Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

If the Universe did start from a single point, then wouldn't all particles be fundamentally entangled? How then could there be a truly "pure" state?

share|improve this question
    
Your question can be rephrased to: "How can there be truly isolated systems?" So the answer is the same :-) –  Siva Apr 20 at 14:39

1 Answer 1

up vote 3 down vote accepted

Pure states are a convenient abstraction for studying tiny, specially-prepared quantum systems. States of complicated systems are never pure.

One makes (very small) systems pure by careful preparation. For example, a simple spin by means of a Stern-Gerlach magnet and a screen where only the up particles can pass. More complex systems need more complex preparation to make them (at least approximately) pure.

Note also that pure states can be entangled; indeed, entanglement is usually defined only for pure states. Thus there is no conflict between entanglement and pureness.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.