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Experiment in Vienna Shows That Ground-to-Satellite Communication with Twisted Light is Possible

The amount of data that a light beam can transmit depends on its frequency range, or bandwidth—the wider it is, the more data you can cram into the beam. By manipulating the polarization of photons (a quantum property known as intrinsic spin angular momentum), which can be either horizontal or vertical, you can double the amount of information transmitted by a beam. There is another quantum property of photons, called orbital angular momentum (AOM), which can, in principle, have a ground state and an infinite number of values. Each of these values is associated with an integer indicating its helicity (represented by the integer l). In the ground state, where  l=0, the wave front of the waves is planar.  For all the other states, the wave front rotates along a “twisted” shape reminiscent of fusilli pasta (a helix); the amount of twist increases with increasing l.

In 2001, Anton Zeilinger, a quantum physicist at the University of Vienna, proposed the idea of using the orbital angular momentum of photons to increase the amount of data that can be transmitted by light beams. Several experiments in laboratories confirmed that data could be transmitted via twisted light beams, but the question of whether the quantum states of light photons would survive turbulence in air over long distances remained an open question.

In 2012, a Swedish-Italian research group successfully transmitted twisted microwaves over 450 meters of free space. But microwaves, although they consist of photons, are impervious to air turbulence. Therefore, Zeilinger and his research colleagues in Vienna decided to put the influence of turbulence—especially the turbulence you find above big cities—to the test.

Earlier this month, they reported in the New Journal of Physics the successful transmission of OAM modes via laser beam through open space over a distance of 3 kilometers. To make the experiment possible, they had to restrict the light beam to 16 OAM modes. “In principle, each single photon can have an unbounded number of different OAM values. We did our experiment with a laser, so we used a lot of photons, and so we are very far away from the single-photon regime,” explains Mario Krenn, a physicist at the University of Vienna and lead author of the New Journal of Physics paper.

The researchers transmitted the beam from the radar tower of the Central Institute for Meteorology and Geodynamics to the rooftop of their own institute building. To create the OAM modes, they reflected the light of a laser off a so-called spatial light modulator. “It is a usual pixel display with about one thousand by one thousand pixels where you can change the reflective index of each pixel,” says Krenn. “This allows us to introduce phase changes of 0 and 2π [pi] for each pixel. When we direct the laser beam at this spatial structure, the laser light undergoes these phase changes and develops these specific intensity energy patterns that we show in the video,” says Krenn.

A high quality lens (the first trials with a lower quality lens killed the OAM modes right away) focuses the reflected light into a 6-centimeter-wide beam that projects the light patterns on a screen 3 km away. A CCD camera recorded each of the 16 different patterns that resulted from the various OAM modes.

To the relief of the researchers, turbulence did not affect the OAM modes significantly, although it caused the patterns’ position on the screen to move. “Some calculations suggested that even after one kilometer, you might have a very big problem because of turbulence,” Krenn recalled. “With our method, we show that up to three kilometers of atmosphere does not destroy the OAM modes. [Because] the effective atmosphere is roughly six km thick, [transmitting through it] will not be a problem," says Krenn. “This result has interesting implications for future ground-satellite communication.”

To ascertain the quality of the transmission, the researchers transmitted small grey-scale images of three famous Viennese: Wolfgang Amadeus Mozart; Ludwig Boltzmann; and of course, Erwin Schrödinger. To recognize the patterns and associate them with the 16 OAM states, the researchers used an artificial neural network. “We recorded some test samples [about 450] of different modes, says Krenn. “This was the input to the network, a so-called unsupervised network. You don't have to tell it how it should learn, you supply the input and it characterizes the different structures by itself," says Krenn.

Holographic Food, Brain-Kitchenware Interface, and Other Future of Home Concepts

The best kinds of concepts are things that are so futuristic that they don’t exist yet, but not so futuristic that you couldn’t convince yourself that just maybe, in five or 10 years, the concept might deliver on its promises. Every year, the Electrolux Design Competition tries to hit this sweet spot, and the theme for 2014 is “Creating Healthy Homes.” Perhaps not the most exciting theme at first glance, but the winner this year is a concept for a system that lets you hunt down holographic fish as they swim through your living room.

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A New Hypersensitive Magnetometer Based on Kinetic Inductance

During the 1960s, researchers at the Ford Motor Company developed one of the early practical applications of superconductors, the SQUID (short for superconducting quantum interference device). A SQUID consists of a superconducting loop incorporating two Josephson junctions—very thin insulating barriers in the superconducting loop. Cooper pairs—paired electrons that are not scattered by atoms and form the supercurrent—jump this gap, even when no voltage exists over the junction, a phenomenon called quantum tunneling. However, when a voltage appears over the Josephson junction, the current starts to oscillate at a very high frequency, with the frequency determined by the voltage.

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Which Mobile Apps Are Worst Privacy Offenders?

You may want to think twice about playing Angry Birds on your Android device.

All mobile applications need to generate money somehow—and for the ones free to download and use, revenue is almost entirely collected through advertisements. Many free apps share contact lists with third parties or use a user’s location to deliver targeted ads.

Most users have no clue, which led to the genesis of, a project spearheaded by the Carnegie Mellon University’s Computer Human Interaction: Mobility Privacy Security (CHIMPS) Lab. The site grades Android apps on a scale of A+ to D based on a model that gauges how much private information an app mines from a user’s device—and how closely that’s in line with a user’s expectations. The model was developed using the preference ratings of 725 different users on 837 free Android app.

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Global Warming: Soon with 50% More Lightning

Lightning may be full of sound and fury, but it also signifies important things on the earth below. Though lightning deaths are falling steadily (there were 23 in the United States in 2013), it continues to ignite some 10,000 wildfires that consume more than 4.1 million acres in the United States each year. Lightning is also, among other things, the leading cause of serious fires in wind turbines. And every flash converts about 7 kilograms of nitrogen into smog-producing nitrogen oxides, to the tune of about 8.6 billion kilograms of NOx per year.

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Smartphone System Combines Gait Measurements, Magnets for Indoor Navigation

Global Positioning Systems have spoiled us. With a glance at our smartphones we expect to immediately know not only exactly where we are but also where we’re going and how to get there. But GPS doesn’t work indoors, which has resulted in what I assume are millions of people getting hopelessly lost in shopping malls worldwide, as they try and figure out how to get from Brookstone to Hot Dog on a Stick.

Researchers at Fraunhofer Portugal think that they have this problem licked, with an innovative cell phone-based indoor localization system that combines magnetic beacons with inertial tracking of your gait to pinpoint your location with sub-meter accuracy.

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IBM Is Redesigning Supercomputers to Solve Big Data Problems

Traditional supercomputers focused on performing calculations at blazing speeds have fallen behind when it comes to sifting through huge amounts of “Big Data.” That is why IBM has redesigned the next generation of supercomputers to minimize the need to shuffle data between the memory  that stores the data and the processors that do the computing work. The tech giant recently earned US $325 million in federal contracts to build the world’s most powerful supercomputers for two U.S. government labs by 2017.

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The Hackaday Prize Awarded to Satellite Ground Station Project

First place in the Hackaday Prize was awarded today to “SatNOGS,” a project aiming to spin up a worldwide network of satellite ground stations—hence the project’s name, which is an abbreviation of “Satellite Network Of Ground Stations.” Its creators will receive either one paid ride into space, when such a ticket becomes available, or $196,418 (whose odd numeric value some astute readers may recognize as being a Fibonacci number).

The SatNOGS team edged out four other contenders in the final round of judging. Second place went to ChipWhisperer (a platform for security testing of embedded systems), third to PortableSDR (a compact software-defined radio), fourth to the Open Source Science Tricorder (a gadget for sensing various environmental parameters), and fifth to ramanPi (a Raman spectrometer based on the Raspberry Pi).

The SatNOGS project aims to serve enthusiasts who like to listen in to transmissions from the many satellites zipping around Earth in low or medium orbits. You can’t, of course, do that with a dish bolted to the side of your house. To do this right, you need a radio coupled to a high-gain antenna that can remain pointed in the right direction as the satellite of interest passes overhead.

The open-source design for such a DIY ground station is the main component of the SatNOGS project. But the project also includes facilities so that people from around the world can monitor satellites cooperatively. The idea is that  the ground stations will communicate with a central server on the internet. The server acts like a network manager, sending instructions to the ground stations and receiving digital copies of the signals they record. Assuming enough people build these ground stations, and that they have good geographic distribution, the system should allow much more comprehensive monitoring of satellite signals than anybody working in isolation could manage.

So congratulations to the SatNOGS team. Not that they need my advice, but I’ll give it anyway: Take the money.


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