Quantum entanglement: from Popper's experiment to quantum eraser

Uncertainty, being perhaps the most basic principle of quantum mechanics, d istinguishes the world of quantum phenomena from the realm of classical phy sics. Quantum entanglement, being perhaps the most surprising consequence o f quantum mechanics, on the other hand apparently suggests paradoxes relati ng to or violations of the quantum mechanical uncertainty principle in some experimental situations. Popper's experiment and quantum eraser are two ex amples. Is this a paradox? Are we confronted by a violation of the uncertai nty principle? These questions are addressed in this paper. (C) 2000 Elsevi er Science B.V. All rights reserved.