The initial observations which motived the theory of quantum mechanics, and later the wide body of results which helped to solidify the theory, were based on experiments performed on large ensembles of quantum systems, such as atoms and photons. In quantum theory, the basic mathematical object describing the state of a physical system, the wave functionevolves deterministically in time with dynamics governed by the Schrödinger equation. When observing some physical property of the system, the wavefunction describes the probability that a measurement of an individual quantum system within an ensemble of identically prepared systems will yield a particular measurement result.