Photons can “sense” each other and coordinate their separate paths through a complex material, new research from the Niels Bohr Institute shows. The scientists demonstrated that photons emitted from a light source embedded in a complex and disordered structure can use their wave properties to coordinate their travel through the medium. The illustration shows how the scattering of photons occurs in a complex photonic medium. Two photons are emitted from a light source in the center and move through a labyrinth to illustrate complex scattering. The photons take different paths through the medium, but they are interdependent in the sense that the chance of observing a photon at one outlet is increased if a photon is observed at the other outlet. “We work with nanophotonic structures in order to control the emission and propagation of photons,” said David García, a postdoctoral quantum photonics researcher at the institute, which is part of the University of Copenhagen. “We have discovered in the meantime that inevitable inaccuracies in the structures lead to random scattering. As a consequence, the transport of photons follow a random path – like a drunken man staggering through the city’s labyrinthine streets after an evening in the pub.” Continuing with the analogy, just because one drunken man gets home safely, it is not certain that a whole crowd of drunken people spreading out from the pub will also find their way safely through the city’s winding streets. There is no relationship between the different random travelers. There is, however, when you are talking about photons. To see how photons can sense each other and coordinate their travel through a material, the scientists inserted a small light source – in this case, a quantum dot – in a nanophotonic structure containing disorder in the form of a random collection of light-diffusing holes, García said. David García works in the quantum photonics laboratory at the Niels Bohr Institute. He experiments with using nanophotonic structures to control the emission and propagation of photons. The research shows that photons can “sense” each other and coordinate their way through a complex material. “The photons are scattered in all directions and are thrown back and forth,” he said. “But photons are not just light particles; they are also waves, and waves interact with each other. This creates a link between the photons, and we can now demonstrate in our experiments that the photons’ path through the material is not independent from the other photons.” Analyzing the photons’ path through the medium could provide valuable insight into microscopic complex structures. “The method could be a new way to measure the spatial properties of complex disordered materials, like biological tissue, and since the light sources are very small, you will be able to place them without destroying the material, and you have the potential for very high spatial resolution,” García said. The results were published in Physical Review Letters (doi: 10.1103/PhysRevLett.109.253902).