The physical resources available to access and manipulate the degrees of freedom of a quantum system define the set $A$ of operationally relevant observables. The algebraic structure of $A$ selects a preferred tensor product structure, i.e., a partition into subsystems. The notion of compoundness for quantum systems is accordingly relativized. Universal control over virtual subsystems can be achieved by using quantum noncommutative holonomies

• Received 7 March 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.077901