February 23, 2026 -- In physics, the classical 'Hall effect,' discovered in the late 19th century, describes how a transverse voltage is generated when an electric current is exposed to a perpendicular magnetic field.

Simply put, the magnetic field causes the electrons, which are negatively charged, to drift sideways, creating a negative charge on one edge of the conducting strip and a positive charge on the opposite side.

For decades, this voltage difference has been used as a diagnostic tool to measure magnetic fields with precision and characterize material doping levels, that is, the addition of a tiny, controlled amount of impurity to a pure material to change how it conducts electricity.

In the 1980s, experiments at ultra-low temperatures with ultra-thin conductors—imagine a sheet of paper—revealed that under intense magnetic fields, this voltage difference increases not in a straight line but in perfectly defined steps.

These plateaus are universal, independent of the material’s composition, shape or microscopic defects. They rely solely on fundamental constants of the universe: the electron charge and the Planck constant.

This is known as the quantum Hall effect, a discovery so important it led to three Nobel Prizes in Physics: in 1985, for the discovery of the quantum Hall effect, in 1998 for the discovery of the fractional quantum Hall effect, and in 2016 for the discovery of topological phases of matter.

Until now, the quantum Hall effect had mainly been observed with electrons, whose electric charge makes them sensitive to electric and magnetic fields. Photons, the particles of light, are electrically neutral and thus immune to these forces.

Replicating the quantum Hall effect with light, therefore, posed a major challenge.

Accomplishing the impossible

Now an international research team has accomplished the hitherto impossible: they have observed a quantized transverse drift for light. Their discovery is reported in the journal Physical Review X.

“Light drifts in a quantized manner, following universal steps analogous to those seen with electrons under strong magnetic fields,” explained Philippe St-Jean, a physics professor at Université de Montréal who co-authored the study.

The implications of this discovery are vast, he added. For example, in metrology—the science of precision measurement—optical systems could become a universal gold standard, either complementing or replacing electronic systems, he believes.

The quantum Hall effect has already been applied in metrology.

“Today, the kilogram is defined on the basis of fundamental constants using an electromechanical device that compares electric current to mass," St-Jean explained. "For this current to be perfectly calibrated, we need a universal standard for electrical resistance.

" The quantum Hall plateaus give us exactly that. Thanks to them, every country in the world shares an identical definition of mass, without relying on physical artifacts.”

St-Jean believes that quantized control over light flow could further unlock new horizons not only in metrology but beyond, in areas such as quantum information processing, and could even pave the way for more resilient quantum photonic computers.

Conversely, a slight deviation from this perfect quantization can signal specific environmental disturbances, making it possible to develop extraordinarily precise sensors.

“Observing a quantized drit of light is uniquely challenging, for photonic systems are inherently out of equilibrium,” St-Jean noted. “Unlike electrons, light demands precise control, manipulation and stabilization.”

The experiment he and his colleagues devised rests on cutting-edge experimental engineering, and according to them it opens up new possibilities for designing and developing next-generation photonic devices for information transmission and processing.