Extracting an entangled state of n−t qubits from ann-qubit entangled state after errors attsites
Chui‐Ping Yang, Julio R. Gea-Banacloche · Physical Review A · 2001
We present a method to protect the entanglement in a maximally entangled n-qubit state of the Greenberger-Horne-Zeilinger or ``Schr\"odinger cat'' form against decoherence, by pairing up each qubit with another one in such a way that each pair of qubits undergoes collective decoherence. Under these conditions, if t pairs experience an error, we show how one can still, in general, extract a maximally entangled state of the remaining $n\ensuremath{-}t$ pairs. Two versions 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 restriction, provides only partial protection (the resulting state is, generally, not a maximally entangled state), but could nonetheless be useful for the important special case in which the source of error is spontaneous emission.