Quantum physics first: Researchers observe single photons in two-slit interferometer experiment

Quantum mechanics is famous for saying that a tree falling in a forest when there’s no one there doesn’t make a sound. Quantum mechanics also says that if anyone is listening, it interferes with and changes the tree. And so the famous paradox: how can we know reality if we cannot measure it without distorting it?

An international team of researchers, led by University of Toronto physicist Aephraim Steinberg of the Centre for Quantum Information and Quantum Control, have found a way to do just that by applying a modern measurement technique to the historic two-slit interferometer experiment in which a beam of light shone through two slits results in an interference pattern on a screen behind.

That famous experiment, and the 1927 Neils Bohr and Albert Einstein debates, seemed to establish that you could not watch a particle go through one of two slits without destroying the interference effect: you had to choose which phenomenon to look for.

“Quantum measurement has been the philosophical elephant in the room of quantum mechanics for the past century,” says Steinberg, who is lead author of Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer, to be published in Science on June 2. “However, in the past 10 to 15 years, technology has reached the point where detailed experiments on individual quantum systems really can be done, with potential applications such as quantum cryptography and computation.”

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. 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 (particle of light) 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 ie. the trajectories it takes to a screen.

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