  In physics, every observation is made with respect to a frame of reference.
Although reference frames are usually not considered as degrees of freedom, in
all practical situations it is a physical system which constitutes a reference
frame. Can a quantum system be considered as a reference frame and, if so,
which description would it give of the world? Here, we introduce a general
method to quantise reference frame transformations, which generalises the usual
reference frame transformation to a "superposition of coordinate
transformations". We describe states, measurement, and dynamical evolution in
different quantum reference frames, without appealing to an external, absolute
reference frame, and find that entanglement and superposition are
frame-dependent features. The transformation also leads to a generalisation of
the notion of covariance of dynamical physical laws, to an extension of the
weak equivalence principle, and to the possibility of defining the rest frame
of a quantum system.
