Editing Wave–particle duality (section)

== Which slit experiments<span class="anchor" id="Which way experiment"></span> ==
In a "which way" experiment, particle detectors are placed at the slits to determine which slit the electron traveled through. When these detectors are inserted, quantum mechanics predicts that the interference pattern disappears because the detected part of the electron wave has changed (loss of [[Coherence (physics)|coherence]]).<ref name=FeynmanIII/> Many similar [[Wheeler's delayed-choice experiment|proposals]] have been made and many have been converted into experiments and tried out.<ref name="MaKoflerZeilinger">{{Cite journal |last1=Ma |first1=Xiao-song |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |date=2016-03-03 |title=Delayed-choice gedanken experiments and their realizations |url=https://link.aps.org/doi/10.1103/RevModPhys.88.015005 |journal=Reviews of Modern Physics |language=en |volume=88 |issue=1 |page=015005 |doi=10.1103/RevModPhys.88.015005 |issn=0034-6861|arxiv=1407.2930 |bibcode=2016RvMP...88a5005M |s2cid=34901303 }}</ref> Every single one shows the same result: as soon as electron trajectories are detected, interference disappears.

A simple example of these "which way" experiments uses a [[Mach–Zehnder interferometer]], a device based on lasers and mirrors sketched below.<ref name=SchneiderLaPuma>{{Cite journal |last1=Schneider |first1=Mark B. |last2=LaPuma |first2=Indhira A. |date=2002-03-01 |title=A simple experiment for discussion of quantum interference and which-way measurement |url=https://digital.grinnell.edu/islandora/object/grinnell%3A47/datastream/OBJ/download/A_Simple_Experiment_for_Discussion_of_Quantum_Interference_and_Which-Way_Measurement.pdf |journal=American Journal of Physics |language=en |volume=70 |issue=3 |pages=266–271 |doi=10.1119/1.1450558 |issn=0002-9505}}</ref>

[[File:Mach Zehnder interferometer schematic diagram.jpg|thumb|center|600px|Interferometer schematic diagram]]

A laser beam along the input port splits at a half-silvered mirror. Part of the beam continues straight, passes though a glass [[quarter wave plate|phase shifter]], then reflects downward. The other part of the beam reflects from the first mirror then turns at another mirror. The two beams meet at a second half-silvered beam splitter.

Each output port has a camera to record the results. The two beams show interference characteristic of wave propagation. If the laser intensity is turned sufficiently low, individual dots appear on the cameras, building up the pattern as in the electron example.<ref name=SchneiderLaPuma/>

The first beam-splitter mirror acts like double slits, but in the interferometer case we can remove the second beam splitter. Then the beam heading down ends up in output port 1: any photon particles on this path gets counted in that port. The beam going across the top ends up on output port 2. In either case the counts will track the photon trajectories. However, as soon as the second beam splitter is removed the interference pattern disappears.<ref name=SchneiderLaPuma/>