What happens when you generalize Probability Theory to allow Minus Signs ...

Over at Shtetl-Optimized, Scott Aaronson has a nice introduction to the field of quantum computing disguised as a series of answers to questions he was asked on his recent job interview tour.  I particularly like the following characterization of quantum computing as:

what happens if we generalize probability theory to allow minus signs, and base it on the L₂ norm instead of the L₁ norm

I now feel I know enough to be able to talk authoritatively on the subject over coffee at work.

Owls at Ascot Station

On Friday afternoon I was waiting for my train at Ascot when I noticed this owl pellet on the platform, just beside a lamp-post.  During the previous night, an owl must have alighted on the lamp-post to regurgitate the hard, indigestible parts of the animals it had eaten.  The pellet appears to mostly consist of beetle wing-cases, and the hair and bones of a small shrew or mouse.  You can see the shoulder blades of the latter in the lower left corner of the photo.