By applying a modern measurement technique to the historic two-slit interferometer experiment, scientists soon may be able to measure reality without distorting it. Quantum mechanics tells us that a tree falling in a forest with no one there doesn’t make a sound. It also states that if anyone is listening, it interferes with and changes the tree. An international team of researchers, led by University of Toronto physicist Aephraim Steinberg of the Centre for Quantum Information and Quantum Control, has found a way to answer the famous paradox: How can we know reality if we cannot measure it without distorting it? The researchers set out to answer the question 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 earlier experiment, along with 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,” Steinberg said. “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.” This three-dimensional plot shows where a quantum particle is most likely to be found as it passes through a double-slit apparatus and exhibits wavelike behavior. The lines overlaid on top of the 3-D surface are experimentally reconstructed average paths that the particles take through the experiment. (Image: University of Toronto) In their modern experiment, the researchers succeeded for the first time in experimentally reconstructing full trajectories that provide a description of how light particles move through the two slits and form an interference pattern. Their technique builds on the 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 in which a photon was moving, conditioned upon where the photon was found. By combining the information about the photon’s direction at many different points, scientists could construct its entire flow pattern; i.e., the trajectories it takes to a screen. Their experiment used a new single-photon source that was developed at NIST (National Institute of Standards and Technology) to send photons one by one into an interferometer that was constructed in Toronto. Using quartz calcite to measure the direction as a function of position, they discovered that their measured trajectories were consistent with Wiseman’s prediction. 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, which states that observing particlelike or wavelike behavior in the double-slit experiment depends on the type of measurement made. The system cannot behave as both a particle and wave simultaneously. The team’s recent experiment, however, suggests that this doesn’t have to be the case: The system can behave as both. “By applying a modern measurement technique to the historic double-slit experiment, we were able to observe the average particle trajectories undergoing wavelike interference, which is the first observation of its kind. This result should contribute to the ongoing debate over the various interpretations of quantum theory,” Steinberg said. “It shows that long-neglected questions about the different types of measurement possible in quantum mechanics can finally be addressed in the lab, and weak measurements such as the sort we use in this work may prove crucial in studying all sorts of new phenomena.” Research partners for the experiment included the University of Toronto’s Centre for Quantum Information and Quantum Control, the Department of Physics and Institute for Optical Sciences, NIST, the Institute for Quantum Computing at the University of Waterloo, Griffith University in Australia and Laboratoire Charles Fabry in Orsay, France. The research, which was published in the June 2 issue of the journal Science, was funded by the Natural Sciences and Engineering Council of Canada, the Canadian Institute for Advanced Research and Quantum Works. For more information, visit: www.utoronto.ca