The measurement postulate is crucial to quantum mechanics. If we measure a quantum system, we can only get one of the eigenvalues of the measured observable, such as position, energy and so on, with a probability. Immediately after the measurement, the system will collapse into the corresponding eigenstate instantly, known as state collapse. It is argued that the non-cloning theorem is actually a result of the measurement postulate, because non-cloning theorem would also hold in classical physics. The possibility of cloning in classical physics is actually the ability to fully measure a classical system, so that a classical state can be measured and prepared.
Click here for original story, Generalizing the measurement postulate in quantum mechanics
Source: Phys.org