Photonics researchers continue to explore the many 'strange' properties of light such as its particle-wave duality, its ability to be slowed and manipulated within photonic crystals or certain optical fibers, the relativity theories that rely upon the speed of light being a constant value, and the process of entangling photons for quantum communications.But a new phenomenon first theorized in 2007 is indeed very unusual: just like atoms group together into myriad molecular configurations, photons can also bunch together and create a 'photonic molecule' that just may end up having practical applications that can only be imagined. The first two-photon systems were demonstrated in 2007 by Stanford researcher Shanhui Fan and colleagues who demonstrated dual-photon transport in a waveguide material per the figure below (Image credit: Stanford University):
The MIT/Harvard team is also looking to create repulsive interactions between photons that could result in ordered "crystalline" photon molecules, a state of light that is indeed fascinating to ponder.
For a much more detailed explanation of two-photon molecules and potential applications, there is a very detailed blog at https://blogs.scientificamerican.com/guest-blog/and-let-there-be-e2809cmoleculese2809d-of-light/ that is worth careful review. To date, the three-photon molecule is the largest structure of its kind. But photonics has a way of advancing rapidly, with four- and five-photon systems seeming likely (unless one of our readers wants to explain whether or not there is a fundamental limit to how many photons you can bunch together in a 'molecular' structure).