Physicists Figure Out How to Make 'Molecules' of Light

 Getting photons to stick together is not easy because they normally pass through each other without interacting. However, a photon has an associated electromagnetic field that can modify its surrounding medium.

These changes can affect nearby photons and create an effective interaction between them. Although this effect is usually tiny, the interactions can be significant if the medium is chosen carefully.

A team at Harvard University and the Massachusetts Institute of Technology had created strong interactions between photons by sending them through a gas of rubidium atoms chilled to a temperature of just a few degrees above absolute zero.

The experiment involved using blue laser light with a carefully chosen wavelength of 479nm, which modifies the rubidium atoms so that a photon can share some of its energy with several atoms and create a collective “Rydberg state”.

This state is like a Rydberg atom – in which an electron is promoted to a very high-energy state but instead, the electron is shared among several atoms.

This Rydberg state propagates through the gas like a sluggish photon with a non-zero mass and when the collective state reaches the opposite edge of the gas cloud, the photon re-emerges at its original energy. 

When a Rydberg state forms, however, it becomes impossible for more Rydberg states to be created nearby, thanks to a process called the Rydberg blockade. So, when two photons are fired into the gas in quick succession, the first forms a Rydberg state but the second does not.

As far as the second photon is concerned, the region of the Rydberg state has a different index of refraction than the rest of the gas, which causes the second photon to stay close to the first as they travel together through the gas. The result is a bound state of two photons – or a molecule – traveling through the atomic gas.

Ref: PhysicsWorld; LiveScience