In modern physics, the understanding of collective phases is a particularly relevant aspect of complex and disordered systems, where many and different interactions are involved. One of the most paradigmatic and fascinating concepts in the physics of complexity is the breaking of replica symmetry: identical copies of the disordered system that manifest a completely different behavior. Theorized by Giorgio Parisi, the phenomenon of replica symmetry breaking defines the glassy phase of a disordered system. 

A team of researchers from Sapienza University of Rome, the Institute of Complex Systems of the National Research Council, and the Hebrew University of Jerusalem, coordinated by Eugenio Del Re and Claudio Conti, has observed for the first time the breaking of replica symmetry for optical waves nonlinearly propagating in a specially designed material.
The discovery is surprizing in that photons, the elementary particles that compose light, do not interact strongly with each other. For this reason light does not normally form collective phases such as a liquid or solid, let alone a glassy state.
“The emergence of a glassy state for light - Davide Pierangeli, one of researchers, explains - is made possible by the strong and disordered interaction that occurs between electromagnetic waves when travelling in particular media. To observe - Piergentili continues - the phenomenon we have investigated the propagation of laser beams in a very thin film of disordered and photorefractive ferroelectric material, where light rays strongly affect each other in a complex fashion We have found that analogous realizations of the system can give rise to completely different properties, even in the same experimental conditions”.

“The result - Claudio Conti, director of the ISC-CNR, continues - represents an important photonic validation of the theory of disordered systems. In fact, our study demonstrates the universality of the replica symmetry breaking phenomenon for classical waves”. “The discovery of a glassy phase for light - Eugenio Del Re of the Physics Department of Sapienza University of Rome concludes - opens new avenues for the experimental investigation of those complex phenomena that rarely can be realized in controlled, laboratory conditions". In fact, understanding the physics of disordered systems has fundamental implications in biology, neuroscience, in social dynamics, as well as in nanotechnology and new materials development.

References:

Observation of replica symmetry breaking in disordered nonlinear wave propagation - Davide Pierangeli, Andrea Tavani, Fabrizio Di Mei, Aharon J. Agranat, Claudio Conti & Eugenio Del Re - Nature Communications 8, Article number: 1501 (2017) doi:10.1038/s41467-017-01612-2

Further Information:

Eugenio Del Re
Department of Physics - Sapienza University of Rome
T (+39) 0649913501
Email eugenio.delre@uniroma1.it