California researchers discovered a way to leverage an unused property of light to apply the unrestricted nature of the quantum domain to wireless communication, creating a new type of channel with infinite capacity that could make looming data limitations irrelevant.

The quantum domain offers something that classical physics can't: the possibility of boundlessness. As such, it is becoming increasingly present in technological applications, particularly with the rise of quantum computing, as technology companies approach their data-manipulation limitations due to a finite number of existing algorithms and channels. In a study published Thursday in Nature Physics, researchers discovered a way of applying this domain to wireless communication by harnessing a quantum property of light called orbital angular momentum.

“The OAM is something else,” said Boubacar Kanté, the principal investigator of the study and a physicist at the University of California, Berkeley. “It's not the frequency, it's not the polarization; it is actually another degree of freedom of light. We just aren't using it right now.”

Orbital angular momentum is a property of light that, in practice, can provide a limitless number of simultaneous communication channels because it follows the laws of quantum mechanics, which permit infinite positions of matter at the same time. That's called superposition, and it's precisely what this property can do.

“In theory, there is absolutely no limit to the amount of information you can send using this technique,” said Kanté.

One way to think about superposition is to imagine flipping a coin: There are always two outcomes — heads or tails — or in this analogy, two communication channels. However, what would happen if the coin is spun on a table, and one tries to explain which channel is in use before it stops spinning? 

The answer is null, because both channels exist at the same time: The coin is in superposition. Orbital angular momentum can achieve this state of superposition with an unimaginable number of channels, removing storage restrictions for wireless communication.

Typically, to send a wireless signal between two phones involves generating an interconnection between the devices; the spoken message must go from point A to point B. This is presently done through the manipulation of radio frequencies, one of light’s more accessible properties. 

However, these frequencies can only be allocated one at a time, unlike a quantum superimposed state; otherwise, callers would cross wires. Such a restriction limits how many frequencies, or channels of communication, are available.

“If you want to listen to a particular station, you tune into the right frequency of the station,” Kanté explained. “If you want to listen to a different station, you actually have to change to another frequency. The problem is, there is almost no frequency left; the spectrum is saturated.”

To solve this issue, Kanté's team successfully reached a state of superposition for communication channels. They were the first to find a way to multiplex, or simultaneously combine, many iterations of a channel formed by orbital angular momentum. As a replacement for conventional frequencies, this new approach would provide an infinite amount of channels in superposition made available from a single signal. 

“Instead of 0 and 1, we have all of them at the same time,” Kanté said, in a nod to quantum computing. “When we are multiplexing, it's exactly superposition.”

To multiplex orbital angular momentum, the researchers designed a tiny antenna made up of concentric circles with lasers that call on the quantum property and form what looks like a vortex. The quicker the vortex of OAM lasers, the more channels in the state of superposition. Kanté says that the speed of the vortex, denoted by a quantum number, is up to the user, and any amount of channels can be drawn upon. 

Not only did the researchers manage to successfully create this platform for data communication, but they were also able to make it compact and ready-to-use. The laser-based device is essentially an antenna close to the size of a human hair, comparable to the lasers already present in iPhones that are used for the Face-ID feature.

“Nobody knew how to generate OAM elements on demand,” Kanté said. “And our platform is so compact that it's not difficult to insert them into everyday applications.”

Kanté further notes that the OAM-backed device can be developed in conventional ways because his lab used materials that are easily accessible by technology companies, such as a semiconductor. 

Progressing toward a world of quantum communication with orbital angular momentum doesn’t need to happen too far into the future, he added, even though more work still needs to be done to perfect it. He says the widespread implementation of quantum sources for wireless channels largely depends on whether technology companies decide to invest in the team's idea.

“It's a lot of work, but with the right people, it can happen in the next 10 years — at most,” Kanté concluded.

The paper, “Photonic quantum Hall effect and multiplexed light sources of large orbital angular momenta” was published Feb. 25 in Nature Physics. It was authored by Babak Bahari, Liyi Hsu, Si Hui Pan, Daryl Preece, Abdelkrim El Amili and Yeshaiahu Fainman, University of California, San Diego; Abdoulaye Ndao and Boubacar Kanté, University of California, Berkeley. The lead author was Boubacar Kanté.