Working with colleagues at the Harvard-MIT Center for Ultracold Atoms, a group led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic have managed to coax photons into binding together to form molecules – a state of matter that, until recently, had been purely theoretical. The work is described in a September 25 paper in Nature.
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.
Read more at: http://phys.org/news/2013-09-scientists-never-before-seen.html#jCp
What is the difference between the ＂molecule of two photons＂and the energy-time entangled correlated photon pair？
Thanks @Qiang Zhou for your interesting question.
What I understood that ; Researchers began by pumped rubidium atoms into a vacuum chamber, then used lasers to cool the cloud of atoms to just a few degrees above absolute zero. Using extremely weak laser pulses, they then fired single photons into the cloud of atoms.
As the photons enter the cloud of cold atoms, 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. 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.
The point I would like to mention is that "this work is a full new physical phenomenon or it an exsiting phonomenon but Professor Lukin and his colleagues enhanced the nonlinear action between photon and photon via this special expriment setups". In my oppion, I think it is the later situation. And also to me, this work is very good, this would be followed by other group for a broadband quantum information application.
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