An exotic physical phenomenon involving synthetic magnetic fields, optical waves and time-reversal have recently been observed directly for the first time, following years of attempts. The researchers said their finding could potentially lead to realizations of topological phases, and in due course to the development of fault-tolerant quantum computers.
The new finding comprises the non-Abelian Aharonov-Bohm Effect as reported in the recent edition of Science journal by the team of Yi Yang, a MIT graduate student, Chao Peng ,a professor at Peking University and MIT visiting scholar, Di Zhu, a MIT graduate student, Professor Hrvoje Buljan from the University of Zagreb in Croatia, Francis Wright Davis, Professor of Physics, John Joannopoulos at MIT, Professor Bo Zhen at the University of Pennsylvania, and MIT professor of Physics Marin Soljacic.
The discovery relates to gauge fields, which define transformations undergone by particles. Gauge fields fall into two classes viz. Abelian and non-Abelian. The Aharonov-Bohm Effect, christened after the two theorists who predicted it in 1959, established that gauge fields is beyond being a pure mathematical aid and possesses physical consequences.
But the findings were only observed in Abelian systems in which gauge fields are commutative and occur the same way both forward and backward in time. In 1975, Tai-Tsun Wu and Chen-Ning Yang generalized the effect to the non-Abelian regime via a thought experiment. However, it continued to be unclear whether it would be possible to ever observe the effect in a non-Abelian system. Physicists did not have ways of creating the effect in the lab, and also lacked technology of detecting the effect even if it could be produced. Now, both of those conundrums have been solved, and the observations have been carried out successfully.
The exotic effect deals with one of the strange and counterintuitive aspects of modern physics which states that virtually all fundamental physical phenomena are time-invariant. Thus the details of the way particles and forces interact can run either forward or backward in time, and a motion picture of how the events unfold can be run in either direction, with no way to tell which is the real version. But some exceptional and exotic phenomena violate this time symmetry.
Soljacic says that generating the Abelian version of the Aharonov-Bohm effects requires breaking the time-reversal symmetry, a challenging task in itself. But achieving the non-Abelian version of the effect requires breaking this time-reversal multiple times, and in diverse ways, posing an even greater challenge.
The researchers use photon polarization to produce the effect. Then, they formed two different kinds of time-reversal breaking. They deployed fiber optics to produce two types of gauge fields that affected the geometric phases of the optical waves, first by transferring them through a crystal biased by powerful magnetic fields, and second by modulating them with time-varying electrical signals, both of these defy the time-reversal symmetry. Consequently, they were able to produce interference patterns that revealed the differences in the way light was affected when sent through the fiber-optic system in opposite directions, clockwise or counterclockwise. Without breaking the time-reversal invariance, the beams would have been indistinguishable, but in its place, their interference patterns exposed specific sets of differences as predicted, signifying the details of the elusive effect.
Yang says, “ The original, Abelian version of the Aharonov-Bohm effect has been observed with a series of experimental efforts, but the non-Abelian effect has not been observed until now. The finding allows us to do many things”.
He refers to a wide variety of potential experiments such as classical and quantum physical regimes to further explore variations of the effect.
Soljacic says, “ The experimental approach devised by this team “might inspire the realization of exotic topological phases in quantum simulations using photons, polaritons, quantum gases, and superconducting qubits. For photonics itself, this could be useful in a variety of optoelectronic applications. In addition, the non-Abelian gauge fields produced a non-Abelian Berry phase and combined with interactions, it may potentially one day serve as a platform for fault-tolerant topological quantum computation”.
Presently, the experiment is principally intended for fundamental physics research, with the objective of gaining a better understanding of some basic underpinnings of modern physical theory.
Soljacic says, “The many possible practical applications will require additional breakthroughs going forward”.
Firstly, for quantum computation, the experiment would need to be scaled up from one single device to possibly a whole lattice of them. And in place of the beams of laser light used in this experiment, it would require working with a source of single individual photons. According to Soljacic, even in its present form, the system can be used to explore questions in topological physics, a very active area of current research.