I take it some common quantum states take the form: (#|A>1(#|B>2 + #|C>2)) + (#|D>1(#|E>2 + #|F>2)) Here particle 1 is in a superposition of |A> and |D> but is also entangled with particle 2 so that finding 1 to be in |A> will collapse 2 into a |B>/|C> superposition while finding 1 to be in |D> will collapse 2 into a |E>/|F> superposition. Firstly, are there realistic examples of this (i.e. where it matter for practical purposes) in which 2's wave function spreads? Here's a fanciful example: an electron gun g that is about to fire an electron is in a superposition of two places (here and there). This would presumably yield: (#|here>g([e spreads over time])) + (#|there>g([e spreads (slightly differently) over time])) Is there any realistic examples of quantum states which has this form?