Section 8.1 Introduction
In our final chapter on quantum mechanics we introduce the concept of entanglement. This is a feature of two-particle states (or multi-particle states) in which the probabilities of the particles are linked in ways that cannot be described by classical physics. Entanglement is at the heart of current research in quantum cryptography, quantum computing, and quantum teleportation. As we shall see, it also leads to some extremely counter-intuitive behavior. We can construct entangled states in which particle 1 is a large distance away from particle 2, and yet a measurement on particle 1 can instantly influence particle 2. Einstein called this “spooky action at a distance.”
Finally, we will be able address the question of completeness of quantum states. When we say a spin has a probability of being measured in the spin up state, is this probability just a reflection of our ignorance — the spin really has some value and we just don't know it — or is the particle's spin fundamentally not determined until we make the measurement? For decades physicists assumed that this was an unanswerable question, but in recent years we have found ways to turn this into an experimental test. To learn what the experiments have found, read on!