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Entangling a Rainbow

A Brazilian team succeeds in the quantum entanglement of three different colors of light

3 min read

28 September 2009—A team of physicists led by Paulo Nussenzveig of the University of São Paulo, in Brazil, has shown for the first time that light of three different wavelengths can be entangled. Entanglement is a quantum mechanics phenomenon that is the key to staggeringly powerful—but still largely theoretical—quantum computers, as well as to unbreakable quantum encryption schemes.

Einstein once called entanglement "spooky action at a distance." Entangled particles are mutually dependent, even when they travel far apart. Quantum theory holds that if you measure one of the photons in an entangled pair, you will also instantly reveal the properties of the other. Entanglement of three light beams has never been demonstrated before. The entanglement produced so far has mostly been between two light beams of the same wavelength, a few atoms of the same element, or molecules of the same chemical. A common way to produce entanglement is to create a pair of photons in such a way that if a measurement shows that one of them is polarized in one direction—say, up—the other will always have a polarization in the other direction—in this case, down.

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Metamaterials Could Solve One of 6G’s Big Problems

There’s plenty of bandwidth available if we use reconfigurable intelligent surfaces

12 min read
An illustration depicting cellphone users at street level in a city, with wireless signals reaching them via reflecting surfaces.

Ground level in a typical urban canyon, shielded by tall buildings, will be inaccessible to some 6G frequencies. Deft placement of reconfigurable intelligent surfaces [yellow] will enable the signals to pervade these areas.

Chris Philpot

For all the tumultuous revolution in wireless technology over the past several decades, there have been a couple of constants. One is the overcrowding of radio bands, and the other is the move to escape that congestion by exploiting higher and higher frequencies. And today, as engineers roll out 5G and plan for 6G wireless, they find themselves at a crossroads: After years of designing superefficient transmitters and receivers, and of compensating for the signal losses at the end points of a radio channel, they’re beginning to realize that they are approaching the practical limits of transmitter and receiver efficiency. From now on, to get high performance as we go to higher frequencies, we will need to engineer the wireless channel itself. But how can we possibly engineer and control a wireless environment, which is determined by a host of factors, many of them random and therefore unpredictable?

Perhaps the most promising solution, right now, is to use reconfigurable intelligent surfaces. These are planar structures typically ranging in size from about 100 square centimeters to about 5 square meters or more, depending on the frequency and other factors. These surfaces use advanced substances called metamaterials to reflect and refract electromagnetic waves. Thin two-dimensional metamaterials, known as metasurfaces, can be designed to sense the local electromagnetic environment and tune the wave’s key properties, such as its amplitude, phase, and polarization, as the wave is reflected or refracted by the surface. So as the waves fall on such a surface, it can alter the incident waves’ direction so as to strengthen the channel. In fact, these metasurfaces can be programmed to make these changes dynamically, reconfiguring the signal in real time in response to changes in the wireless channel. Think of reconfigurable intelligent surfaces as the next evolution of the repeater concept.

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