Hadamard Operator
The Hadamard operator is the also known as the √NOT or the square
root of the NOT operator. This is because visually the operator rotates the qubit around the y-axis by π / 2 radians
followed by a rotation along the x-axis by π. The H operator is defined as

which is unitary and Hermitian. Even though it is called the square root of NOT operator,
applying it twice does not create the NOT operator seen in classical computation. Since the
operator is both unitary and Hermitian, applying the operator twice is the same as applying
the identity operator and does not affect the state of the qubit. The figure below is an illustrated
example of the Hadamard operator [1].

[1] Michael A. Nielsen and Isaac L. Chuang. Quantum Computation and Quantum Infor-
mation. Cambridge University Press, 2000.