Y gate

Y gate

From the state certain to read 0, Y writes the state certain to read 1, and the bar is amber because the whole state carries a factor of i. A measurement on its own still reads 1.

The Y gate swaps the two amplitudes, then multiplies the new amplitude for 0 by minus i and the new amplitude for 1 by i. Here i is the imaginary unit, the number whose square is minus one. On the Bloch sphere, the globe used to draw a single qubit, that rule is a half turn about the y axis. In the usual textbook drawing that axis points to the right.

From the state certain to read 0, Y produces the state certain to read 1 with a factor of i on the whole state. A measurement on its own still reads 1, and the factor starts to matter when another amplitude is present for it to meet.

Used in