Linear and angular momenta of photons in the context of "which path" experiments of quantum mechanics

Research output: Chapter in Book/Report/Conference proceedingConference contribution


In optical experiments involving a single photon that takes alternative paths through an optical system and ultimately interferes with itself (e.g., Young's double-slit experiment, Mach- Zehnder interferometer, Sagnac interferometer), there exist fundamental connections between the linear and angular momenta of the photon on the one hand, and the ability of an observer to determine the photon's path through the system on the other hand. This paper examines the arguments that relate the photon momenta (through the Heisenberg uncertainty principle) to the "which path"(German: welcher Weg) question at the heart of quantum mechanics. We show that the linear momenta imparted to apertures or mirrors, or the angular momenta picked up by strategically placed wave-plates in a system, could lead to an identification of the photon's path only at the expense of destroying the corresponding interference effects. We also describe a thought experiment involving the scattering of a circularly-polarized photon from a pair of small particles kept at a fixed distance from one another. The exchange of angular momentum between the photon and the scattering particle in this instance appears to provide the "which path"information that must, of necessity, wipe out the corresponding interference fringes, although the fringe-wipe-out mechanism does not seem to involve the uncertainty principle in any obvious way.

Original languageEnglish (US)
Title of host publicationOptical Trapping and Optical Micromanipulation XIX
EditorsKishan Dholakia, Gabriel C. Spalding
ISBN (Electronic)9781510653801
StatePublished - 2022
EventOptical Trapping and Optical Micromanipulation XIX 2022 - San Diego, United States
Duration: Aug 21 2022Aug 24 2022

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
ISSN (Print)0277-786X
ISSN (Electronic)1996-756X


ConferenceOptical Trapping and Optical Micromanipulation XIX 2022
Country/TerritoryUnited States
CitySan Diego

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Computer Science Applications
  • Applied Mathematics
  • Electrical and Electronic Engineering


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