Photon Quantum Mechanics Experiments for Undergraduates

 Introduction

Superposition lies at the heart of quantum mechanics.  Feynman has described it as the “only mystery” of quantum mechanics.  But it is mystery enough.  Einstein, Podolsky and Rosen noted that in quantum mechanics superposition of two-particle states implies that the theory is non local.  Although they rejected this possibility, subsequent experiments based on John Bell’s tests for nonlocality, the so-called Bell’s inequalities, have confirmed it.  “Spooky action at a distance,” as Einstein derogated it, seems to occur.  Schrödinger recognized that such non-local properties are consequences of  the superposition of two-particle states that can not be factored into two single-particle states in any representation. He called such non-factorable superpositions “entangled states.”

During the past decade there have been significant advances in the experimental production and study of entangled states.  There are prospects that such states can be used for unbreakable cryptography, innovative parallel computation, and for transferring the properties of a quantum state from one object to another some distance away – so called “quantum teleportation.”  The experiments and techniques developed to explore these potentialities have also made it possible to exhibit basic features of quantum superposition directly and unambiguously.  For example, most traditional experimental demonstrations of the existence of the photon have possible alternative explanations in terms of waves; by using entangled states the possible classical explanations can be eliminated and the quantum nature of the photon can be convincingly demonstrated.  Entangled states make it possible to produce experimentally such consequences of two-particle superposition states as of two-photon interference effects and the violation of Bell’s inequalities.

Parallel to the increasing ease with which various superposition phenomena can be produced there has been the creation of a small body of pedagogical material that helps to make these ideas accessible to undergraduates.  There is an excellent article “Multiparticle interferometry and the superposition principle” by Greenberger, Horne, and Zeilinger¹ and there are David Mermin’s several delightful and insightful articles on Bell’s inequalities especially his “Bringing home the atomic world: Quantum mysteries for any body.”² There is enough such material to support a series of experiments exhibiting the phenomena they explain. 

 Given the existence of suitable pedagogical materials and the advances in the technology for producing entangled states and detecting and counting single photons, it is the right time to develop and implement a set of experiments and the supporting pedagogy that will help undergraduates to understand the some of the above described ramifications of the superposition principle in quantum mechanics.

¹Daniel M. Greenberger, Michael A. Horne, and Anton Zeilinger, “Multiparticle interferometery and the superposition principle,” Phys. Today 46(8) pp. 22-29 (1993).

²N. D. Mermin, “Bringing home the atomic world: Quantum mysteries for anybody,” Am. J. Phys. 49(10) 940- 943 (1981).

C.H. Holbrow/ E.J. Galvez/Colgate U.