Work as an external quantum observable and an operational quantum work fluctuation theorem

e propose a definition of externally measurable quantum work in driven systems. Work is given as a quantum observable on a control device which is forcing the system and can be determined without knowledge of the system Hamiltonian (H) over cap (S). We argue that quantum work fluctuation theorems which rely on the knowledge of (H) over cap (S) are of little practical relevance, contrary to their classical counterparts. Using our framework, we derive a fluctuation theorem which is operationally accessible and could in principle be implemented in experiments to determine bounds on free energy differences of unknown systems.