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Lights, Camera, Acrimony!

Physicists and engineers face off over how to make room for more data.

A crowd of thousands had gathered at St. Mark’s Square in Venice, drawn by the prospect of witnessing a telecommunications revolution.…By Mark Peplow

A crowd of thousands had gathered at St. Mark’s Square in Venice, drawn by the prospect of witnessing a telecommunications revolution. In the warm summer evening of June 24, 2011 they watched a video, projected onto the wall of the medieval Doge’s Palace, explaining a new technology that promised to multiply the volume of data carried by radio waves at a stroke.

Some 442 meters across the lagoon, a pair of unusual antennas had been mounted on the lighthouse on St George’s Island. At the flick of a switch, a signal leapt from the antennas, accompanied by a rifle shot—the same signal used by Guglielmo Marconi to confirm the first radio transmission in 1895. An instant later, the message reached its destination and was flashed onto the palace’s gothic facade. “Segnale Ricevuto,” it read. “Signal received.”

The crowd, already whipped up by a light show and an increasingly hysterical commentator, broke into wild applause.

Signal ReceivedWith echoes of Marconi, Tamburini and Thidé light up the wall of the Doge’s Palace in Venice. They project the words “Segnale Ricevuto” when two radio waves, one of which was twisted, were transmitted across 442 meters and received by two separate channels.Simonetta Soglio

Fabrizio Tamburini and Bo Thidé, the scientists who orchestrated the Venice demonstration, triumphantly wrote up their results. The transmission, they explained, used two radio beams, one of which had been carved into a helix shape. Even though the beams had the same wavelength, the twist meant that receivers could distinguish between them—as if the beams were separate data channels. In principle, the scientists said, many more beams could be added, each with different degrees of twist in the helix, to provide even more channels. Tamburini was soon being styled by the Italian media as “il nuovo Marconi.”

More data channels are exactly what the telecommunication industry desperately needs. With the growing burden of images, movies, and online gaming, mobile devices have become voracious consumers of bandwidth. Telecom company Cisco predicts that by 2018, global traffic will reach 2.7 gigabytes per smartphone per month, totaling 15.9 exabytes (15.9 x 1018 bytes), all carried over finite radio spectra and straining wireless networks.

The Venice experiment promised a solution, potentially multiplying the data capacity of a single frequency at a stroke. But rather than global acclaim, their paper was met with skepticism—even outrage—from other researchers who argued that their technique was not new, and would do nothing to boost communication capacity. “This will not allow us to increase the spectral efficiency of wireless communication,” says Julien Perruisseau-Carrier, an electrical engineer at the Swiss Federal Institute of Technology in Lausanne. “What is sure: we have a new ‘cold-fusion’ subject that will survive for years or decades,” says Laszlo Kish, a physicist at Texas A&M University, College Station.

“I don’t understand how it ended up being so acrimonious,” says Miles Padgett, a professor at School of Physics and Astronomy at the University of Glasgow who has pioneered research into the helical light.

The grandstanding, misunderstanding, acrimony, and accusation that characterize this episode harken back to the days before professionalized science, when showmanship was an integral part of advancing a research agenda.

But on closer examination, the story of Tamburini and helical light transmission also highlights the latent burden of specialization, offering a revealing glimpse of how science works when fields collide. Working at the borders of physics and electrical engineering, Tamburini and his critics almost speak in different languages when describing these phenomena. With scientists sometimes talking past each other, passionate rhetoric can boil over into invective. Throw in the prospects of huge profits from solving one of the biggest conundrums in telecommunications, and tensions are stoked even higher.

Contrary to appearances, science and engineering do not always proceed in an orderly, rational stepwise fashion, but can have just as much showmanship as a circus—and, consequently, as much acrimony as a courtroom.


Things could have turned out very differently for Tamburini, an astrophysicist at the University of Padua, in Italy. While studying physics as an undergraduate, Tamburini moonlighted as an Alfa Romeo test driver—a tempting career option. But then he met Dennis Sciama, a father figure of modern cosmology, who persuaded Tamburini to abandon his cars and study the stars. 

Tamburini’s FerrariTamburini’s work on orbital angular momentum helped him realize a childhood dream: owning a classic Ferrari. This specimen is a Ferrari 208 Turbo GTB with 400 Bhp, racing kit, and an adjustable wheel-chassis setup. “I went crazy to find one as brand new,” Tamburini told me. “It looks like the Ferrari of Magnum P.I.”Nicoletta Stacco

Starlight is packed with information, if you know how to read it. Its color can reveal a star’s temperature, its composition, or even its distance; polarization of the light can signal the presence of magnetic fields or vast interstellar dust clouds. In 2003, Cornell University astrophysicist Martin Harwit suggested another way to squeeze more data from starlight. He predicted that the matter around pulsars, or the rapid rotation of certain black holes, could imbue the light with something called orbital angular momentum (OAM).

Beams of light typically travel in straight lines, and the energy that they carry follows the same trajectory. But it is also possible for the energy to trace out a corkscrew path around the beam’s direction of travel, which gives the light orbital angular momentum. This staggers the electromagnetic waves in the beam, so that their peaks collectively trace out the shape of a helix. Add more momentum and it becomes a double helix, and then a triple helix—the shape of fusilli pasta. In theory, there is no limit to the amount of twist you could give to a beam.

Even single photons can have orbital angular momentum. As quantum entities, it’s impossible to define their position and their momentum at the same time—but if you were able freeze a movie of a “twisted photon” in flight, its momentum would appear to be angled away from its direction of travel. Advance the photon’s journey frame by frame, and the arrow representing its momentum seems to rotate around the line of flight—almost as if the light itself were spiraling around an axis. “The energy moves like a tornado,” says Juan Torres, physicist and telecommunications expert at the Institute of Photonic Science in Barcelona, who works with twisted light.

Light beams carrying orbital angular momentum are a remarkably recent discovery, considering that a related property, spin angular momentum, has been known about for more than a century. Spin angular momentum arises when light is circularly polarized, so that each wave’s electric field rotates as it travels. In 1936, Richard Beth at Princeton University measured that spin angular momentum for the first time by passing circularly polarized light through a quartz disc hung from a fiber—he found that the lens started to rotate as the light transferred its momentum.

Yet it was only in 1992 that scientists realized that twisted laser beams could be made quite easily in the lab. Three years later, a team of Australian scientists used twisted light to make floating microscopic particles orbit around the beam’s axis.

Science and engineering do not always proceed in an orderly, rational stepwise fashion, but can have just as much showmanship as a circus.

One way to give light a twist is to pass it through a fork-shaped pattern in a liquid crystal. As the light waves diffract through the pattern, they interfere with each other in just the right way to give the light orbital angular momentum; changing the diffraction pattern produces different amounts of twist. Shining light through a clear plate with one face shaped into a spiral can also twist light in a similar way.

Harwit’s prediction that black holes could imbue light with orbital angular momentum offered a potential route to the first observational evidence that black holes can spin, just as the theory of general relativity predicts. Inspired by this idea, Tamburini paired with Thidé, a physicist at the Swedish Institute of Space Physics in Uppsala, to search for starlight with non-zero orbital angular momentum. In 2010, the pair proved that man-made radio waves could carry such modes, using a bowl-shaped transmitter twisted into a very shallow helix. But they could not get funding to test their technique on astronomical radio signals—so they decided to apply it to terrestrial communication. “A star is just like a mobile phone,” says Tamburini. “So if we can’t get funding with pure physics, let’s go applied.”

The target was a rich one. If each orbital angular momentum mode could be used as a separate data channel, it might be possible to add dozens, even hundreds, of extra channels to the same radio frequency. Solve the capacity problem that modern telecoms are facing, and the rewards could be huge.

After some successful trial runs, Tamburini and Thidé started to plan the Venice demonstration. “It’s like having a nicely-colored race car—it was for advertising,” says Tamburini. “We were trying to grab the attention of private investors—and it worked.” In 2012, the scientists founded a company called Twist-off srl to develop the technology in partnership with SIAE Microelettronica, one of the oldest telecom companies in Europe. The deal is worth €5m (about $7 million), with payouts dependent on meeting key milestones, Tamburini says.

“As telecommunication engineers, we were astonished the first time we saw this,” says Piero Coassini, microwave laboratories director at SIAE, who now leads a small team working with Twist-off srl. SIAE specializes in point-to-point communication systems, particularly between mobile phone towers, and Coassini says that OAM radio is a blue-skies project for the company. Nevertheless, he is optimistic that after three years of development work, short-range communication links might begin to incorporate OAM—perhaps wirelessly streaming high-definition video to a display screen. “OAM is a new dimension in electromagnetic communication,” he says.

In September 2013, Francois Rancy, Director of the Radiocommunications Bureau at the International Telecommunications Union (ITU), which develops international regulations and standards on spectrum use, visited Padua to see the system in action. This time the experiment used three transmitters—one a clockwise helix, one a conventional bowl, and the third a counterclockwise helix—to generate three OAM modes, all carried by 17 gigahertz radio waves over 150 meters. The three transmissions used quadrature amplitude modulation, showing that OAM could be compatible as an add-on to existing communication systems. “I was very impressed by what I saw.” Rancy concluded. “It shows that this technique could multiply the spectrum by three or more.”

Two of the ITU’s specialist committees will be taking a closer look at OAM radio—modeling what would happen to networks if a significant number of these antennas were deployed, for example. “This has the potential to bring considerable change to the world of communications,” says Rancy. “But we have to avoid being too optimistic and raising false expectations.”


False expectations is exactly what many scientists believe have been raised by the Venice demonstration. “This claim sounded like an amazing discovery,” says Perruisseau-Carrier. Yet he was convinced that this “breakthrough” was nothing of the sort, and that Venice was a publicity stunt that would draw public funding towards a technological dead end. So he and his colleagues wrote up a critique of Tamburini’s paper, joining two other ripostes that had just been published.

One of their main objections is that although orbital angular momentum can indeed carry information in radio waves, it won’t make for a useful technology. “It does not offer a practical means to increase communication capacity”, says Ove Edfors, an information technology researcher at Lund University who wrote one of the papers.

Beams of light, or radio waves, are typically brightest at the center. But the reverse is true in beams carrying orbital angular momentum—they have a dark spot at their heart, surrounded by a ring that contains most of the beam’s power. Since the beam spreads out as it travels, the hole is magnified to enormous proportions over long distances. A receiver might capture a readable signal if the antennas were just meters apart, and aligned very precisely. But over miles, the signal’s power would be dissipated across a huge ring that could only be captured by an impractically vast receiver dish. Even worse, adding more twist to the beam also makes the hole grow bigger, and harder to receive reliably. So much for an unlimited number of extra channels.

A star is just like a mobile phone.

Also contentious is that Tamburini and Thidé transmit each mode through separate antennas. Perruisseau-Carrier and others argue that this is merely a form of multiple-input multiple-output (MIMO) communication, already used to increase wireless capacity, and therefore not new. MIMO uses several antennas set up so that signals take different paths—bouncing off buildings and trees, for example—so that they reach their destination at slightly different times. This means they can use the same wavelength yet be read separately, as if each signal was traveling along its own “virtual wire.” “Each OAM mode in the Venice experiments plays the role of an antenna in MIMO. If someone tells you this isn’t MIMO, they’re wrong,” says Perruisseau-Carrier.

The ensuing discussions have grown increasingly irate. “These people are not telecommunication engineers, and unfortunately they haven’t taken the time to fully understand what else has been done in the field,” says Perruisseau-Carrier. Tamburini and his colleagues have retorts for each of these complaints. “They can call it MIMO, MIMO-OAM, Joe, Zorro, or whatever else one wants. We do not care,” retorted Tamburini in an email to me last year on the topic of whether his approach was new. “What is important is that it is working and we make money out of it.”

Perruisseau-Carrier says he’s had a lot of support from the telecommunications community, who all agree with his objections. Thidé refutes that, arguing that they only have a handful of critics, and that they get far more positive than negative feedback. Late last year, Thidé presented their work at the Wireless World Research Forum in Vancouver, Canada, and says that he satisfied delegates’ queries about the technology. “I went away from Vancouver with a good feeling,” he says. “Finally we are getting the message through.”

Torres points out that some1 criticisms share a common assumption: that the twisted beams are being detected in a region known as the “far field.” As the waves emitted by an antenna fan out, they form a curved wave front. But eventually, the fan grows so large that its front is essentially flat, and looks like an infinite plane to any receiver. At this range, a receiver would not be able to distinguish any twisty structure in the waves.

“What is important is that it is working and we make money out of it.”

But far-field means more than being far away, says Torres. “Indeed, a receiving antenna can be hundreds of meters or even tens of kilometers away from the emitting antenna and still be in the electromagnetic near field,” he says. As long as the receiver is big enough, it should still be able to discern the structure of the radio beam, he says. Some of the deepest objections to using OAM for free-wave radio communications, including a claim by Texas A&M’s Kish that it violates the second law of thermodynamics, appear to make this mistake, Torres says.

And as for the hole-in-the-middle problem, SIAE’s Coassini says it is not a fundamental barrier—it’s just another engineering issue to solve, perhaps by stopping the beam from spreading out so much. Keeping the beam tight would also ensure that the required receiver was not too large. Even if technological fixes cannot overcome the “donut hole” problem, the technology could still be used to boost capacity over a few hundred meters, says Torres, perhaps handling the sudden surge of calls when a cruise ship docks in port, or when a football team scores in a packed stadium.

Tamburini adds they are already well on the way to developing a solution to the donut problem, and expect a practical demonstration within a year. “It’s a very smart idea,” he enthuses. Now that he is working with SIAE, though, he cannot reveal the breakthrough (although he may publish once a patent is granted). That commercial secrecy means that Tamburini’s promises are unlikely to cut much ice with detractors, leaving the debate hanging unresolved. 


Tamburini was clearly playing on Marconi’s legacy when he staged his Venice demonstration. But the parallels with the wireless communication pioneer go much deeper than he anticipated.

Marconi began his work on radio waves in Italy, before traveling to England in 1896 to present his system to potential buyers from the Navy, Army, and Post Office. But it took a public show at Toynbee Hall in London to really cement his reputation. The newspapers were overawed by the showmanship of his demonstration, calling his equipment “Signor Marconi’s magic box.” The publicity undoubtedly helped him to commercialize the technology.

But it also generated a lot of ill-feeling. “There was an element of suspicion about Marconi seeking publicity,” says Elizabeth Bruton, a historian of science at the University of Leeds, and Marconi expert. “It definitely annoyed quite a lot of scientists.” Marconi had some training in physics, but he was very much an outsider to the telecommunications field. “They felt like here was a jumped-up Italian, who didn’t have the right background, who hadn’t engaged with the community,” says Bruton.

Similarly, Perruisseau-Carrier is clear that if Tamburini’s light show in Venice hadn’t attracted so much publicity, he probably would have ignored the research. “It’s not the first time I’ve read an article I thought was incorrect,” he says. But the extraordinary nature of Tamburini’s claim and the uncritical press coverage that followed prompted him to speak out.

Public demonstrations like Marconi’s—and Tamburini’s—were once an integral part of science, says Iwan Morus, historian of science at Aberystwyth University. But by the 1930s, when science had become much more professionalized, “the notion that you publicize a new discovery by staging a performance starts to look more dubious,” says Morus. “The notion grows that this is disreputable, beneath the dignity of science.”

“It’s a shame that we now have a scientific culture that doesn’t regard that sort of thing as part and parcel of scientific life,” Morus adds. “I think we have lost something—in the 19th century there was a more immediate connection between science and its publics.”

The developments in communication technologies in the 19th century were also accompanied by heated debate between physicists and electrical engineers, about how telegraph wires work, for example, with one side arguing from theory, the other from practice. As those fields have become more specialized, the potential for culture clashes at their borders has grown, according to Morus. “I think it does lead to more disputes,” he says.

If Tamburini’s light show in Venice hadn’t attracted so much publicity, he probably would have ignored the research.

Tamburini sees the communication breakdown as a result of people only being trained in the language of their own narrow fields. “It’s a problem of super-specialization,” he says. Torres agrees that people from physics and engineering backgrounds have tended to talk past each other on twisted radio waves. “This can create confusion,” he says. “In physics they are more interested in the fundamentals,” he adds. “For electrical engineers, they’re just interested in the channels they get.”

Echoing Marconi’s outsider status, Tamburini also wonders if some critics are annoyed that the idea came from radio astronomy, rather than their own field. Thidé agrees: “I can’t help but feel these are sour grapes feelings—‘shit, why didn’t we think of that.’ ”

It’s telling that while the debate around radio OAM has been colored by antagonism, it has been positively cordial at higher frequencies, in the infrared part of the spectrum. Alan Willner, an electrical engineer at the University of Southern California, is twisting beams of laser light to combine data streams and increase the amount of data carried through free space or down optical fibers. “But it’s not clear to me there are many fundamental differences,” he says.

There are big differences, however, in how Willner has approached his research. As a communications expert, Willner says that he has benefitted from working closely with physicists studying twisted light, including Padgett. “We come at it from a different perspective, and everyone benefits. You get physicists to think about engineering systems, and engineers have to think about fundamental physics,” he says. “There’s a blurring of the lines.”

As well as reaching across the disciplinary divide, Willner also has an impeccable scientific pedigree: He is vice president of the Optical Society, the leading professional association in optics and photonics. And while the work has been published—with all the requisite caveats—in leading scientific journals, there have been no dazzling public light shows.

Even some of Tamburini’s critics say that Willner’s technology could have potential in these circumstances. “If we get to really high frequencies—light rather than radio—the orbital angular momentum communication concept makes more sense and may be a viable solution in some special situations,” says Edfors.


Back in Italy, research and development work on twisted radio waves is progressing independently of the debate. And it has already helped Tamburini realize a childhood ambition. “I finally own the classic Ferrari I dreamed of since was a boy,” Tamburini wrote to me in December. A few weeks later, a photo followed—the scientist, looking relaxed in yellow shirt and brown loafers, his hand resting proprietorially on a gleaming red sports car. “I’ve had two heart attacks—I must be calm,” he says. “But I still drive racecars.”


Mark Peplow is a science journalist based in Cambridge, U.K.

Issue 011

Light

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