Extracting an entangled state of n-t qubits from an n-qubit entangled state after errors at t sites - art. no. 032309

We present a method to protect the entanglement in a maximally entangled n- qubit state of the Greenberger-Horne-Zeilinger or "Schrodinger cat" form ag ainst decoherence, by pairing up each qubit with another one in such a way that each pair of qubits undergoes collective decoherence. Under these cond itions, if t pairs experience an error, we show how one can still, in gener al, extract a maximally entangled state of the remaining n-t pairs. Two ver sions of the method are considered: one of them applies when the locations of the decohered qubits are known, and it might be useful, for instance, in secret-sharing protocols. Another version, which is free from this restric tion, provides only partial protection (the resulting state is, generally, not a maximally entangled state), but could nonetheless be useful for the i mportant special case in which the source of error is spontaneous emission.