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Entangled photons promise light-speed computers

Properties of entangled photon pairs are interlinked in a bizarre way that is dictated by the rules of quantum physics. European funded researchers in the Atesit project have developed a new method for creating these particles of light.

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Entanglement is playing an increasingly important role in present-day physics. In fact, it is the irrevocable signature of quantum affinity between distant particles. This paradigm, which is more than 70 years old, has up until now, been the fundamental cornerstone in our still uncertain understanding of the Universe. Recently, quantum physics has also opened the way to a radically new technology for the manipulation of information, its transmission and storage. Taking advantage of quantum mechanical phenomena, quantum computers can receive all possible states of entangled particles as input simultaneously. In this respect, it appears that so far, the most promising realisations of entanglement has been obtained in the field of quantum optics. Like virtuosos and their violins, the Atesit project partners tuned the assembly of engineered crystals, mirrors and lenses and harmonised the production of entangled photons to reach an unprecedented rate. As part of the research, scientists sent a light beam through a special, nonlinear crystal. Every so often, pairs of photons the same colour interacted in a process called 'four-wave-mixing', converting them into two new, entangled photons – one that is redder and the other that is bluer than the original. A major production problem they faced initially was that entangled photons were emitted in many directions and with a wide range of polarisation-phase relationships - each acting like an individual singer in a choir. They needed to come up with a way of further harmonising the production of entangled photon pairs. By passing the photons produced through another crystal, scientists removed distortions in the quality of entanglements, in the same manner that a corrector lens in a telescope removes colour distortion and improves the image quality. Quantum entanglement was demonstrated by inserting their experimental results into a mathematical inequality, the Bell's inequality. Until that time, scientists, using a low-brightness source of entangled photon pairs, could look into the quantum world as through a foggy window. The Atesit project partners hope that this bright light source will allow them to see phenomena they couldn't see before. Moreover, this research, still in its theoretical stage, could potentially enable scientists to continue upgrading computers even after traditional manufacturing procedures have been exhausted.

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