In a new publication in `Physical Review Letters’, researchers at IQOQI Vienna tackle the question of what makes a good quantum reference frame. That is, what properties of a reference system allow for a good quantum description of the world. Surprisingly, they find that it is solely the amount of entanglement between the reference frame and the rest of the world that is responsible for giving a good account of the physical situation. Their results allow to better understand the role of entanglement in a relational formulation of quantum physics.
In a fully relational world, relata such as rods and clocks are treated as physical systems and are included into the description. Treating them fundamentally as quantum systems, they become quantum reference frames and allow for a description of the systems of interest from an internal perspective. As an example, in the Page-Wootters formalism, a system of interest S and a clock system R are described jointly in terms of a time-less state. The evolution of the system S in time then arises from conditioning its state on different states of the clock. In an ideal world, the clock system used in such a description would be a perfect one. However, a more realistic account of reference systems calls for a description of imperfect reference frames. This led researchers at IQOQI to the question of whether there are better or worse imperfect reference frames. Generally, a reference frame is considered the “better” the more asymmetry it induces on the system it describes. They found that it is the amount of entanglement between the quantum reference frame and the system of interest which determines the asymmetry induced on the system. Thus, the more entangled the frame is with the system that is described, the better it can “resolve” its properties. When applied to quantum clocks, that is, temporal quantum reference frames, these results provide a quantitative understanding of the interplay between entanglement and emergent time.