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Complementarity and Information in "Delayed-choice for entanglement swapping"

Brukner, C, Aspelmeyer, M, Anton Zeilinger

Published Jan 1, 2004• 18 citations
OpenAlex

Abstract

Building on Peres's idea of "Delayed-choice for extanglement swapping" we show that even the degree to which quantum systems were entangled can be defined after they have been registered and may even not exist any more. This does not arise as a paradox if the quantum state is viewed as just a representative of information. Moreover such a view gives a natural quantification of the complementarity between the measure of information about the input state for teleportation and the amount of entanglement of the resulting swapped entangled state.

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•Nov 28, 2012•0 citations
Building on Peres's idea of "Delayed-choice for extanglement swapping" we show that even the degree to which quantum systems were entangled can be defined after they have been registered and may even not exist any more. This does not arise as a paradox if the quantum state is viewed as just a representative of information. Moreover such a view gives a natural quantification of the complementarity between the measure of information about the input state for teleportation and the amount of entanglement of the resulting swapped entangled state.
Entanglement swapping is a fascinating generalization of quantum teleportation, where the entanglement between pairs is interchanged leading to entanglement between particles that never interacted. Extending the idea of Peres, we propose an experimental setup where the choice which quantum systems are finally entangled is made at a time after they have been registered and do not even exist anymore. Our proposed setup can be used in Third-Man Quantum Cryptography where a third party can control whether Alice and Bob are able to communicate secretly, but he does know their secret key.
Delayed-choice experiments in quantum mechanics are often taken to undermine a realistic interpretation of the quantum state.More specifically, Healey has recently argued that the phenomenon of delayed-choice entanglement swapping is incompatible with the view that entanglement is a physical relation between quantum systems.This paper argues against these claims.It first reviews two paradigmatic delayed-choice experiments and analyzes their metaphysical implications.It then applies the results of this analysis to the case of entanglement swapping, showing that such experiments pose no threat to realism about entanglement.
It is well known that Wheeler proposed several delayed choice experiments in order to show the impossibility to speak of the way a quantum system behaves before being detected. In a double-slit experiment, when do photons decide to travel by one way or by two ways? Delayed choice experiments seem to indicate that, strangely, it is possible to change the decision of the photons until the very last moment before they are detected. This led Wheeler to his famous sentence: No elementary quantum phenomenon is a phenomenon until it is a registered phenomenon, brought to a close by an irreversible act of amplification. Nevertheless some authors wrote that backward in time effects were needed to explain these results. I will show that in delayed choice experiments involving only one particle, a simple explanation is possible without invoking any backward in time effect. Delayed choice experiments involving entangled particles such as the so called quantum eraser can also be explained without invoking any backward in time effect but I will argue that these experiments cannot be accounted for so simply because they rise the whole problem of knowing what a measurement and a collapse are. A previously presented interpretation, Convivial Solipsism, is a natural framework for giving a simple explanation of these delayed choice experiments with entangled particles. In this paper, I show how Convivial Solipsism helps clarifying the puzzling questions raised by the collapse of the wave function of entangled systems.
It is shown that quantum teleportation and entanglement swapping can be easily derived from Einstein, Podolsky and Rosen correlations and special relativity (SR). A recent proposal of entanglement swapping via delayed-choice experiment is discussed by means of a concrete example. Experiments in decoherence in high-Q cavities are briefly examined from the point of view of SR. It is argued that although decoherence can explain why we never see a Schrödinger ‘cat’ which is dead and alive at the same time, this approach apparently does little to explain the cat's ‘real’ state.
Bohr's complementarity notion is at the heart of quantum physics, and suggests that in a quantum experiment, the observed phenomena are inherently linked to the type of measurement that is performed. For example, sending single photons to an open or closed Mach-Zehnder interferometer leads to the observation of particle or wave behaviours. In 1984, Wheeler proposed a 'Gedanken' experiment in which the experimentalist chooses the interferometer configuration, at will, only after the photon has already passed the input beam-splitter (BSin) of the device. This experiment was realized using a diamond-based single photon source, and it was shown that Bohr's complementarity notion was still obeyed, as expected. It was recently proposed theoretically to take Wheeler's experiment one step further by employing a 'quantum beam-splitter' (QBS) at the interferometer output. In this case, the output beam-splitter (BSout) is placed in a coherent superposition of being absent and present, which also allows to further delay the choice of the measurement type. The type of measurement is determined only after having measured the state of the QBS, which can be (in principle) infinitely delayed. We demonstrate the realization of such an experiment by exploiting entanglement. In particular, we take advantage of pairs of polarization entangled photons. The behaviour of one of the paired photons, called the test photon, is analysed by sending it to a Mach-Zehnder interferometer having an entanglement-enabled QBS at its output. This device is made of a polarization dependent beam-splitter followed by polarization state erasers. The other photon, called the corroborative photon, allows determining the actual state of the QBS, and consequently, which test photon behaviour (wave, particle, or both) is measured. By manipulating the corroborative photon, we demonstrate a continuous morphing from wave to particle behaviour, which refutes too simple views of single photons behaving exclusively as waves or particles. The measurements on the test photon and the interferometer configuration are performed in a strictly delayed manner. The state of the QBS is measured only after the test photon already left the interferometer, and is detected, which means that it is destroyed before the type of measurement it under- goes is determined. We find that even in this configuration, Bohr's complementarity notion, and its extension, are perfectly obeyed. The space-like separation between the measurements invalidates local-hidden variable models associated with pre-existing information about the measurement out- comes. In other words, when the test photon is detected, no information is available about the type of measurement that was performed. This assertion is confirmed by the violation of the Bell's inequalities with more than 10 standard deviations. The presence of entanglement shows furthermore that genuine quantum behaviour for the test photons has been observed. As a conclusion, we have demonstrated the experimental realization of the quantum version of Wheeler's experiment. Trying to explain our observations in classical terms causes severe contradictions, but the results are in perfect agreement with quantum physics, in which the measurement timing order does not matter. The beauty in such experiments is that space and time do not seem to play any role.