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This 3D plot shows where a quantum particle is theoretically most likely to be found as it passes through the two fibres. The lines overlaid on top of the 3D surface are the average paths that the particles take through the experiment, as reconstructed from the measurements. (Courtesy of Krister Shalm and Boris Braverman)

Canadian researchers have traced the average path of single light particles through two slits, probing the limits of a famous physics principle that seemed to suggest doing so wasn't possible.

"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.

The Heisenberg uncertainty principle states that you can never precisely measure the position, speed and direction of a tiny particle, such as a photon of light, at the same time. According to the principle, if you measure where it is, you disturb it in a way that changes its velocity — its speed in a given direction.

But Steinberg and his team at the U of T centre found that you can still take measurements of photons at each point along a given path and know which way they were moving at the time.

"While it doesn't go against the rigorous theorem that Heisenberg proved, it goes against the way most of us were brought up and educated to think about the meaning of the theorem," Steinberg said in an interview Thursday.

"What the conclusion is for us is that although the principle when you read it really carefully is correct, many people have been interpreting it a bit too strongly."

The technique relies on multiple "weak measurements" rather than precise ones that significantly change the particles' position and trajectory.

Quantum computing applications

Steinberg thinks  the discovery could give a boost to the development of quantum computing devices — computer systems that rely on very small particles and quantum physics — by providing a way for engineers to figure out what is happening inside devices that need to separate some particles from others.

Double slit experiment

When light is sent through a screen that has two slits, it produces a pattern of light and dark stripes. That is similar to the ripples that are produced when water passes between two breaks and waves overlap to form an interference pattern. The fact that light forms interference patterns was strong evidence that light behaves like a wave, even though it also behaves as though it is made up of particles. Particles of light are called photons.

"What I believe is that this technique that we're using here will prove to be the right technique for catalyzing those kinds of devices," he said.

The experiment was designed to test the theory developed by Tel Aviv University physicist Yakir Aharonov and his team, who proposed that after detecting a particle, it should be possible to find out the average velocity of all particles that reached that point.

Steinberg's team used devices called quantum dots, created by U.S. collaborators at the National Institutes of Standards and Technology, to create single photons of light.

The photons were fed through a fibre optic circuit, and at one point, the light could end up in one of two different fibres. The two fibres acted like two slits, resulting in an interference pattern of light and dark stripes after the light passed through.

The photons also passed through a crystal that measures the direction the photon had been travelling at different points along the path before hitting the crystal. That information could be used to calculate the average momentum and the average paths of all the photons.

The system was set up and measurements were taken mainly by Sacha Kocsis, Boris Braverman and Sylvain Ravets, who were students in Steinberg's lab at the time.