The Only Constant Thing...

In his Special Theory of Relativity, Albert Einstein postulated the cosmic speed limit: The speed of light. Nothing with mass can match or move faster than 299,792 km per second (186,282 m/per second). But, as we have seen, this is not always the case. Some things can, under specific situations, move faster than light.

To that end, we know that light sometimes slows down.

Now, a team of researchers from the University of Ottawa have discovered another example of light experiencing a cosmic slowdown.  They found that twisted light travels slower than the speed of light in a vacuum. The corkscrew-twisted light traveled 0.1 percent off Einstein's speed.

The team started to take notice of the difference when Gaussian laser lights and light with 10 twists did not arrive at their destination at the same time. Frequency-resolved optical gating (FROG) was used to measure the delay. Using FROG, the researchers discovered that the twisted light lagged behind the Gaussian beam by 23 femtoseconds. The delay is caused by a slight tilt brought by the twisting, which means the light does not take the fastest route to the detector.

A femtosecond is a quadrillionth of a second. Twenty-three of them may not seem like much, but the delay could mean twisted light might not be as effective in applications such as quantum computing.

Credit: E-karimi

Twisted Lights

Twisted lights show enormous potential in the field of quantum computers. As of now, they are used to encode information by either varying the number of photons emitted or switching between light’s two polarization states. Each spiral in twisted light can encode a different value or letter, giving it the ability to carry immense amounts of data. 

The thing with twisted light, and quantum computing in general, is that it's incredibly precise. Even a small disturbance, such as one caused by slow light, could garble the information being sent.

“Anyone who wants to use twisted light for quantum communication should be aware of this effect,” said Ebrahim Karimi, leader of the research team. “If they don’t compensate for the slow-light effect, information coded on twisted light might not arrive in the right order. Propagation speeds can significantly affect many protocols related to quantum communication.”


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