A collection of atom pairs inside an optical cavity formed by a pair of mirrors facing each other. The light trapped between the mirrors turns pairs of atoms into molecules in a coherent way. Credit: Ella Maru studio.

Quantum physics is gradually becoming the plinth in this era of technologies, but for that, we have to conquer the ability to make light interact with matter—or more technically, photons with atoms. In a first of its kind experiment, researchers have come up with a novel way for photons to interact with a pair of atoms. This is a crucial breakthrough in the field of cavity quantum electrodynamics (QED).

Quantum electrodynamics (QED) is already used in quantum networks and quantum information processing, even though their is still a long way to go. At present light-matter interactions are limited to individual atoms, which limits our ability to study them in the sort of complex systems involved in quantum-based technologies.

According to the paper by Hideki Konishi, Kevin Roux et al. ‘Universal pair-polaritons in a strongly interacting Fermi gas’ published in the journal Nature on 25 August 2021, physicists at EPFL were able to get photons to ‘mix’ with pairs of atoms at ultra-low temperatures.  Experts used Fermi gas, a state of matter made of atoms resembling electrons in materials.

As stated by Jean-Philippe Brantut at EPFL’s School of Basic Sciences “In the absence of photons, the gas can be prepared in a state where atoms interact very strongly with each other, forming loosely bound pairs. As light is sent onto the gas, some of these pairs can be turned into chemically bound molecules by absorbing with photons.”

“A key concept in this new effect is that that it happens ‘coherently,’ which means that photon can be absorbed to turn a pair of atoms into a molecule, then emitted back, then reabsorbed multiple times.”

“This implies the pair-photon system forms a new type of ‘particle’ – technically an excitation – which we call ‘pair-polariton.’ This is made possible in our system, where photons are confined in an ‘optical cavity’ – a closed box that forces them to interact strongly with the atoms.”

The hybrid pair-polaritons capture some properties of photons meanwhile, optical methods can capture them. They also take some of the properties of the Fermi gas, like the number of atom pairs it had initially before the incoming photons.

"Some of the very intricate properties of the gas are translated onto optical properties, which can be measured in a direct way, and even without perturbing the system," says Brantut.

"A future application would be in quantum chemistry, since we demonstrate that some chemical reactions can be coherently produced using single photons."