For the first time, researchers have demonstrated light behaving like the quantum hall effect, a phenomenon previously observed only in electrons. Photons now drift sideways in quantized steps determined by fundamental constants. This breakthrough could enhance precision measurements and advance quantum photonic technologies.
The hall effect, discovered in the late 1800s, involves a voltage appearing sideways across a material when an electric current flows through it under a perpendicular magnetic field. This occurs as the magnetic field deflects negatively charged electrons to one side, creating a charge buildup and measurable voltage difference. Scientists have long used this effect to measure magnetic fields accurately and assess material doping levels.
In the 1980s, experiments with ultra-thin conductors at very low temperatures and strong magnetic fields revealed the quantum hall effect. Here, the sideways voltage forms distinct plateaus that rise in steps, independent of the material's details and set solely by the electron charge and Planck's constant. This discovery earned Nobel Prizes in Physics in 1985 for the quantum hall effect, 1998 for its fractional version, and 2016 for related topological phases of matter.
Replicating this with light has been challenging because photons, unlike electrons, carry no electric charge and do not respond directly to electric or magnetic fields. An international team, including researchers from Université de Montréal, has now succeeded in observing a quantized transverse drift of light. Their work appears in Physical Review X.
"Light drifts in a quantized manner, following universal steps analogous to those seen with electrons under strong magnetic fields," said Philippe St-Jean, a physics professor at Université de Montréal and co-author of the study.
The quantum hall effect underpins modern metrology, such as defining the kilogram using fundamental constants via electromechanical devices calibrated by electrical resistance standards from these plateaus. St-Jean noted, "Today, the kilogram is defined on the basis of fundamental constants using an electromechanical device that compares electric current to mass. For this current to be perfectly calibrated, we need a universal standard for electrical resistance. The quantum Hall plateaus give us exactly that."
This light-based version could provide optical references for measurements, potentially alongside or replacing electronic ones. It may also support quantum information processing and more robust photonic computers. Small deviations from quantization might enable sensitive sensors detecting environmental changes.
St-Jean added, "Observing a quantized drift of light is uniquely challenging, for photonic systems are inherently out of equilibrium. Unlike electrons, light demands precise control, manipulation and stabilization." The achievement points to new designs for photonic devices in information transmission and processing.