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Quantum Electronics and Laser Science Conference

Experimental realization of Popper's experiment

Y.-H. Kim, Yanhua H. Shih

Published Jan 20, 2003• 0 citations
Publisher

Abstract

Summary form only given. In early 1930's, Karl Popper, a metaphysical realist, proposed a thought experiment to support his realistic interpretation of quantum mechanics, which shared the same goal of the EPR gedankenexperiment. Based on the same EPR two-particle entangled state, Popper asked: if the precise position of particle 2 is known due to the precise measurement of the position for its entangled twin particle 1, does particle 2 experience a greater uncertainty in its momentum? This question has been ignored for many years. One may not like Popper's philosophy of quantum mechanics; however, his question is closely related to an important problem in recent hot applications: what one can predict for the remaining parts of an entangled state if one of the subsystems is measured alone? We have realized Popper's experiment with the use of entangled two-photon source of spontaneous parametric down conversion. We conclude from this experiment: in an entangled two-particle system, the precise knowledge of the position of a particle, which is determined by the two-particle entanglement, does not necessarily produce an uncertainty in its momentum.

DOI
10.1109/qels.1999.807308

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Summary form only given. In early 1930's, Karl Popper, a metaphysical realist, proposed a thought experiment to support his realistic interpretation of quantum mechanics, which shared the same goal of the EPR gedankenexperiment. Based on the same EPR two-particle entangled state, Popper asked: if the precise position of particle 2 is known due to the precise measurement of the position for its entangled twin particle 1, does particle 2 experience a greater uncertainty in its momentum? This question has been ignored for many years. One may not like Popper's philosophy of quantum mechanics; however, his question is closely related to an important problem in recent hot applications: what one can predict for the remaining parts of an entangled state if one of the subsystems is measured alone? We have realized Popper's experiment with the use of entangled two-photon source of spontaneous parametric down conversion. We conclude from this experiment: in an entangled two-particle system, the precise knowledge of the position of a particle, which is determined by the two-particle entanglement, does not necessarily produce an uncertainty in its momentum.
An entangled pair of photons (1 and 2) are emitted to opposite directions. A narrow slit is placed in the path of photon 1 to provide precise knowledge of its position on the yyy axis and this also determines the precise yyy position of its twin, photon 2, due to quantum entanglement. Is photon 2 going to experience a greater uncertainty in momentum, i.e., a greater Δpy\Delta p_{y}Δpy​, due to the precise knowledge of its position yyy? The experimental data shows $\Delta y\Delta p_{y}<\hbar $ for photon 2. Can this recent realization of the historical thought experiment of Karl Popper signal a violation of the uncertainty principle?
An entangled pair of photon 1 and 2 are emitted in opposite directions along the positive and negative x -axis. A narrow slit is placed in the path of photon 1 which provides precise knowledge about its position along the y -axis and because of the quantum entanglement this in turn provides precise knowledge of the position y of its twin, photon 2. Does photon 2 experience a greater uncertainty in its momentum, i.e., a greater Δ p y , due to the precise knowledge of its position y ? This is the historical thought experiment of Sir Karl Popper which was aimed to undermine the Copenhagen interpretation in favor of a realistic viewpoint of quantum mechanics. Thispaper reports an experimental realization of the Popper’s experiment. One may not agree with Popper’s position on quantum mechanics; however, it calls for a correct understanding and interpretation of the experimental results. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
An entangled pair of photons (1 and 2) are emitted to opposite directions. A narrow slit is placed in the path of photon 1 to provide precise knowledge of its position on the yyy axis and this also determines the precise yyy position of its twin, photon 2, due to quantum entanglement. Is photon 2 going to experience a greater uncertainty in momentum, i.e., a greater ΔpyΔp_{y}Δpy​, due to the precise knowledge of its position yyy? The experimental data shows $ΔyΔp_{y}
The realization of Karl Popper’s EPR‐like experiment by Shih and Kim (published 1999) produced the result that Popper hoped for: no “action at a distance” on one photon of an entangled pair when a measurement is made on the other photon. This experimental result is interpretable in local realistic terms: each photon has a definite position and transverse momentum most of the time; the position measurement on one photon (localization within a slit) disturbs the transverse momentum of that photon in a non‐predictable way in accordance with the uncertainty principle; however, there is no effect on the other photon (the photon that is not in a slit) no action at a distance. The position measurement (localization within a slit) of the one photon destroys the coherence (entanglement) between the photons; i.e. decoherence occurs. This realistic (albeit retrodictive) interpretation of the Shih‐Kim realization of what Popper called his “crucial experiment” is in accord with Bohr’s original concept of the nature of the uncertainty principle, as being an inevitable effect of the disturbance of the measured system by the measuring apparatus. In this experiment the impact parameter of an incident photon with the centerline of the slit is an uncontrollable parameter of each individual photon scattering event; this impact parameter is variable for every incident photon, the variations being a statistical aspect of the beam of photons produced by the experimental arrangement. These experimental results are also in accord with the proposition of Einstein, Podolski and Rosen’s 1935 paper: that quantum mechanics provides only a statistical, physically incomplete, theory of microscopic physical processes, for the quantum mechanical description of the experiment does not describe or explain the individual photon scattering events that are actually observed; the angle by which an individual photon is scattered is not predictable, because the photon’s impact parameter with the centerline of the slit is not observable, and because the electromagnetic interaction between the photon and the matter forming the walls of the slit is not calculable.
The realization of Karl Popper's experiment by Shih and Kim (published 1999) produced the result that Popper hoped for: no ``action at a distance'' on one photon of an entangled pair when a measurement is made on the other photon. This experimental result is interpretable in local realistic terms: each photon has a definite position and transverse momentum most of the time; the position measurement on one photon (localization in a slit) disturbs the transverse momentum of that photon in a non-predictable way in accordance with the uncertainty principle; however, there is no effect on the other photon (the photon that is not in a slit) no action at a distance. The position measurement (localization within a slit) of the one photon destroys the coherence (entanglement) between the photons; i.e. decoherence occurs. This can be understood physically as an electromagnetic interaction between the photon in the slit and the electrons of the atoms in the surface of the solid that forms the slit.
Popper invented a thought experiment that is alleged to test the Copenhagen interpretation of Quantum mechanics, though not the theory itself. In particular it is alleged to test the application of the Heisenberg uncertainty principle to indirect measurements. The experiment is similar to the EPR thought experiment, but it suffers from some technical flaws. These flaws are examined to see if they affect the validity of the argument; so long as they can be alleviated by technical means, the argument retains its force. The experiment has been recently instantiated, with some resolutions of these technical flaws. The results, on face value seem to vindicate Popper and to indicate a violation of the uncertainty principle. Nevertheless, Kim and Shih, who carried out the experiment, give their own interpretation that suggests that the principle should be considered intact but that the correct way to understand the results is to adopt a new metaphysics. Others suggest different reasons why the results do not amount to a violation of the uncertainty principle. Despite a variety of responses, which are carefully examined, the general point that an interpretation cannot alter the empirical basis of a theory is shown to be true. The importance of Rigolid's work in this connection is emphasized.