Complex number

A complex number has a real part and an imaginary part, so it carries a size and an angle. An amplitude may be complex, and the chance of a reading is the square of the size, which leaves the angle out of the click.

The imaginary unit i is the number whose square is minus one. The S gate multiplies the amplitude for 1 by i, and the Y gate brings in i and minus i as it swaps. A bare measurement in the 0-and-1 arrangement does not print those angles.