Editing Quantum superposition (section)

==Consequences==
[[Paul Dirac]] described the superposition principle as follows:
<blockquote>The non-classical nature of the superposition process is brought out clearly if we consider the superposition of two states, ''A'' and ''B'', such that there exists an observation which, when made on the system in state ''A'', is certain to lead to one particular result, ''a'' say, and when made on the system in state ''B'' is certain to lead to some different result, ''b'' say. What will be the result of the observation when made on the system in the superposed state? The answer is that the result will be sometimes ''a'' and sometimes ''b'', according to a probability law depending on the relative weights of ''A'' and ''B'' in the superposition process. It will never be different from both ''a'' and ''b'' [i.e., either ''a'' or ''b'']. ''The intermediate character of the state formed by superposition thus expresses itself through the probability of a particular result for an observation being intermediate between the corresponding probabilities for the original states, not through the result itself being intermediate between the corresponding results for the original states.''<ref name="Dirac1947">{{cite book|author=P.A.M. Dirac|title=The Principles of Quantum Mechanics|year=1947|publisher=Clarendon Press|page=12|edition=2nd}}</ref></blockquote>

[[Anton Zeilinger]], referring to the prototypical example of the [[double-slit experiment]], has elaborated regarding the creation and destruction of quantum superposition:

<blockquote>"[T]he superposition of amplitudes ... is only valid if there is no way to know, even in principle, which path the particle took. It is important to realize that this does not imply that an observer actually takes note of what happens. It is sufficient to destroy the interference pattern, if the path information is accessible in principle from the experiment or even if it is dispersed in the environment and beyond any technical possibility to be recovered, but in principle still ‘‘out there.’’ The absence of any such information is ''the essential criterion'' for quantum interference to appear.<ref name=Zeilinger>{{cite journal | author = Zeilinger A | year = 1999 | title = Experiment and the foundations of quantum physics | journal = Rev. Mod. Phys. | volume = 71 | issue = 2 | pages = S288–S297 | doi=10.1103/revmodphys.71.s288|bibcode = 1999RvMPS..71..288Z }}</ref></blockquote>