[DonGateley]

An amplitude is not a complex number. It is the real number one gets taking the root of the sum of the squares of the real and imaginary part of a complex number. In QM they will be between 0 and 1 and are probabilities.

[davorak]

From memory of quantum class the amplitude is a complex number and are not directly probabilities:

Most convenient reference:

> In quantum mechanics, a probability amplitude is a complex number ...

https://en.wikipedia.org/wiki/Probability_amplitude#A_basic_...

[loup-vaillant]

No matter, that's still a physical quantity. Probability is in the mind.

http://lesswrong.com/lw/oj/probability_is_in_the_mind/