Speed of light can be reduced to zero at ‘exceptional points’, say scientists

Light normally travels at nearly 300,000 kilometres per second through space, but scientists have proposed a new way to bring it to a complete standstill.

In 2001, researchers worked out how slow light down to a fraction of its original speed by trapping it in a cloud of ultra-cold sodium atoms.

Another way to achieve the same result involves slowing light down using materials called photonic crystals.

Now in the journal Physical Review Letters, scientists have outlined another theoretical method that makes use of a phenomenon termed “exceptional points”.

Exceptional points are when two different varieties or “modes” of light wave come together and combine into one mode, or “coalesce”.

When this happens, light stops in its tracks, but in most systems much of the light itself is also lost at these points.

In their new paper, Dr Tamar Goldzak and Dr Nimrod Moiseyev at the Technion – Israel Institute of Technology, and Dr Alexei Mailybaev of Brazil’s Institute for Pure and Applied Mathematics, proposed a way to stop light waves while preventing this loss using waveguides with something called “parity-time symmetry”.

A waveguide is a physical structure that, as its name suggests, is used to guide the movement of waves. An optical fibre is an example of a waveguide that is used to transmit telephone signals.

According to the scientists, waveguides could be used to adjust the two waves of light travelling through them, so that they balance each other out exactly.

This would mean the light intensity remains constant as it approaches the exceptional point and stops.

“We show that the group velocity vanishes, i.e., a light pulse is fully stopped if the waveguide is designed exactly at the exceptional point,” the researchers wrote in their paper.

They suggested that the parameters could be tuned to work at any frequency of light, and could also work with other types of waves beside light – such as sound.

In addition, though this work was entirely theoretical, it has the potential for practical, technological applications.

“This result opens conceptually new possibilities for designing slow light devices, which exploit generic properties of the exceptional point and, therefore, may offer much larger freedom for technical implementation and operational capability,” the researchers wrote.