When light attracts light

Brazilian researchers show that two photons can couple, exhibiting similar behavior to electron pairs in superconductors

Cassiano Rabelo Red laser passes through a container of water and produces bonded photon pairsCassiano Rabelo

A group of Brazilian physicists has discovered that at room temperature, some photons (particles of light) can exhibit a property typical of electrons in superconducting materials. When passing through transparent liquids, a small number of photons exchange vibrations with the molecules of the medium and join together in pairs. The process involves an exchange of energy between the photons: the energy lost by one light particle passes into the medium, which immediately passes it on to the other particle. The exact energy released by one photon is thus received by the other, creating a bond between the two particles. In superconductors, when two electrons interact in this way and unite, they form a Cooper pair, a quantum effect first described in 1956. A similar phenomenon had never been described for particles of light until a study by researchers from the Federal University of Minas Gerais (UFMG) and the Federal University of Rio de Janeiro (UFRJ) was published in scientific journal Physical Review Letters on November 9, 2017.

When superconducting materials are cooled to temperatures close to absolute zero (-273 °C), the presence of Cooper pairs nullifies their electrical resistance. “Based on our experiment, we are not looking at superconductivity of light,” says physicist Ado Jorio from UFMG. Jorio coordinated the experiments, which recorded the occurrence of photon pairs in eight different transparent liquids, including water. “We are simply saying that particles of light can produce the same phenomenon as these electron pairs.”

In the experiments, Jorio and his colleagues directed ultrafast and intense pulses of a red laser beam toward a transparent container containing a colorless liquid. Two detectors measured the output time and energy of each photon that passed through the system. Two particles of light leaving at exactly the same time, with one having gained energy and the other having lost it—as recorded by the study—indicates that the two photons are paired. The experiment was also successfully repeated using transparent solids that do not absorb light, such as diamond and quartz, in place of the liquid medium.

Although similar to the formation of Cooper pairs in superconductors, the phenomenon observed in particles of light was much less intense. Each second, about 10 quadrillion photons were emitted by the laser and hit the transparent liquid medium. Of this total, only 10 pairs were produced. “The formation of photon pairs observed so far suggests it is a very rare phenomenon,” says physicist Belita Koiller from UFRJ, who coauthored the study used as a theoretical basis for the experiments.

Attractive vibrations
About a year before the experiments began, Koiller’s research group used mathematical modeling to demonstrate that under certain conditions, particles of light could behave similarly to electron pairs. “We wanted to obtain experimental evidence of the phenomenon before we published anything,” Koiller explains. The mechanism that causes the particles of light to interact and couple together when passing through a transparent medium is similar to the formation of Cooper pairs in superconductors.

In free space, two electrons—which are negatively charged particles—repel each other, as do any two particles with equal charges. But two electrons inside a superconductor can be attracted to one another. A simplified explanation of this quantum phenomenon is that as it moves through a material, an electron causes the nuclei of the nearby atoms that compose the medium to vibrate and move towards the electron. If another electron passes close to where the atomic nuclei are oscillating, it is attracted to the vibration and thus drawn towards the first electron. This attraction causes the electrons to bond, forming Cooper pairs. Since light is not a superconductor, the formation of photon pairs caused by a laser beam passing through water was unexpected, even with the much lower intensity observed compared to electrons in superconductors. But according to the article by Jorio, Koiller, and their colleagues, two photons interacting with vibrating H20 molecules can also generate paired particles of light.

In superconductors, the absence of electrical resistance has been used in the development of various technologies, such as the magnets for mass spectrometers and magnetic resonance imaging (MRI) machines. It is too early to know if the existence of photon pairs in transparent media could be used in a practical application. “This is an amazing discovery. The consequences of this attraction between photons are not yet clear. The authors mention that the photon pairs are entangled, meaning that the properties of one depend on the other,” says physicist Luiz Nunes de Oliveira, from the São Carlos Institute of Physics at the University of São Paulo (IFSC-USP), who did not participate in the study. “This is interesting and will lead to many more studies. For now, it seems unlikely that there is any practical consequence of the discovery, but the research fulfills the greater goal of science, which is to help us better understand nature.”

Scientific article
SILVEIRA, A. et al. Photonic counterparts of cooper pairs. Physical Review Letters. Nov. 9, 2017.