Quantum detectives

By Robert Matthews IN A startling demonstration of the weirdness of quantum theory, researchers at the Los Alamos National Laboratory in New Mexico announced last week that they had detected and studied a variety of objects using laser light which never went anywhere near them. Common sense dictates that to find out anything about an object, there must be some form of interaction—even if you only shine light on it. However, the quantum nature of light—in particular, its ability to behave both as a wave and a particle—allows so-called interaction-free measurements to be made. In 1993, Avshalom Elitzur and Lev Vaidman of Tel Aviv University suggested that this trick could be achieved by allowing light to go along two different paths, one of which can contain the object (see diagram). When the object is absent, the setup can be arranged so that a particle-like photon of light can travel via either path with equal probability. Its wave-like properties then produce destructive interference, so the emerging photon continues along the same path and thus no signal is registered at the detector. Once an object blocks one of the paths, a photon travelling along it can’t complete its journey and create the destructive interference needed to stop activation of the object detector. So any photon that chooses the path without the object can then make a choice when it reaches the beam splitter, and there is a 50 per cent chance that a signal will be detected—allowing the photon to reveal the presence of an object despite never interacting with it in any way. Interaction-free detection has since been demonstrated in the lab, prompting physicists to dream up real-life applications (see “Crazy logic”, 2 May, p 38). Now Andrew White and his colleagues at Los Alamos have taken a key step towards such practical devices by measuring the dimensions of small objects, despite the fact that about half the photons emitted effectively never interacted with the target. In a paper in a forthcoming issue of Physical Review A, the team describe how they measured the thickness of objects, which included a knife-edge, a wire and human hair, by passing them across one of the optical paths. And at an international conference on quantum electronics held in San Francisco last week, the team announced that a more sophisticated system has boosted the level of non-interaction to over 70 per cent. “The next logical step for us is to use this higher-efficiency system to produce one-dimensional images of objects,” said team leader Paul Kwiat. “If you can do that, you can also produce 2D images—it’s not really any more difficult”. According to Kwiat, that would open the way to a radically new form of imaging, in which sensitive targets—such as living cells—could be examined using virtually no photons at all. Vaidman says the new work is impressive. “There are many technical difficulties involved in such experiments,
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