The delayed-choice experiments of the type introduced by Wheeler and extended by Englert, Scully, Süssmann and Walther (ESSW) (1992 Z. Naturf. a 47 1175–86), and others, have formed a rich area for investigating the puzzling behaviour of particles undergoing quantum interference. The surprise provided by the original delayed-choice experiment led Wheeler to the conclusion that 'no phenomenon is a phenomenon until it is an observed phenomenon', a radical explanation which implied that 'the past has no existence except as it is recorded in the present'. However Bohm et al (1985 Nature 315 294–97) have shown that the Bohm interpretation (BI) gives a straightforward account of the behaviour of the particle without resorting to such a radical explanation. The subsequent modifications of this experiment led both Aharonov and Vaidman (1996 Bohmian Mechanics and Quantum Theory: An Appraisal ed J T Cushing, A Fine and S Goldstein (Dordrecht: Kluwer) pp 141–54) and ESSW to conclude that the resulting Bohm-type trajectories in these new situations produce unacceptable properties. For example, if a cavity is placed in one arm of the interferometer, it will be excited by a particle travelling down the other arm. In other words, it is the particle that does not go through the cavity that gives up its energy! If this analysis is correct, this behaviour would be truly bizarre and could only be explained by an extreme non-local transfer of energy that is even stronger than that required in an Einstein, Podolsky and Rosen (EPR)-type process. In this paper, we show that this conclusion is not correct and that if the BI is used correctly, it gives a local explanation, which actually corresponds exactly to the standard quantum mechanics explanation offered by ESSW.
The delayed-choice experiments of the type introduced by Wheeler and extended by Englert, Scully, Süssmann and Walther (ESSW) (1992 Z. Naturf. a 47 1175–86), and others, have formed a rich area for investigating the puzzling behaviour of particles undergoing quantum interference. The surprise provided by the original delayed-choice experiment led Wheeler to the conclusion that 'no phenomenon is a phenomenon until it is an observed phenomenon', a radical explanation which implied that 'the past has no existence except as it is recorded in the present'. However Bohm et al (1985 Nature 315 294–97) have shown that the Bohm interpretation (BI) gives a straightforward account of the behaviour of the particle without resorting to such a radical explanation. The subsequent modifications of this experiment led both Aharonov and Vaidman (1996 Bohmian Mechanics and Quantum Theory: An Appraisal ed J T Cushing, A Fine and S Goldstein (Dordrecht: Kluwer) pp 141–54) and ESSW to conclude that the resulting Bohm-type trajectories in these new situations produce unacceptable properties. For example, if a cavity is placed in one arm of the interferometer, it will be excited by a particle travelling down the other arm. In other words, it is the particle that does not go through the cavity that gives up its energy! If this analysis is correct, this behaviour would be truly bizarre and could only be explained by an extreme non-local transfer of energy that is even stronger than that required in an Einstein, Podolsky and Rosen (EPR)-type process. In this paper, we show that this conclusion is not correct and that if the BI is used correctly, it gives a local explanation, which actually corresponds exactly to the standard quantum mechanics explanation offered by ESSW.
The delayed choice experiments of the type introduced by Wheeler and extended by Englert, Scully, Süssmann and Walther [ESSW], and others, have formed a rich area for investigating the puzzling behaviour of particles undergoing quantum interference. The surprise provided by the original delayed choice experiment, led Wheeler to the conclu-sion that “no phenomenon is a phenomenon until it is an observed phenomenon”, a radical explanation which implied that “the past has no existence except as it is recorded in the present”. However Bohm, Dewdney and Hiley have shown that the Bohm interpretation gives a straightforward account of the behaviour of the particle without re-sorting to such a radical explanation. The subsequent modifications of this experiment led both Aharonov and Vaidman and [ESSW] to conclude that the resulting Bohm-type trajectories in these new sit-uations produce unacceptable properties. For example, if a cavity is
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.
Abstract David Bohm published his “Suggested Interpretation of Quantum Theory in Terms of Hidden Variables” some twenty five years after Louis de Broglie first presented his similar Pilot Wave theory of quantum mechanics. In the following 30 years what became known as the de Broglie–Bohm approach to quantum theory was to a large extent ignored within the physics community. Even David Bohm himself became somewhat disillusioned with the lack of impact of his interpretation of quantum theory and he directed his interest elsewhere. But some 27 years after Bohm had published his interpretation of quantum theory, interest was rekindled in part by new, detailed calculations that demonstrated clearly and graphically, exactly how his interpretation explained quantum phenomena in terms of well defined individual particle trajectories. These computations encompassed two-slit interference, quantum tunnelling, neutron interferometry, Wheeler’s delayed choice experiment, orbital and intrinsic angular momentum, quantum measurement and Einstein–Podolsky–Rosen nonlocal correlations for orbital angular momentum, intrinsic angular momentum and correlated particle interferometry. Since then, the acceptance of the validity of de Broglie–Bohm theory has steadily grown, as has the interest in the consequences of the approach. For my contribution to the current celebratory volume I was asked to provide a personal review specifically of this novel work within its historical context of the last quarter of the twentieth century.
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.
In recent years there has been an upsurge in the discussion of so-called delayed-choice experiments, as exemplified in the interferometer experiment discussed by Wheeler [1]. In commenting on this experiment Bohm et al. [2] claim that their quantum potential interpretation gives results entirely compatible with Bohr's views on wholeness and complementarity. It seems to me, however, that this discussion to a large extent completely misses the fundamental point in Bohr's argumentation as given in his famour answer [3] to Einstein, Podolsky and Rosen as well as in his paper on his discussions with Einstein [4]. Expressing it a bit paradoxically one might say that in the examples discussed the experiments are not sufficiently delayed. I shall therefore here discuss a different thought experiment, much closer to Bohr's own examples, which brings out this point.
A two-slit experiment with atoms, proposed by Englert, Scully, and Walther, appears to permit experimenters to choose, after each atom has made its mark on the screen, whether the atom has passed through a particular slit or has, in some sense, passed through both of them. Through a misleading wording these authors even appear to endorse this interpretation. In actual fact, this choice exists only until the atom hits the screen. The said experiment thus is a ‘‘delayed-choice’’ experiment only in the semantically contingent sense of Wheeler.