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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 Melbourn. 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.|coauthors=Rudolph, Terry and 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>
One final remark may be made on the existence of a quantum reference frame. After all, a reference frame, by definition, has a well-defined position and momentum, while quantum theory, namely [[uncertainty principle]], states that one cannot describe any quantum system with well-defined position and momentum simultaneously, so it seems there is some contradiction between the two. It turns out, an effective frame, in this case a classical one, is used as a reference frame, just as in Newtonian mechanics a nearly inertial frame is used, and physical laws are assumed to be valid in this effective frame. In other words, whether motion in the chosen reference frame is inertial or not is irrelevant.
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