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Furtive Approach Rolls Back the Limits of Quantum Uncertainty

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Science  05 Aug 2011:
Vol. 333, Issue 6043, pp. 690-693
DOI: 10.1126/science.333.6043.690

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Twenty-three years ago, three theorists invented a scheme that, they predicted, would enable experimenters to make "weak measurements" that do not disturb a quantum object. That idea flies in the face of the standard quantum theory. For decades, weak measurement remained a controversial sidelight little noticed by most physicists. But lately, experimenters have used weak measurements to make measurements with mind-boggling precision, to resolve apparent paradoxes posed by quantum mechanics, and even to probe things previously thought impossible to probe directly. Weak measurements provide a way to wring more information from quantum theory and to sidestep some of the prohibitions drilled into the heads of physics students. However, weak measurements themselves can be as mind-bending as other aspects of quantum mechanics. For example, they provide self-consistent explanations of paradoxical experiments, but those explanations rely on negative probabilities, a concept that many physicists find unpalatable.