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Scientific Reports

Experimental Limits of Ghost Diffraction: Popper’s Thought Experiment

Paul‐Antoine Moreau, Peter A. Morris, Ermes Toninelli, Thomas Gregory, Reuben S. Aspden, Gabriel C. Spalding, Robert W. Boyd, Miles J. Padgett

Published Aug 29, 2018• 16 citations
Publisher

Abstract

Quantum ghost diffraction harnesses quantum correlations to record diffraction or interference features using photons that have never interacted with the diffractive element. By designing an optical system in which the diffraction pattern can be produced by double slits of variable width either through a conventional diffraction scheme or a ghost diffraction scheme, we can explore the transition between the case where ghost diffraction behaves as conventional diffraction and the case where it does not. For conventional diffraction the angular extent increases as the scale of the diffracting object is reduced. By contrast, we show that no matter how small the scale of the diffracting object, the angular extent of the ghost diffraction is limited (by the transverse extent of the spatial correlations between beams). Our study is an experimental realisation of Popper's thought experiment on the validity of the Copenhagen interpretation of quantum mechanics. We discuss the implication of our results in this context and explain that it is compatible with, but not proof of, the Copenhagen interpretation.

DOI
10.1038/s41598-018-31429-y

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Quantum ghost diffraction harnesses quantum correlations to record diffraction or interference features using photons that have never interacted with the diffractive element. By designing an optical system in which the diffraction pattern can be produced by double slits of variable width either through a conventional diffraction scheme or a ghost diffraction scheme, we can explore the transition between the case where ghost diffraction behaves as conventional diffraction and the case where it does not. For conventional diffraction the angular extent increases as the scale of the diffracting object is reduced. By contrast, we show that no matter how small the scale of the diffracting object, the angular extent of the ghost diffraction is limited (by the transverse extent of the spatial correlations between beams). Our study is an experimental realisation of Popper's thought experiment on the validity of the Copenhagen interpretation of quantum mechanics. We discuss the implication of our results in this context and explain that it is compatible with, but not proof of, the Copenhagen interpretation.
Abstract In an effort to challenge the Copenhagen interpretation of quantum mechanics, Karl Popper proposed an experiment involving spatially separated entangled particles. In this experiment, one of the particles passes through a very narrow slit, and thereby its position becomes well-defined. This particle therefore diffracts into a large divergence angle; this effect can be understood as a consequence of the Heisenberg uncertainty principle. Popper further argued that its entangled partner would become comparably localized in position, and that, according to his understanding of the Copenhagen interpretation of quantum mechanics, the ‘mere knowledge’ of the position of this particle would cause it also to diffract into a large divergence angle. Popper recognized that such behavior could violate the principle of causality in that the slit could be removed and the partner particle would be expected to respond instantaneously. Popper thus concluded that it was most likely the case that, in an actual experiment, the partner photon would not undergo increased diffractive spreading and thus that the Copenhagen interpretation is incorrect. Here, we report and analyze the results of an implementation of Popper’s proposal. We find that the partner beam does not undergo increased diffractive spreading. Our work helps to clarify the issues raised in Popper’s proposal, and it provides further insight into the nature of entanglement and its relation to the uncertainty principle as applied to correlated particles.
High-resolution ghost image and ghost diffraction are performed by using a single source of pseudo-thermal speckle light divided by a beam splitter. By only operating on the optical setup of the light in the reference arm, that never interacted with the object, we are able to pass from the image to the diffraction pattern. The product of spatial resolutions of the ghost image and ghost diffraction experiments is shown to overcome a limit which was formerly thought to be achievable only with entangled photons. A complementarity between the spatial coherence of the beams and their mutual correlation is demonstrated by showing a complementarity between ghost diffraction and ordinary diffraction patterns.
Abstract An experiment to verify the adequacy of the Heisenberg uncertainty principle, as proposed by Karl Popper and implemented in practice, is considered. As in the Einstein–Podolsky–Rosen paradox, the quantum properties of an entangled pair of elementary particles are used. In this case, a ghost image of a narrow slit is actually formed. The results of the experiment, at first glance, support a violation of the uncertainty principle. However, analysis of the spatial resolution of the slit ghost image shows that this is not correct. A more correct description of diffraction in the case of spatially limited light beams, gives no violation of the uncertainty principle. The results can be also used to estimate the extreme quality of diffraction limited ghost images.
Abstract: Many problems exist in the Double Slit Experiment when you consider that the elements in the test are moving as waves. Outside of the pattern resembling peeks and troughs that overlap each other; there is no real scientific explanation on how this is taking place. In a more accurate and classical way, can we explain how the dark and light areas of the interference pattern are being made? Why is an Interference pattern being made when particles are being sent through the exercise one at a time? And how is it that through observation the Interference pattern collapses in on itself. Waves add more questions than they do answers, a very typical outcome of an incorrect solution. Maybe the unanswered questions left behind by the wave theory is a confirmation of its inaccuracy and not the mystique of its results. Rather than double down on the wave theory with time traveling particles, elements of duality, and the Copenhagen Interpretation; l reconsidered that elements tested in the double slit experiment behave as particles. But this time, I weighed the thought that these bits of matter are much smaller than previously thought. This new logic allowed particles with straight trajectories to interact with the edge of materials used in the double slit wall in a very detailed way. This created a storm of new ways to look at what is really taking place in the test; revealing the competence of the detectors utilized and the capacity of the Nonlinear Optical Crystal. Everything solved from just one idea, that quantum move as particles but on a much smaller scale. This is the enigma of the Double Slit Experiment.
We present a detailed analysis of a previously published realization of Popper’s experiment using entangled ghost imaging. Our analysis, which is based on optical diffraction integrals, shows that, for the configuration previously described, the transverse spread of an unmeasured particle (the signal photon here) does not increase in inverse proportion to the width of its virtual confinement when its entangled twin (the idler photon) is confined in a transverse dimension by a physical slit. Rather, we show that the spread of the unmeasured particle carries no dependence on the width of its virtual confinement in the published configuration. Instead, it spreads geometrically at a rate determined by the numerical aperture of the ghost imaging system. We further propose an alternative configuration for which the spread of the unmeasured particle does increase in inverse proportion to the width of its virtual confinement. We discuss the relation of these results to the predictions of Popper.
Karl Popper had proposed an experiment to test the standard interpretation of quantum mechanics. The proposal survived for many year in the midst of no clear consensus on what results it would yield. The experiment was realized by Kim and Shih in 1999, and the apparently surprising result led to lot of debate. We review Popper's proposal and its realization in the light of current era when entanglement has been well studied, both theoretically and experimentally. We show that the "ghost-diffraction" experiment, carried out in a different context, conclusively resolves the controversy surrounding Popper's experiment. Quanta 2012; 1: 19–32.