The strange quantum dance of electrons

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http://www.elpais.com/articulo/sociedad/The/strange/quantum/dance/of/electrons/elpepusoc/20111006elpepusoc_8/Tes





Despite being one of the founding fathers of quantum theory, Albert Einstein was never able to reconcile it's predictions with his philosophy of nature. Arguably, the animal in the quantum zoo that most clearly contested his views was the phenomenon of entanglement - a "spooky action at distance"....

Quantum theory predicts that for two distant, entangled particles, measuring the properties of one particle instantaneously determines the properties of the other particle. Since the days of Einstein, entanglement has continued to spark vivid debates on the foundations of quantum theory. During the last decades, entanglement has emerged as the prime resource for quantum computers. Quantum computers are, potentially, extremely powerful computers with working principles based on quantum theory. The prospect of developing quantum computers and then integrating them with present day computers has stimulated intense theoretical and experimental research on entanglement in electronic systems. The ultimate goal is to use the quantum properties of individual electrons as the elementary building blocks of a quantum computer. My research is focused on understanding how to generate and detect entangled electrons in nanometre sized electrical conductors. In a recent experiment our theoretical prediction were confirmed, providing the first glimpse of evidence that electrons can indeed perform the strange entanglement dance.

In his famous work in the mid 1930's, Einstein, together with colleagues Rosen and Podolsky, formulated his critique of quantum theory - it can not be a complete theory of nature since it does not fully describe the properties of all its constituents. This was most apparent for a pair of entangled particles where quantum theory only ascribes certain properties to the pair, but not to the individual particles. Niels Bohr, the leading scientist in quantum theory at the time, answered within a year. There is no reason to demand of a physical theory that it fully should describe nature as it really is; we can only demand that it shall predict the outcomes of all possible measurements we can do, Bohr argued. Einstein, as well as many physicists over the years, never felt content with this. There have been numerous attempts, inspired by Einstein's ideas, to construct alternatives to quantum theory. However, whenever making predictions different from quantum theory, experiments have always decided in favour of quantum theory.

With the appearance of quantum information theory in the 1990's, the interest in entanglement took a new turn. Suddenly, fundamental properties of quantum theory were married with information science, presenting the prospect of a new, powerful breed of quantum computers. Motivated by the breathtaking development of classical computers in the last half century, researchers started to investigate the possibilities for quantum computing in electronic systems. This went hand in hand with the rapid development of nanotechnology, boosting the vision of a quantum computer based on the quantum properties of individual electrons in nanometre sized systems. It soon became clear that the road to a working electronic quantum computer would be long. In order to control the quantum properties of a single electron experiments needed to go to extreme conditions. Temperatures a fraction of a degree above absolute zero and meticulous efforts to shield the electrons from disturbances from the surrounding world were required.