Entanglement and the nature of reality

This lecture takes a deeper look at entanglement.  Professor Susskind begins by discussing the wave function, which is the inner product of the system's state vector with the set of basis vectors, and how it contains probability amplitudes for the various states.  He relates these probability amplitudes to the expectation values of observables discussed in previous lectures.

He then examines more deeply the difference between product and entangled states.  For product states, the wave function factorises which allows the two (or more) sub-systems to be treated as independent systems.  He also describes the properties of a maximally entangled two spin system, and introduces the concept of density matrices, which express everything we can know about one part of an entangled system.

Professor Susskind then moves on to discuss measurement versus entanglement.  There are two views of measurement: one in which the measuring apparatus becomes entangled with the system under measurement, and the other in which the wave function of the system under measurement collapses when measured.

He then discusses locality beginning with Einstein's famously skeptical phrase "spooky actions at a distance."  He distinguishes between actual instantaneous action at a distance - which is impossible - and simple correlation.  What is strange about quantum mechanics is not correlation in entangled states, but rather that we can know everything about this system as a whole, without knowing anything about the individual states of the entangled elements.  

Professor Susskind concludes the lecture by revisiting the example of the computer simulation from the last lecture, which is an example of Bell's theorem that local hidden variables are not sufficient to explain quantum mechanics.