Quantum Measurements

Abstract

Measurements of quantum states are a key feature of QIS and QC. Polarized light, optical polarization devices, and light detectors illustrate how we describe quantum measurements and how the results of measurements are related to the properties of the quantum states describing the photon qubits. The key distinction between quantum physics and classical physics is that the results of individual quantum measurements cannot be predicted; quantum mechanics gives us only the probabilities of observing each of the possible measurement outcomes. The famous Born rule expresses the probability interpretation of quantum states. Those states carry all the information about the probabilities of all possible measurements on the corresponding qubit. In particular, the probabilities are proportional to the squares of the amplitudes of the basis states appearing in the overall quantum state. Bob points out that state preparation and state measurement are two sides of the same coin.