Team applies weak-measurement principles to two-slit interferometer experiment

Toronto, QC, Canada--A group of researchers at the University of Toronto and other institutions worldwide has redone the historic two-slit interferometer experiment, in which light passing through two slits and interfering allows scientists to probe the photon's dual wave/particle nature. That famous experiment, and the 1927 Niels Bohr and Albert Einstein debates, seemed to establish that one could not watch a particle go through one of two slits without destroying the interference effect: one had to choose which phenomenon to look for.

The original double-slit experiment played a central role in the early development of quantum mechanics, leading directly to Bohr's formulation of the principle of complementarity. Complementarity states that observing particle-like or wave-like behaviour in the double-slit experiment depends on the type of measurement made: the system cannot behave as both a particle and wave simultaneously. The new experiment suggests this doesn't have to be the case: the system can behave as both.

In the experiment, a single-photon source developed at the National Institute of Standards and Technology (Boulder, CO) was used to send photons one by one into an interferometer constructed at the University of Toronto. The light then passed through a quartz crystal that had an effect on light that depended on the direction the light was propagating; this allowed measurement of the direction as a function of position. "Our measured trajectories are consistent, as Wiseman had predicted, with the realistic but unconventional interpretation of mechanics of such influential thinkers as David Bohm and Louis de Broglie," said University of Toronto physicist Aephraim Steinberg, lead author of a paper published in Science June 2.

"With this new experiment, the researchers have succeeded for the first time in experimentally reconstructing full trajectories which provide a description of how light particles move through the two slits and form an interference pattern," said Steinberg. "Their technique builds on a new theory of weak measurement that was developed by Yakir Aharonov's group at Tel Aviv University. Howard Wiseman of Griffith University proposed that it might be possible to measure the direction a photon was moving, conditioned upon where the photon is found. By combining information about the photon's direction at many different points, one could construct its entire flow pattern (the trajectories it takes to a screen)."

In essence, probabalistic measurements can lead to measurements of wave- and particle-like properties in the same experiment.

Research partners include the University of Toronto's Centre for Quantum Information and Quantum Control, Department of Physics and Institute for Optical Sciences, the National Institute of Standards and Technology in Boulder, Colorado, the Institute for Quantum Computing at the University of Waterloo, Griffith University, Australia, and the Laboratoire Charles Fabry in Orsay, France. Research was funded by the Natural Sciences and Engineering Research Council of Canada, the Canadian Institute for Advanced Research and QuantumWorks.