We derive a necessary and sufficient condition for two pure states, each entangled in two remote systems, to be clonable by the sequential access to the two systems. The result shows to what extent the correlation to other systems can be read out from a subsystem without altering its marginal density operators. This extends the standard no-cloning theorem to the case of a subsystem correlated to others.

• Received 1 May 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.4264