Editing Double-slit experiment (section)

===Mach-Zehnder interferometer===
{{Main|Mach–Zehnder interferometer}}
[[File:Mach–Zehnder interferometer.gif|thumb|Light in Mach–Zehnder interferometer produces interference (wave-like behavior) even when being detected one photon at a time (particle-like behavior)]]

The Mach–Zehnder interferometer can be seen as a simplified version of the double-slit experiment.<ref>{{Cite journal |last1=Maries |first1=Alexandru |last2=Sayer |first2=Ryan |last3=Singh |first3=Chandralekha |date=2020-07-01 |title=Can students apply the concept of "which-path" information learned in the context of Mach–Zehnder interferometer to the double-slit experiment? |url=https://pubs.aip.org/ajp/article/88/7/542/1044560/Can-students-apply-the-concept-of-which-path |journal=American Journal of Physics |language=en |volume=88 |issue=7 |pages=542–550 |doi=10.1119/10.0001357 |issn=0002-9505|arxiv=2005.07560 |bibcode=2020AmJPh..88..542M }}</ref> Instead of propagating through free space after the two slits, and hitting any position in an extended screen, in the interferometer the photons can only propagate via two paths, and hit two discrete photodetectors. This makes it possible to describe it via simple linear algebra in dimension 2, rather than differential equations.

A photon emitted by the laser hits the first beam splitter and is then in a superposition between the two possible paths. In the second beam splitter these paths interfere, causing the photon to hit the photodetector on the right with probability one, and the photodetector on the bottom with probability zero.<ref>{{Cite journal |last1=Marshman |first1=Emily |last2=Singh |first2=Chandralekha |date=2016-03-01 |title=Interactive tutorial to improve student understanding of single photon experiments involving a Mach–Zehnder interferometer |url=https://iopscience.iop.org/article/10.1088/0143-0807/37/2/024001 |journal=European Journal of Physics |volume=37 |issue=2 |pages=024001 |doi=10.1088/0143-0807/37/2/024001 |issn=0143-0807|arxiv=1602.06162 |bibcode=2016EJPh...37b4001M }}</ref> Blocking one of the paths, or equivalently detecting the presence of a photon on a path eliminates interference between the paths: both photodetectors will be hit with probability 1/2. This indicates that after the first beam splitter the photon does not take one path or another, but rather exists in a quantum superposition of the two paths.<ref name="vedral">{{cite book |first=Vlatko |last=Vedral |title=Introduction to Quantum Information Science |date=2006 |publisher=Oxford University Press |isbn=978-0-19-921570-6 |oclc=442351498 |author-link=Vlatko Vedral}}</ref>