What is a Photonic Molecule?

By Isabelle Robinson, M.Sc.Jan 4 2019

A photonic molecule is a group of photons bound together in order to make a molecule made of light.

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Physicists in the United States have artificially bound a group of 3 photons in order to make the first photonic molecule. The molecule was created by firing a laser into an atomic gas. The research team behind this new discovery hopes that this technique could have applications in creating quantum entangled photons of quantum-information systems, a topic which has been much discussed in recent times.

This feat is significant due to the fact that under normal circumstances, photons do not interact with each other, therefore do not naturally create molecule structures. Scientists define the phenomenon as creating an interaction so strong that the photons act as if they have mass. The theory has drawn parallels to the fictional ‘lightsaber’ from the movie franchise, Star Wars.

This is not the first time that photonic molecules have been created in the United States. In 2013, Mikhail Lukin and Vladan Vuletic from Harvard University and Massachusetts Institute of Technology respectively, were able to create a photonic molecule consisting of two photons by firing a laser through extremely cold gas atoms.

Rydberg polariton is at the heart of the technique which is used to create these photonic particles. This is a collective state found in atomic gas in which a highly-excited electron is shared by several atoms. It works by travelling through the gas in a similar fashion to a slow-moving photon until it is converted back to light. The polariton prevents the nearby photons from creating there on excitations and therefore causes the photons to group together with the polariton, therefore creating the photonic molecule.

The new experiment showed an attractive interaction between the three photons, something which was confirmed with phase measurements. The interactions were much stronger than previous experiments. Vuletic and his team are currently looking into the creation of a repulsive interaction, which could be used to develop a crystalline lattice of light.

The hope is that in the future this technology could be used to create a system which is able to manipulate quantum information. In addition to this, it could optimize quantum processing and aid in the development of efficient quantum computers which are able to manipulate photons instead of electrons, thereby requiring much less power than conventional computer systems and also maximizing the speed of processing.

In a 2018 paper by Mian Zhang and team, the Chinese researchers discuss the possibility of electronically programming a photonic molecule. In their paper, they demonstrate a technique of controlling the artificial light molecule using external microwave excitations. The experiment shows that by doing so, they could create “on-demand photon storage and retrieval in optical microresonators by reconfiguring the photonic molecule into a bright-dark mode pair.”

The team believe that the ability to control these photonic molecules could lead to future applications in microwave photonic signal processing, optical computing concepts and simulations of complex systems using quantum technology. (Mian Zhang, 2018)

References:

1. Havard University. (2013, September 25). Seeing light in a new light: Scientists create never-before-seen form of matter. Retrieved from ScienceDaily.com: https://www.sciencedaily.com/releases/2013/09/130925132323.htm
2. Johnston, H. (2018, February 20). Three photons bind together to make a ‘molecule’ of light. Retrieved from physicsworld.com: https://physicsworld.com/a/three-photons-bind-together-to-make-a-molecule-of-light/
3. Mian Zhang, C. W.-A. (2018, September 23). Electronically Programmable Photonic Molecule. Retrieved from arxiv.org: https://arxiv.org/abs/1809.08638
4. Firstenberg, O.; Peyronel, T.; Liang, Q. Y.; Gorshkov, A. V.; Lukin, M. D.; Vuletić, V. (2013). "Attractive photons in a quantum nonlinear medium". Nature: 71–75.

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