Quantum disentanglers

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Authors	BŮŽEK Vladimír HILLERY M.
Year of publication	2000
Type	Article in Periodical
Magazine / Source	Physical Review A
MU Faculty or unit	Faculty of Informatics
Citation
Field	Theoretical physics
Keywords	CLONING; STATES; INFORMATION; ENSEMBLES
Description	It is not possible to disentangle a qubit in an unknown state \ psi > from a set of N-1 ancilla qubits prepared in a specific reference state \0 >. That is, it is not possible to perfectly perform the transformation (\ psi ,0,...,0 > + \0,psi,...,0 > +...+\0.0,...,0 psi >) --> \0,...,0 > circle times \ psi >. The question is then how well we can do? We consider a number of different methods of extracting an unknown state from an entangled state formed from that qubit and a set of ancilla qubits in a known state. Measuring the whole system is, as expected, the least effective method. We present various quantum ''devices'' which disentangle the unknown qubit from the set of ancilla qubits. In particular, we present the optimal universal disentangler which disentangles the unknown qubit with a fidelity that does not depend on the state of the qubit, and a probabilistic disentangler which performs the perfect disentangling transformation, but with a probability less than 1.
Related projects:	Informatics as the third methodology Non-sequential Models of Computing -- Quantum and Concurrent Distributed Models of Computing