section_path: Definitions and theories/Quantum mechanics
section_name: Quantum mechanics
paragraph_text: Quantum mechanics
  
  In quantum mechanics, a measurement is an action that determines a particular property (position, momentum, energy, etc.) of a quantum system. Quantum measurements are always statistical samples from a probability distribution; the distribution for many quantum phenomena is discrete. Quantum measurements alter quantum states and yet repeated measurements on a quantum state are reproducible. The measurement appears to act as a filter, changing the quantum state into one with the single measured quantum value. The unambiguous meaning of the quantum measurement is an unresolved fundamental problem in quantum mechanics; the most common interpretation is that when a measurement is performed, the wavefunction of the quantum system " collapses " to a single, definite value.