Two-Photon Entanglement and Quantum Reality

Abstract

Abstract One of the most surprising consequences of quantum mechanics is the entanglement of two or more distant particles. In 1935, Einstein-Podolsky-Rosen suggested the first classic two-particle entangled state, and proposed a gedanken-experiment . What was surprising about the EPR state and the outcome of the EPR gedankenexperiment is the following: The value of an observable for neither single particle is determined. However, if one of the particles is measured to have a certain value for that observable, the other one is 100% determined. A simple yet fundamental question was then asked by EPR: “Does a single particle have definite value for an observable, in the course of its travel, regardless of whether we measure it or not?” Quantum mechanics answers: “No.” EPR thought: “It should!” In 1964, J. S. Bell proofed a theorem to show that an inequality must be obeyed by any theories that subject to Einstein's local realism. It is this work that made possible the real-life experimental testing. The progression from gedanken to real experiment in recent years has been greatly aided by the use of Spontaneous Parametric Down Conversion (SPDC). The distinctive entanglement quantum nature of the resulting two-photon state of SPDC has allowed us to demonstrate the “spooky” EPR phenomenon as well as the violation of Bell's inequalities. In addition to reviewing several recent experiments, we introduce a new concept of “biphoton” in this chapter, which may be considered as a different approach to challenge the EPR puzzle.