The issue of the ► wave-particle duality of light and matter is commonly illustrated by the ► double-slit experiment, in which a quantum object of relatively well defined momentum (such as a photon, electron, neutron, atom, or molecule) is sent through a diaphragm containing two slits, after which it is detected at a capture screen. It is found that an interference pattern characteristic of wave behaviour emerges as a large number of similarly prepared quantum objects is detected on the screen. This is taken as evidence that it is impossible to ascertain through which slit an individual quantum object has passed; if that were known in every individual case and if the quantum objects behaved as free classical particles otherwise, an interference pattern would not arise.
The notion that a description of atomic objects in terms of definite classical particle trajectories is not in general admissible is prominent in Werner Heisenberg's seminal paper [1] of 1927 on the ► Heisenberg uncertainty principle; there he notes: “I believe that one can fruitfully formulate the origin of the classical ‘orbit’ in this way: the ‘orbit’ comes into being only when we observe it.” In the same year, in his famous Como lecture, Niels Bohr introduced the ► complementarity principle, which entails that definite particle trajectories cannot be defined or observed for atomic objects because according to it their spatiotemporal and causal descriptions are mutually exclusive [2]. Bohr cited the uncertainty relation as a symbolic expression of complementarity but recognized that this relation also offered room for approximately defined simultaneous values of position and momentum. Still in the same year, at the 1927 Solvay conference, Albert Einstein questioned the impossibility of determining the path taken by an individual particle in a double-slit interference experiment [21]; he proposed an experimental scheme wherein he considered it possible to infer through which slit the particle passed, without thereby destroying the interference pattern by measuring the recoil of the double-slitted diaphragm. This was the first instance of a welcher-weg or which-way experiment. As Bohr reported in his 1949 tribute to Einstein [3], he was able to demonstrate that Einstein's proposal was in conflict with the principles of quantum mechanics.
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Preview
Unable to display preview. Download preview PDF.
Primary Literature
Heisenberg, W.: Ü ber den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik. Zeitschrift für Physik 43, 172–198 (1927). The quotation is taken from the English translation in [20].
Bohr, N.: The quantum postulate and the recent development of atomic theory. Nature 121, 580–590 (1928).
Bohr, N.: Discussion with Einstein on Epistemological Problems in Atomic Physics, in Albert Einstein: Philosopher-Scientist, ed. P. Schilpp (Library of Living Philosophers, Evanston, 1949).
Wootters, W.K./Zurek, W.H.: Complementarity in the double-slit experiment: Quantum non-separability and a quantitative statement of Bohr's principle. Physical Review D 19, 473–484 (1979).
Wheeler, J.A.: The past and the delayed-choice double-slit experiment, in Marlow, A.R. (ed.), Mathematical Foundations of Quantum Theory (Academic Press, New York, 1978).
Jacques, V./Wu, E./Grosshans, F./Treussart, F./Grangier, P./Aspect, A./Roch, J.-F.: Experimental realization of Wheelers delayed-choice GedankenExperiment, arxiv:quant-ph/0610241 (2006).
Deutsch, D.: Uncertainty in Quantum Measurements. Physical Review Letters 50, 631–633 (1983).
Mittelstaedt, P./Prieur, A./Schieder, R.: Unsharp particle-wave duality in a photon split beam experiment. Foundations of Physics 17, 891–903 (1987).
Greenberger, D.M./Yasin, A.: Simultaneous wave and particle knowledge in a neutron interferometer. Physics Letters A 128, 391–394 (1988).
Scully, M.O./Englert, B.-G./Walther, H.: Quantum optical tests of complementarity. Nature 351, 111–116 (1991).
Scully, M.O./Drühl, K.: Quantum eraser — A proposed photon correlation experiment concerning observation and delayed choice in quantum mechanics. Physical Review A 25, 2208–2213 (1982).
Kim, Y.-H./Yu, R./Kulik, S.P./Shi, Y./Scully, M.O.: Delayed ‘Choice’ Quantum Eraser. Physical Review Letters 84, 1–5 (2000).
Wheeler, J. A.: Law without law, in Wheeler, J.A./Zurek, W.H. (eds.), Quantum Theory and Measurement (Princeton, New Jersey, 1983, pp. 182–213).
Scully, M.: Quantum Optics (Cambridge University Press, Cambridge, 1997, Section 20.1).
Dürr, S./Nonn, T./Rempe, G.: Origin of quantum-mechanical complementarity probed by a ‘which-way’ experiment in an atom interferometer. Nature 395, 33–37 (1998).
Jaeger, G./Shimony, A./Vaidman, L.: Two interferometric complementarities. Physical Review A 51, 54–67 (1995).
Englert, B.-G.: Fringe Visibility and which-way information: An inequality. Physical Review Letters 77, 2154–2157 (1996).
Englert, B.-G./Bergou, J.A.: Quantitative quantum erasure. Optics Communications 179, 337–355 (2000).
Vigier, J.-P. /Rauch, H.: Proposed neutron interferometry test of Einstein's ‘Einweg’ assumption in the Bohr-Einstein controversy. Physics Letters A 151, 269–275 (1990).
Secondary Literature
Wheeler, J.A./Zurek, W.H. (eds.), Quantum theory and measurement (Princeton, New Jersey, 1983).
Bacciagaluppi, G./Valentini, A.: Quantum mechanics at the crossroads: Reconsidering the 1927 Solvay Conference (Cambridge University Press, UK, 2009).
Feynman, R./Leighton, R./Sands, M.: The Feynman Lectures on Physics Vol. III (Addison Wesley, 1965).
Busch, P./Shilladay, C.R.: Complementarity and uncertainty in Mach-Zehnder interferometry and beyond. Physics Reports 435, 1–31 (2006).
Rauch, H./Werner, S.: Neutron interferometry (Oxford, 2000).
Falkenburg, B.: Particle Metaphysics — A Critical Account of Subatomic Reality (Springer 2007, Sec. 7.4 and 7.5.).
Editor information
Editors and Affiliations
Rights and permissions
Copyright information
© 2009 Springer-Verlag Berlin Heidelberg
About this chapter
Cite this chapter
Busch, P., Jaeger, G. (2009). Which-Way or Welcher-Weg-Experiments. In: Greenberger, D., Hentschel, K., Weinert, F. (eds) Compendium of Quantum Physics. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-70626-7_237
Download citation
DOI: https://doi.org/10.1007/978-3-540-70626-7_237
Published:
Publisher Name: Springer, Berlin, Heidelberg
Print ISBN: 978-3-540-70622-9
Online ISBN: 978-3-540-70626-7
eBook Packages: Physics and AstronomyPhysics and Astronomy (R0)