http://www.elpais.com/articulo/sociedad/journey/across/the/quantum-classical/border/elpepusoc/20111109elpepusoc_10/Tes
A narrow staircase, guarded by a bust of grim-looking Ludwig Boltzmann, connects the rest of the world to the cellar of the historic building that houses the faculty of physics. The cellar, dominated by rattling recirculation pumps and dust-covered heating tubes, radiates an almost morbid charm that immediately evokes the famous underground scenes in the movie The Third Man. And the people who work in this underground vault are also in hot pursuit of a double agent?matter. Just as the movie's protagonists, who used the underground network of tunnels to roam the occupied zones in post-war Vienna, matter can change sides and switch its identity from the wave form to the particle form and back as the situation demands...
But the trap has been set: behind the otherwise unimposing door number 37, a surprised visitor will find a modern laboratory environment that stands in amazing contrast to the gloomy atmosphere of the cellar. This is the site for the world's currently most powerful matter-wave interferometer. With this device, a team of physicists, led by Markus Arndt, is investigating the wave-particle duality of objects of increasing complexity and mass to tackle the question: How our everyday world can appear "normal" while the behaviour of its constituent parts is anything but normal? In the quantum regime, objects may be found in a superposition of not only multiple but also mutually exclusive states. Properties such as position and momentum cannot be measured independently with arbitrary precision: generally speaking, the properties of an object are not independent of observations and the design of the experiment.
On the other hand, our everyday experience tells us that a given object at a given moment occupies only one, well-defined, location and that different properties of the object can be measured independently of one another and are unaffected by the observer. It is the striking conflict between this "normal" or "classical" behaviour, as physicists call it, and the often counterintuitive nature of quantum phenomena that has fascinated and unsettled many people since the introduction of quantum mechanics.
Matter-wave interferometry now allows us to explore the quantum-classical border and to verify the wave behaviour of objects of increasing mass, size, and complexity. In a metal tank that is immersed in surreal green laser light during the experiment, the physicists let molecules propagate through a set of three consecutive gratings. The bars of the gratings are ten-millionth of a metre apart and have to be manufactured with a precision a thousand times greater?precision that corresponds approximately to the size of a single hydrogen atom! Only when all gratings are perfectly adjusted, a characteristic interference pattern arises as a consequence of the superposition of the matter waves. The pattern depends on the mass and momentum of the molecules as well as, indirectly, on the molecules' structure.
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