Lower bounds for attainable fidelities in entanglement purification

We derive lower bounds for the attainable fidelity of standard entanglement purification protocols when local operations and measurements are subjecte d to errors. We introduce an error parameter which measures the distance be tween the ideal completely positive map describing a purification step and the one in the presence of errors. We derive nonlinear maps for a lower bou nd of the fidelity at each purification step in terms of this parameter. [S 1050-2947(99)01104-X].