An Introduction to Quantum Optics

10Quantum Entanglement

In quantum theory, a particle is allowed to be in a state of coherent superposition between a set of orthogonal states. A vivid picture of this concept is the Schrödinger cat: a cat is alive and dead simultaneously. This would be no surprise if one means a large number of an ensemble of cats with 50% alive and 50% dead. But, Schrödinger cat is a cat. We are talking about a cat being alive and dead simultaneously. In mathematics, the best word to characterize the states of “alive” and “dead” is perhaps “orthogonal.” In quantum mechanics, the superposition of orthogonal states is used to describe the state of a quantum object, or a particle. The superposition principle is indeed a mystery compared to our everyday classical ...