shine two laser beams at each other, he said, and they simply pass through one another.

“‘Photonic molecules’, however, behave less like traditional lasers and more like something you might find in science fiction – the light saber.

“‘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,’ Lukin said. ‘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 light sabers,’ Lukin added. ‘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.’

“To get the normally-massless photons to bind to each other, [they] couldn’t rely on something like the Force – they instead turned to a set of more extreme conditions.

“As the photons enter the cloud of cold atoms, Lukin said, its energy excites atoms along its path, causing the photon to slow dramatically. As the photon moves through the cloud, that energy is handed off from atom to atom, and eventually exits the cloud with the photon.

“‘When the photon exits the medium, its identity is preserved,’ Lukin said. ‘It’s the same effect we see with refraction of light in a water glass. The light enters the water, it hands off part of its energy to the medium, and inside it exists as light and matter coupled together, but when it exits, it’s still light. The process that takes place is the same it’s just a bit more extreme – the light is slowed considerably, and a lot more energy is given away than during refraction.’

“When Lukin and colleagues fired two photons into the cloud, they were surprised to see them exit together, as a single molecule.”