Scientists at the University of Bath in the UK have found a way to bind together two photons of different colours, paving the way for important advancements in quantum-electro­dynamics. In time, the team's findings are likely to impact developments in optical and quantum communi­cation, and precision measure­ments of frequency, time and distances.

The wave-particle duality, proposed by French physicist Louis de Broglie in 1924, is a powerful concept that describes how every particle or quantum entity can be described as either a particle or a wave. Many quasiparticles have been discovered that combine either two different types of matter particles, or light waves bound to a particle of matter. A list of exotic quasi­particles includes phonons, plasmons, magnons and polaritons. The team of physicists at Bath has now reported a way to create quasi­particles that bind together two differently coloured particles of light. They have named these forma­tions photon-photon polaritons.

The oppor­tunity to discover, and manipulate, photon-photons is possible thanks to the relatively new development of high-quality micro­resonators. For light, micro­resonators act as miniature racetracks, with photons zipping around the internal structure in loops. The signature left by photon-photons in the light exiting the micro­resonator can be linked to the Autler-Townes effect, a peculiar phenomenon in quantum theory that describes strong photon-atom inter­actions. To achieve this effect in microresonators, a laser is tuned to the specific resonance frequency where a photon is expected to be absorbed, yet no resonance absorption happens. Instead, the photon-photon inter­action makes up two new resonance frequencies away from the old one.

A signi­ficant feature that has emerged from the Bath research is that the micro­resonator provided a whole set of split resonances, where each photon-photon pair displayed its own momentum and energy, allowing the researchers to apply the quasi­particle concept and calculate mass. According to the researchers' predictions, photon-photons are 1,000+ times lighter than electrons. Dmitry Skryabin, the physicist who led the research, said: “We now have a situation where micro­resonators behave like giant atoms. The artificial atoms concept is rapidly gaining ground in the quantum-electro­dynamics of microwaves in super­conducting circuits, while here we are looking at the similar oppor­tunity in the optical range of frequencies. The small mass of photon-photons could lead to further developments of many important analogies between light and fluids, where other families of quasi­particles have already being used.”

PhD student Vlad Pankratov, who parti­cipated in the project, said: “After a year of running models and collecting data, these are incredibly exciting findings for us. The potential appli­cations of our results are in the terabit and quantum optical communi­cation schemes, and in the area of precision measure­ments.” (Source: U. Bath)

Reference: D. V. Skryabin et al.: Photon-photon polaritons in χ(2) microresonators, Phys. Rev. Res. 3, L012017 (2021); DOI: 10.1103/PhysRevResearch.3.L012017

Link: Centre for Photonics and Photonic Materials, Dept. of Physics, University of Bath, Bath, UK