Wootters [Phys. Rev. Lett. 80, 2245 (1998)] has given an explicit formula for the entanglement of formation of two qubits in terms of what he calls the concurrence of the joint density operator. Wootters’s concurrence is defined with the help of the superoperator that flips the spin of a qubit. We generalize the spin-flip superoperator to a “universal inverter,” which acts on quantum systems of...

The interest on quantum entanglement has dramatically increased during the last two decades due to the emerging field of quantum information theory [1, 2]. It turns out that quantum entanglement may be used as basic resources in quantum information processing and communication. The prominent examples are quantum cryptography, quantum teleportation, quantum error correction codes and quantum com...

In both classical mechanics and quantum mechanics, one can de ne a pure state to be a state that is as completely speci ed as the theory allows. In classical mechanics a pure state might be represented by a point in phase space. In quantum mechanics it is a vector in a complex vector space. Perhaps the most remarkable feature of quantum mechanics, a feature that clearly distinguishes it from cl...