Science

Twisting puts the brakes on light in a vacuum

Artist's illustration of two twisted beams on the left traveling slower than the Gaussian beam on the right
Artist's illustration of two twisted beams on the left traveling slower than the Gaussian beam on the right
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Artist's illustration of two twisted beams on the left traveling slower than the Gaussian beam on the right
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Artist's illustration of two twisted beams on the left traveling slower than the Gaussian beam on the right

The speed of light in a vacuum is a universal constant, but, according to scientists at the University of Ottawa, not that constant. A team of researchers led by assistant professor Ebrahim Karimi has discovered that twisted light traveling through a vacuum moves slower than the speed set by Einstein's theory of relativity, which has implications for quantum computing and communications.

One of the perennial challenges of telecommunications has been how to cram more information into any given transmission line. No sooner was telegraphy invented than engineers started working on multiplexing to carry more than one message at a time. Over the years, messages ranging from the trivial to the tremendous have been crammed into coaxial cables, microwave conduits, fiber optics, and along laser beams.

Twisted light is based on the Orbital Angular Momentum of light or OAM, which allows a beam of a particular color – or wavelength – to be twisted into a corkscrew shape to dramatically increase the amount of information that can be transmitted. This is possible because, instead of varying the number of photons emitted, the information is encoded by switching between light's two polarization states. Since each twist can encode a different value, and the number of twists are theoretically infinite, much more information can be transmitted using less light.

Unlike conventional laser signals, which are essentially on/off patterns, twisted light has a tremendous potential for quantum-based communication and computing, not only for its ability to carry immense amounts of data, but also for energy savings and more secure encryption. The trouble is that physicists are working on a level that requires such precision that an unexpected factor can have huge implications.

In this case, the Ottawa team discovered that twisted light travels slower than the speed of light, which is 299,792,458 meters per second (186,282.4 miles per second) in a vacuum. In the case of twisted light, it travels through a vacuum 0.1 percent slower than regular light.

According to the team, the phenomenon first became apparent when they compared Gaussian laser light and light with 10 twists.

"We realized that the two beams didn't arrive at the detector at the same time," says Karimi. "The twisted light was slower, which was surprising until we realized that the twists make the beam tilt slightly as it propagates. This tilt means that the twisted light beam doesn't take the straightest, and thus fastest, path between two points."

This time delay was so short that it only amounted to tenths of a femtosecond or quadrillionths of a second. In order to properly measure this, the team needed to use a method for studying ultra-short laser pulses called Frequency-Resolved Optical Gating (FROG).

The problem was that the usual way to time an event like a laser pulse is to compare it to something of shorter duration, like using a strobe light to photograph a popping balloon. However, laser pulses are about as short as something can get. Sidestepping a lot of mathematics, FROG works by splitting a laser pulse in two and passing one through an optical medium to form an "auto-spectrogram," which allows the pulse to essentially compare its speed against itself.

Using FROG, the team discovered that in comparison to Gaussian laser beams, the twisted beams were delayed by up to 23 femtoseconds. According to the team, the ability to slow down light by altering its structure implies that it may also be possible to speed it up by around one femtosecond faster than the speed of light in a vacuum. In the meantime, the measurement of slow twisted light has major implications for the development of quantum communications.

"Anyone who wants to use twisted light for quantum communication should be aware of this effect," says Karimi. "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."

The team's research was published in Optica.

Source: Optica

11 comments
Hassan Syed
It appears to me that light is actually not slower. Instead of taking the straight line, it is just taking a longer route, hence arriving little late.
Jacob Shepley
"it may also be possible to speed it up by around one femtosecond faster than the speed of light in a vacuum" WHAT?! how could you include this and not elaborate at all??? I say the quoted passage is complete rubbish because nothing can go faster than the speed of light in a vacuum. but it's such a large claim, I want more information
Roger Garrett
Once again, a science article gets it wrong. The limit to the speed of light is an UPPER limit. Nothing can go faster than "the speed of light in a vacuum". It has always been known that it can indeed go SLOWER. So this article claiming that there's something NEW about the speed of light is just dumb.
Aloisius
Lightspeed is NOT a constant! This was proven by Hubble only a few years after Einstein's erroneous assumption. It is a shame that this fact is ignored even by today's most famous scientists.
Paul Anthony
This might be trivial but given the information in the article the speed of the light is not slower but rather the distance traveled is greater. The speed of the twisted light is likely the same speed as the non-twisted light, the difference is that it did not travel the shorter direct path from A-B. "According to the team, the ability to slow down light by altering its structure implies that it may also be possible to speed it up by around one femtosecond faster than the speed of light in a vacuum." this claim is false, again using only the information in the article, the twisted light is traveling further but at the same speed as the non twisted light. Unless they figured out how to shorten the distance from A-B (and if they have then NASA is interested) the twisted light is not going to get from A-B faster than the non-twisted light. Or am I missing something here? I know Quantum theory is not my expertise and I may be thinking only in Newtonian physics.
PaulMulwitz
The implications of this discovery (that light speed is not a constant) go much further than communications theory. This could change the entire notion of the nature of the universe. Hubble used the red shift apparent in astronomical observations to prove the universe is expanding. The underlying theory is that the speed of light is a universal constant and therefore the only possible explanation for red shift is an expanding universe. I have always hated that logic since we have never been able to observe any of this stuff from anywhere other than our home on Earth to verify the theory. Now that it seems there are ways to slow down light in a vacuum it might be time to reexamine the notion that the universe is expanding. The existing conclusion is flawed by the realization that the speed of light isn't constant.
Jeff Layton
Your interpretation seems to be correct, Paul Anthony - at least based on the information provided within the article. I think I'll try looking more into the "tilting" that was described to get more information about it.
David A Galler
Could Einstein be wrong?
JimDelton
I may have the terms wrong but this is just showing the difference between the vector speed of the actual particles versus the gross speed of the "beam" composed of the particles. Non-spinning light has a vector of "straight ahead" and goes "straight ahead" at the speed of light. Spinning light has two vectors, the vector in the angular direction plus the vector in the "straight ahead" direction. The geometric sum of those two vectors should be the actual "speed of light" . Because the actual speed of light is a constant, the only way to have light "vectored" in two directions is for each of those vector speeds to be something less than the actual "speed of light" AS MEASURED in the direction of each vector. The combined vector speeds will be the actual expected speed of light as we think of it.
James Oss
Well duh. If the photon is corkscrewing then of course it is not going to cover a given distance compared to a photon traveling in a strait line. The velocity of both remains the same.