Photonic molecules are a synthetic form of matter in which photons bind together to form molecules. According to Mikhail Lukin, individual (massless) photons "interact with each other so strongly that they act as though they have mass". The effect is analogous to refraction. The light enters another medium, transferring part of its energy to the medium. Inside the medium, it exists as coupled light and matter, but it exits as light.
Gaseous rubidium atoms were pumped into a vacuum chamber. The cloud was cooled using lasers to just a few degrees above absolute zero. Using weak laser pulses, small numbers of photons were fired into the cloud of atoms.
As the photons entered the cloud, their energy excited atoms along their path, causing them to lose speed. Inside the cloud medium the photons dispersively coupled to strongly interacting atoms in highly excited Rydberg states. This caused the photons to behave as massive particles with strong mutual attraction (photon molecules). Eventually the photons exited the cloud together as normal photons (often entangled in pairs).
The effect is caused by a so-called Rydberg blockade, which in the presence of one excited atom, prevents nearby atoms from being excited to the same degree. In this case, as two photons enter the atomic cloud, the first excites an atom, but must move forward before the second can excite nearby atoms. In effect the two photons push and pull each other through the cloud as their energy is passed from one atom to the next, forcing them to interact. This photonic interaction is mediated by the electromagnetic interaction between photons and atoms.
The interaction of the photons suggests that the effect could be employed to build a system that can preserve quantum information, and process it using quantum logic operations.
The system could also be useful in classical computing, given the much-lower power required to manipulate photons than electrons.
It may be possible to arrange the photonic molecules in such a way within the medium that they form larger three-dimensional structures (similar to crystals).
The term photonic molecule was also used earlier for an unrelated phenomenon involving interacting microcavities.
"A micrometer-sized piece of semiconductor can trap photons inside it in such a way that they act like electrons in an atom. Now the 21 September PRL describes a way to link two of these “photonic atoms” together. The result of such a close relationship is a “photonic molecule,” whose optical modes bear a strong resemblance to the electronic states of a diatomic molecule like hydrogen."
"Photonic molecules, named by analogy with chemical molecules, are clusters of closely located electromagnetically interacting microcavities or “photonic atoms”."
"Optically coupled microcavities have emerged as photonic structures with promising properties for investigation of fundamental science as well as for applications."
- "Seeing light in a new light: Scientists create never-before-seen form of matter". Sciencedaily.com. Retrieved 2013-09-27.
- Firstenberg, O.; Peyronel, T.; Liang, Q. Y.; Gorshkov, A. V.; Lukin, M. D.; Vuletić, V. (2013). "Attractive photons in a quantum nonlinear medium". Nature. doi:10.1038/nature12512.