From quantum no-cloning to wave-packet collapse

One aspect of the longstanding 'mystery and weirdness' of wave-packet collapse in quantum mechanics has recently been dissolved by Zurek from an information transfer perspective (Phys. Rev. A 76 (2007) 052110). This result is a significant extension of the original quantum no-cloning theorem (Nature 299 (1982) 802). In this Letter we provide two closely related, but alternative, informational approaches to the orthogonality in wave-packet collapse: The first justifies and refines Zurek's derivation by relaxing the repeatability postulate to a more intuitive and simple one, the second replaces the repeatability postulate by a covariant condition of measuring apparatus. Our derivations illuminate the informational nature of wave-packet collapse.