Subsystem entanglement and separability in quantum reference frames

We find a necessary condition for subsystems to become entangled after a quantum reference frame transformation.

We then distinguish between subsystems that admit separable descriptions relative to a quantum reference frame, and those that do not.

On the one hand, we show that separable descriptions maximize the entanglement internal to the subsystem, and relate our results with the conservation of entanglement and coherence under quantum reference frame transformations.

On the other hand, systems that do not admit a separable description relative to any subsystem display a form of entanglement that is genuine to the quantum reference frame formalism.