Editing Double-slit experiment (section)

==="Which-way" experiments and the principle of complementarity===
A well-known [[thought experiment]] predicts that if particle detectors are positioned at the slits, showing through which slit a photon goes, the interference pattern will disappear.<ref name="Feynman" /> This which-way experiment illustrates the [[complementarity (physics)|complementarity]] principle that photons can behave as either particles or waves, but cannot be observed as both at the same time.<ref>{{cite web
  | last = Harrison
  | first = David
  | title = Complementarity and the Copenhagen Interpretation of Quantum Mechanics
  | work = UPSCALE
  | publisher = Dept. of Physics, U. of Toronto
  | year = 2002
  | url = http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Complementarity/CompCopen.html
  | access-date = 21 June 2008}}</ref><ref>{{cite web
  | last = Cassidy
  | first = David
  | title = Quantum Mechanics 1925–1927: Triumph of the Copenhagen Interpretation
  | work = Werner Heisenberg
  | publisher = American Institute of Physics
  | year = 2008
  | url = http://www.aip.org/history/heisenberg/p09.htm
  | access-date = 21 June 2008
  | archive-date = 14 January 2016
  | archive-url = https://web.archive.org/web/20160114094044/https://www.aip.org/history/heisenberg/p09.htm
  }}</ref><ref>{{cite conference
  | first = María C.
  | last = Boscá Díaz-Pintado
  | title = Updating the wave–particle duality
  | book-title = 15th UK and European Meeting on the Foundations of Physics
  | date = 29–31 March 2007
  | location = Leeds, UK
  | url = http://philsci-archive.pitt.edu/archive/00003568/
  | access-date = 21 June 2008}}</ref>
Despite the importance of this thought experiment in the history of quantum mechanics (for example, see the discussion on [[Bohr–Einstein debates#Post-revolution: First stage|Einstein's version of this experiment]]), technically feasible realizations of this experiment were not proposed until the 1970s.<ref name=Bartell1980>{{Cite journal | last1 = Bartell | first1 = L. | title = Complementarity in the double-slit experiment: On simple realizable systems for observing intermediate particle-wave behavior | doi = 10.1103/PhysRevD.21.1698 | journal = Physical Review D | volume = 21 | issue = 6 |pages=1698–1699 | year = 1980 |bibcode = 1980PhRvD..21.1698B }}</ref> (Naive implementations of the textbook thought experiment are not possible because photons cannot be detected without absorbing the photon.) Currently, multiple experiments have been performed illustrating various aspects of complementarity.<ref name="Zeilinger1999">{{Cite journal |last1=Zeilinger |first1=A. |author-link=Anton Zeilinger |year=1999 |title=Experiment and the foundations of quantum physics |journal=Reviews of Modern Physics |volume=71 |issue=2 |pages=S288–S297 |bibcode=1999RvMPS..71..288Z |doi=10.1103/RevModPhys.71.S288}}</ref>

An experiment performed in 1987<ref name="Mittelstaedt">{{cite journal
  | author=P. Mittelstaedt
  |author2=A. Prieur |author3=R. Schieder
   | title=Unsharp particle-wave duality in a photon split-beam experiment
  | journal=Foundations of Physics
  | volume=17
  | issue=9
  | pages=891–903
  | year=1987
  | doi=10.1007/BF00734319
|bibcode = 1987FoPh...17..891M |s2cid=122856271 }}</ref><ref>D.M. Greenberger and A. Yasin, "Simultaneous wave and particle knowledge in a neutron interferometer", ''Physics Letters'' '''A 128''', 391–4 (1988).</ref> produced results that demonstrated that partial information could be obtained regarding which path a particle had taken without destroying the interference altogether. This "wave-particle trade-off" takes the form of an [[Wave–particle duality relation|inequality]] relating the visibility of the interference pattern and the distinguishability of the which-way paths.<ref>{{Cite journal |last=Sen |first=D. |date=2014 |title=The uncertainty relations in quantum mechanics |url=https://www.jstor.org/stable/24103129 |journal=Current Science |volume=107 |issue=2 |pages=203–218 |jstor=24103129 |issn=0011-3891 |quote="However, the 'wave-particle trade-off is now expressed in terms of an inequality, known as Englert-Greenberger duality or simply wave-particle duality relation". See also ref 24 in this work.}}</ref>