The first "molecules" made from two photons have been created by physicists in the US. Their experiment involves firing pairs of photons through an ultracold atomic gas, where an attractive interaction causes the photons to stick together and become quantum-mechanically entangled. The breakthrough could allow both conventional and quantum computers to encode and process information using photons.
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.
In the new study, a team led by Mikhael Lukin at Harvard University and Vladan Vuletić at the Massachusetts Institute of Technology has 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 479 nm, 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.
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