Controlled NOT

A controlled NOT, often written CNOT, acts on two qubits. It flips the second qubit when the first is in the state that reads 1, and it leaves the second qubit alone when the first is in the state that reads 0.

On a pair that is already certain of a single reading, the rule is the ordinary NOT with a control bit in front of it. On a split of the control, which is what H produces just before this gate in the note on two wires, the flip rewrites one term of the joint amplitudes and leaves the other term standing, so 10 becomes 11 while 00 stays put.