By Casey Johnston
Heisenberg meets a qubit.
A quantum memory may be all scientists need to beat the limit of Heisenberg's uncertainty principle, according to a paper published in Nature Physics. According to a group of researchers, maximally entangling a particle with a quantum memory and measuring one of the particle's variables, like its position, should snap the quantum memory in a corresponding state, which could then be measured. This would allow them to do something long thought verboten by the laws of physics: figure out the state of certain pairs of variables at the exact same time with an unprecedented amount of certainty.
Our ability to observe particles at the quantum level is currently limited by Heisenberg's uncertainty principle. Heisenberg noticed that when someone measured one variable of a particle, such as its position, there were some other variables, like momentum, that could not be simultaneously measured with as much precision—there was a small amount of uncertainty applied to one or both of the measurements.
The physical reasoning behind this is hard to follow. But Paul Dirac, another physicist, made up a scenario to illustrate why some variables have this contentious relationship.
Dirac pointed out that one of the only ways to measure a particle's position is by bouncing a photon off of it, and seeing where and how that photon lands on a detector. How the photon lands completely describes the particle's position, but by hitting it, the measurement changes the particle's momentum.
Likewise, a measure of momentum would change the particle's position. Because of this quirk, scientists thought it was impossible to know certain pairs of variables that affect one another at the same exact time with a very high degree of precision.
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Read more here/Leia mais aqui: ARSTECHNICA
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Nature Physics
Published online: 25 July 2010 | doi:10.1038/nphys1734
The uncertainty principle in the presence of quantum memory
Mario Berta1,2, Matthias Christandl1,2, Roger Colbeck1,3,4, Joseph M. Renes5 & Renato Renner1
The uncertainty principle, originally formulated by Heisenberg1, clearly illustrates the difference between classical and quantum mechanics. The principle bounds the uncertainties about the outcomes of two incompatible measurements, such as position and momentum, on a particle. It implies that one cannot predict the outcomes for both possible choices of measurement to arbitrary precision, even if information about the preparation of the particle is available in a classical memory. However, if the particle is prepared entangled with a quantum memory, a device that might be available in the not-too-distant future2, it is possible to predict the outcomes for both measurement choices precisely. Here, we extend the uncertainty principle to incorporate this case, providing a lower bound on the uncertainties, which depends on the amount of entanglement between the particle and the quantum memory. We detail the application of our result to witnessing entanglement and to quantum key distribution.
Institute for Theoretical Physics, ETH Zurich, 8093 Zurich, Switzerland
Faculty of Physics, Ludwig-Maximilians-Universität München, 80333 Munich, Germany
Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, Ontario N2L 2Y5, Canada
Institute of Theoretical Computer Science, ETH Zurich, 8092 Zurich, Switzerland
Institute for Applied Physics, Technische Universität Darmstadt, 64289 Darmstadt, Germany
Correspondence to: Roger Colbeck1,3,4 e-mail: rcolbeck@perimeterinstitute.ca
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