Quantum reference frame: Difference between revisions

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For example, if a [[spin-1/2]] particle is said to be in the state <math>|\uparrow z \rangle</math>, a reference frame is implicitly implied, and it can be understood to be some reference frame with respect to an apparatus in a lab. It is obvious that the description of the particle does not place it in an absolute space, and doing so would make no sense at all because, as mentioned above, absolute space is empirically unobservable. On the other hand, if a magnetic field along y-axis is said to be given, the behaviour of the particle in such field can then be described. In this sense, ''y'' and ''z'' are just relative directions. They do not and need not have absolute meaning.

One can observe that a ''z'' direction used in a laboratory in Berlin is generally totally different from a ''z'' direction used in a laboratory in MelbournMelbourne. Two laboratories trying to establish a single shared reference frame will face important issues involving alignment. The study of this sort of communication and coordination is a major topic in [[quantum information theory]].

Just as in this [[spin-1/2]] particle example, a quantum reference frames are almost always treated implicitly in the definition of quantum states, and the process of including the reference frame in a quantum state is called quantisation/internalisation of reference frame while the process of excluding the reference frame from a quantum state is called dequantisation{{Citation needed|reason="Dequantisation" does not appear to be a common term in this field when applied to reference frames. Please find an example in the literature.|date=April 2014}}/externalisation of reference frame. Unlike the classical case, in which treating a reference internally or externally is purely an aesthetic choice, internalising and externalising a reference frame does make a difference in quantum theory.<ref>{{cite journal|last=Barlett|first=Stephen D. |author2=Rudolph, Terry |author3=Spekkens, Robert W.|title=Dialogue concerning two views on quantum coherences: factist and fictionist|journal=Int. J. Of Quant. Info|volume=4:17|year=2006}}</ref>