Taking Apart A Quantum Routine Doesn't Help To Understand It
The case of the HZH-subroutine
Looking at each part of a program individually is the usual approach when we learn a new programming language. So, if you look closely at a Quantum Circuit, you will see a chain of Quantum Gates connected in series. Most examples can even be reduced to three simple Quantum Gates that reflect the Pauli Operators plus the Controlled-NOT Operator. Each Quantum Gate seems to have a fixed purpose: X Gate flips a Bit, Z-Gate flips a Quantum Phase, Y Gate does both, and Controlled-NOT Operator entangles Qubits. This seems clear and predictable. Until it doesn’t.