Photons are caught behaving like superconducting electrons

Light is a fan of the buddy system. Photons, or particles of light, have been spotted swapping energy with partners. This chummy behavior resembles how electrons pair up in materials that conduct current without resistance, known as superconductors, researchers report in a paper accepted in Physical Review Letters.

Although the photons exchange energy like electrons do, it’s unknown whether the particles are actually bound together as electrons are, and whether photons could produce an effect analogous to superconductivity. “This is a door that is opened,” says study coauthor Ado Jorio, a physicist at the Universidade Federal de Minas Gerais in Brazil. Now, he says, the questions that must be addressed are, “How far can we push this similarity? Can we find with photons incredible results like we find for electrons?”

In certain solid materials cooled to extremely low temperatures, electrons form partnerships called Cooper pairs (SN: 6/13/15, p. 8), which allow superconductivity to occur. Although the negatively charged particles typically repel one another, two electrons can bind together by exchanging phonons, or quantum packets of vibration, via the lattice of ions within these materials. This alliance coordinates the electrons’ movements and thereby eases their passage through the material, allowing them to flow without resistance. Superconductivity’s potential technological applications — which include energy-efficient power transmission, superstrong magnets and levitating trains — have attracted heaps of scientific interest in the phenomenon.

Now, Jorio and colleagues have shown photons behaving similarly to superconducting electrons. When the researchers shined a laser on water, pairs of photons that emerged from the liquid at the same time tended to have complementary energies. While one photon had lost a little energy, another had gained the same amount of energy, indicating that they were exchanging quantum vibrations. The effect appeared in a variety of transparent materials, says Jorio, and it was observed at room temperature, unlike electron pairing in superconductors.

The team also showed that the exchanged quantum vibrations were “virtual” — appearing only for fleeting moments — just like the vibrations exchanged by electrons. The theory that explains the interaction “is exactly the same as for the electrons,” Jorio says.

Scientists already knew that photons can lose or gain energy via vibrations, but the similarity with Cooper pairs is a new and interesting way of thinking about the effect, says physicist Ian Walmsley of Oxford University, who was not involved with the research. “It’s a field that has not yet been explored.”

It is still too early to know how far the analogy with superconducting electrons extends, says physicist Ben Sussman of the National Research Council of Canada in Ottawa, who was not involved with the research. But the connection seems worth investigating: “This is an interesting rabbit hole indeed.”