Contact: Mary Ann Meyers, Ph.D., Senior Fellow
The “weirdness” and “mysteries” of quantum mechanics have been recognized for nearly a century. Advances in our understanding of this branch of physics occurring over the past two years suggest that there are fundamental features of quantum mechanics, specifically those related to the measurement of photons, that have been, somewhat surprisingly, under investigated until now. Anticipating the International Year of Light in 2015, the purpose of this symposium is to explore these and other foundational issues concerning the nature and effects of light as quantized photons, placing special attention on questions that are suitable for laboratory investigation using an optical approach, which is the preferred methodology for such studies. A specific goal will be to establish a comprehensive strategy, including specific experimental protocols, for addressing these basic measurement questions about quantum phenomena in light with the intention of formulating a major research initiative.
A very general working hypothesis of the proposed work is that it will lead to a more comprehensive understanding of quantum mechanics. While modern physics has learned to deal with numerous, highly counterintuitive features of quantum mechanics, including the concepts of entanglement, wavefunction collapse, the role of which-way information in quantum interference, quantum uncertainty, quantum indeterminism, local realism, interaction-free measurements, the Einstein-Podolsky-Rosen effect, Hardy’s paradox, and Popper’s thought experiment, two developments occurred over the past two years that presented the possibility of shedding new light on aspects of these enigmas even as they raised new questions.
The first was an experiment conducted by Ralf Menzel that proved an intriguing new addition to the literature on wave-particle duality. Dr. Menzel and his colleagues observed that they could seemingly violate a well-established tenet of quantum mechanics stating that quantum interference, of the sort that might be measured in a variant of Thomas Young’s classic double-slit experiment, will disappear if an experiment can be performed to determine which opening the particle passed through. By making use of the strong correlations of entangled photon pairs, the Berlin-based physicists showed that if the two photons are entangled in position then the measurement of the position of one photon reveals the position of the other photon, and in this way one can determine through which opening in a barrier with two slits the photon has passed. But despite the fact that the experimentalist “knew” through which slit the photon passed, strong interference effects were observed in contradistinction to the prevailing view. Work currently in progress suggests that Dr. Menzel’s results are in fact consistent with standard quantum mechanics. Nonetheless, this experiment provides yet another example of the strongly counterintuitive effects that can occur in quantum mechanics.
These experimental results lead immediately to questions to be addressed in the symposium: Do the interference effects persist because the position of the photon was not truly “measured” but rather deduced by means of the properties of entanglement? Can similar results be observed for other independent physical parameters, known as degrees of freedom, and other forms of entanglement? Could the nature of the measurement (weak versus strong, for instance, as in Lev Vaidman’s interaction-free measurement experiment) influence the outcome of the experiment?
The second recent development involves work in quantum optics that challenges the generally accepted view that one cannot directly measure the wavefunction of a quantum particle, which could describe how a quantum system evolves over time, because a measurement of the function at one point will cause the entire wavefunction to “collapse.” Or to put it differently, determining the quantum state of a photon requires knowledge of both its position and its momentum, but in the process of measuring the position of the photon, one would lose all knowledge of its momentum. Traditionally, scientists measure a quantum state by means of quantum-state tomography, which is an extremely time consuming procedure. Recently, however, an alternative procedure for the direct measurement of the photon wavefunction was proposed and, for the first time, implemented by J.S. Lundeen and his associates at the National Research Council of Canada. They performed a “weak measurement” on one degree of freedom (the position) followed by a “strong measurement” of the complementary degree of freedom (the momentum). Because the first measurement was performed in a gentle way using a method in which the measured system was very weakly coupled to the measuring device, it did not collapse the wavefunction, and thus the momentum could be measured with high accuracy. This procedure was then repeated many times to determine the position with high accuracy as well. Detailed analysis showed that this measurement procedure is much more time-efficient than tomography. In a series of experiments, Robert Boyd’s research team at the University of Ottawa and the University of Rochester has recently extended the Lundeen method to allow the measurement of the state of a qubit, the fundamental unit of information in quantum information science, thereby providing a second example of a situation in which direct measurement can be used to determine a quantum wavefunction.
Follow-up questions flowing from this work include: Can these methods be applied to still other degrees of freedom and especially to complex systems described by a very large Hilbert space? And, more broadly: Can the direct measurement of the wavefunction, which has often been taken to be an abstract mathematical concept, lead to new conceptual understandings of the nature of quantum mechanics and its philosophical implications?
The probe for answers brings together twelve physicists and philosophers from eight countries, located on four continents, in Ottawa, Canada’s capital city.