Entangled entanglement

In entangled systems values cannot be assigned to all quantum-mechanical observables of individual members of the system independent of the measurement context of the whole system. While various cases are known where properties like spin, momentum, energy, etc. can be entangled, entanglement itself is usually considered to be an objective property of the system. We show that situations can arise where this is no longer the case, and where therefore entanglement itself becomes an entangled property. © 1996 The American Physical Society.