Quantum networkers, watch out: Twisting light slows it down

One technique for putting more data on networks creates another problem, researchers say

Slowing down the speed of light may sound exciting if you're a sci-fi author, but if you're an optical engineer, it might just make your workdays longer.

Scientists at the University of Ottawa say twisting light into a corkscrew shape slows it down. Doing this can cut 0.1 percent off the speed of light, a tiny tap of the brakes that nevertheless could create a traffic jam for future networks and even quantum computing.

Twisted light is emerging as a possible boost for optical networking, which is used in long-haul fiber networks and other types of connections. Today, the networks rely on different wavelengths, or colors of light, to distinguish one message from another. But as the amount of video, social media and other data on those networks grows, wavelengths alone may not be enough to pack more bits onto the fiber.

Quantum networking adds other ways of encoding data, like varying the number of photons that are emitted. It can also prevent certain kinds of network intrusions.

By putting a twist on beams of light and changing how they travel through space, engineers can create another variable for encoding information. Each kind of twist creates a different corkscrew pattern, and theoretically there's no limit to the number of patterns that can be created, said University of Ottawa Assistant Professor Ebrahim Karimi, who led the research team.

With that extra form of encoding, networks could carry as much as four times as much data as they do now, just using fiber already in the ground, Karimi said. More advanced fiber could allow for many more times the data.

Twisted light may also play a role in quantum computing, the fledgling field of number-crunching using more than just binary zeroes and ones.

But now there's a hurdle to overcome before it can be used reliably. Karimi and his team say they've discovered that twisting light slows it down and measured the effect.

In a vacuum, the researchers measured beams of light and found that ones they twisted reached their destinations slightly later than regular beams. Twisting a light beam more made it slower, and they achieved delays as long as 23 femtoseconds. Their results are coming out in a paper published Wednesday in Optica, the journal of the Optical Society of America.

A femtosecond is only one quadrillionth of a second, but the delay is long enough to cause problems, Karimi said. Bits speeding over a network have to arrive at exactly the right time if they're going to be reconstructed as meaningful data, unless the system know how to correct for the delay. 

That's not impossible, but it's an extra challenge that the Ottawa team has uncovered, Karimi said. It's now working on ways to solve the problem and expects to have something to show off next year.

It might seem obvious that light beams traveling a longer distance as they twist around an axis would take longer to arrive at a destination, but that effect hadn't shown up in published tests, he said. The delay is so small that it took Karimi's group a year to find a way to measure it.

If twisting light slows it down, that could be a problem in both networking and quantum computing, said Nino Walenta, principal research scientist for quantum technologies at the Battelle Memorial Institute, a research organization working in areas including optics. But if it's possible to change the speed of a light beam by twisting it, that might have useful applications, too, he said.

Still, there's some time. Optical networking with twisted light beams is still at the demonstration stage and not yet commercially practical, Walenta said.

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