Chapter 2 clarifies the connection between Bananaworld correlations and quantum correlations with an introduction to the simplest quantum systems, qubits, using polarized photons – particles of light – as the standard example. A “bit” is the basic unit of classical (Shannon) information, but the term is also used to refer to an elementary classical system that can be in one of two states. The corresponding elementary quantum system is referred to as a “qubit,” short for “quantum bit.” A photon is a qubit with respect to its state of polarization. The Born rule specifies the probability that a photon in a certain initial polarization state, prepared by passing the photon through a polarizing filter, will be detected by a photon detector after being directed to pass through a second polarizer called an “analyzer.” A pair of qubits can be in an “entangled” state. Two separated “maximally entangled” photons exhibit nonclassical correlations for appropriate polarization measurements analogous to Bananaworld correlations.
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