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Jun2-11, 09:22 PM
P: 720
Role of the Observer

"We are all just thrilled to be able to see, in some sense, what a photon does as it goes through an interferometer, something all of our textbooks and professors had always told us was impossible," Aephraim Steinberg, a physicist at the University of Toronto's Centre for Quantum Information and Quantum Control, said in a statement.

The results were published Thursday in Science.
EDIT: more details:

Science, 3 June 2011:
Vol. 332 no. 6034 pp. 1170-1173
DOI: 10.1126/science.1202218

Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer

Sacha Kocsis1,2,*,
Boris Braverman1,*,
Sylvain Ravets3,*,
Martin J. Stevens4,
Richard P. Mirin4,
L. Krister Shalm1,5, and
Aephraim M. Steinberg1,†


A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.