Research shows the behavior of light in an effective magnetic field

The results of the study, with Brazilian participation, were published in the journal Nature Photonics.

By José Tadeu Arantes

Agência FAPESP – A recent experiment conducted at Cornell University in the United States has shown that photons, although they are electrically neutral, can behave like electrons in the presence of a magnetic field. The results of the study, in which Brazilian Paulo Alberto Nussenzveig participated, have just been reported in an article that appeared in the online issue of the journal Nature Photonics.

Nussenzveig is a full professor at the Physics Institute of the University of São Paulo (USP) and participated in the Cornell experiment with the support of a FAPESP research scholarship for study abroad.

One possible application of the discovery is the construction of optical isolators that would only allow the passage of light in one direction and not in the other. A device of this nature could be added to silicon chips, combining optical transmission with electronic processing, i.e., integrating communications and computer processing in a single environment.

“The group led by Michal Lipson, who conducted the experiment, is breaking new ground,” Nussenzveig told Agência FAPESP. The team, which also included Lawrence Tzuang, Kejie Fang and Shanhui Fan, performed an experiment similar to that proposed in the 1950s by physicists David Bohm (1917-1992) and Yakir Aharonov (1932-) but using photons in place of electrons.

One of the leading participants in the development of quantum physics and the author of two alternative theories to the dominant model proposed by the so-called Copenhagen School led by Niels Bohr, the American Bohm lived in Brazil and became a naturalized Brazilian citizen during the 1950s, when he was forced to leave the United States to escape political persecution brought about by McCarthyism. He later moved to Israel before finally settling down in England. It was in Israel where he studied the effect, later named Aharonov-Bohm, in partnership with his then advisee Aharonov.

“When an electron propagates itself in a region of space in which there is a magnetic field, the field causes the trajectory of the electron to be altered. This is classical physics. What Aharonov and Bohm did was to consider the phenomenon within the context of quantum physics and demonstrate that the altered trajectory could occur even if the magnetic field were zero,” Nussenzveig said.

“Such quantum effect occurs as long as the potential associated with the magnetic field, also called the ‘vector potential’, is not zero and presents different values at two distinct points,” he went on to say.

The explanation is that the energy that the potential confers upon the electron carries a phase change in the wave associated with the electron displacement. We need to remember that all material entities can be described as waves. The determination of the dual nature, at once corpuscular and undulatory, of entities from the atomic and subatomic worlds was exactly one of the points of departure for the approach and development of quantum theory.

“If the phase variations in the coming and going of the electron on a closed route were equal, it would return to the original point with the same phase it started with because the changes that occurred in the two passages would compensate for each other. However, what occurs in the case of the Aharonov-Bohm effect is that the trip out is different from the trip back, and this yields a non-zero result,” the researcher commented.

“What we did was replicate that experiment using light instead of electrons,” Nussenzveig said. “And we determined a non-reciprocal phase displacement, in other words, different in one way and in another, as if it were induced by a magnetic flux. The fact that the phase displacement is not reciprocal means that the phase plus the light is different depending on how the light is injected by one side or the other of the device.”

It is this non-reciprocal behavior that enables the building of optical isolators in which the light is completely transmitted in one direction and not transmitted in another.

“Our study started from a proposed Aharonov-Bohm photonic effect, not electronic, presented by the two co-authors of the study, Kejie Fang and Shanhui Fan, in the journal Physical Review Letters in 2012.”

“We implemented this proposal experimentally, using a process of optical interferometry known as Ramsey interferometry,” Nussenzveig said.

“That was how I made my main contribution to the study, which consisted of presenting the analogy of the experimental process to Ramsey interferometry. This analogy was very useful for adequately characterizing the experimental device and observing the effect,” he said.

Paulo Alberto Nussenzveig and Michal Lipson have, in a manner of speaking, physics in their DNA. Nussenzveig is the son of Herch Moysés Nussenzveig, a name widely known to modern physics in Brazil. Lipson is the daughter of another renowned physicist, Reuven Opher, full professor in the Astronomy Department of USP. Born in Israel, Michal studied in Brazil and earned her undergraduate degree in physics at USP, completing it at the Israel Institute of Technology (Technion) before settling in the United States, where she is currently a professor at Cornell.

The article, Non-reciprocal phase shift induced by an effective magnetic flux for light (doi:10.1038/nphoton.2014.177), by Lawrence D. Tzuang and colleagues can be read by subscribers to Nature Photonics at www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2014.177.html.