Quantum mechanics and the covariance of physical laws in quantum reference frames

In physics, every observation is made with respect to a frame of reference. Although refer-ence frames are usually not considered as degrees of freedom, in all practical situations it is aphysical system which constitutes a reference frame. Can a quantum system be consideredas a reference frame and, if so, which description would it give of the world? Here, weintroduce a general method to quantise reference frame transformations, which generalisesthe usual reference frame transformation to a“superposition of coordinate transformations”.We describe states, measurement, and dynamical evolution in different quantum referenceframes, without appealing to an external, absolute reference frame, andfind that entangle-ment and superposition are frame-dependent features. The transformation also leads to ageneralisation of the notion of covariance of dynamical physical laws, to an extension of theweak equivalence principle, and to the possibility of defining the rest frame of a quantumsystem.