We are quite used to the idea of measuring quantities: for a recipe, we might need 200 grams of flour and a couch shall not be longer than 2.8 m to still fit into the room. In all of these measurement processes, we compare the object that we want to measure against a reference. In the cases above, these references are a gram and a meter, [[SI Units|units]] of mass and length. And we tacitly make one assumption: the object that we want to measure does not change when we measure it. Flour does not get influenced if we place it on a scale and the couch does not change its length. It is this assumption that breaks when we start measuring systems that obey the laws of [[Quantum Mechanics|quantum mechanics]]. A quantum mechanical system can be in a [[Superposition|superposition]]. The classical measurement device, however, can only show exactly one value. While we can predict the precise change of the [[Wave Function|wave function]] of a quantum system, we cannot predict which state a [[Superposition|superposition]] collapses into. This feature of quantum mechanics is actively used in [[Quantum Communication|quantum communication]] protocols and quantum [[Encryption|encryptions]] to ensure security. While we cannot predict which state a given superposition collapses into, we can predict the probability for it to collapse into a certain state (see also the Q&A article [[Quantum is completely random]]). This idea is not specific to [[Quantum Mechanics|quantum mechanics]]: if a coin is fair, we cannot say whether it will show heads or tails, but we can be sure that both appear the same number of times in the long run. ![[coins.excalidraw.light.svg]] Interestingly, the precise process of measurement is not described by the basic rules of quantum mechanics. It is just assumed that the measurement process follows certain rules. The precise measurement process is a topic of intense debate. Is it a coupling process between the quantum system and a much bigger classical system or does the consciousness of the observer matter? >[!read]- Further Reading >- [[Quantum Mechanics]] >- [[Bloch Sphere]] >- [[Superposition]] >[!ref]- References