Debugging Quantum Circuits

Most quantum circuit bugs come from how the circuit is written, not from the physics. Common ones are a swapped control and target, sign errors a normal histogram can't show, a measurement placed too early, or a misread bit order. Debug step by step. Test the simplest input with a sure answer first. Look at the amplitudes before measurement. And remember that more shots never fix a logic error.

Why do quantum circuits break in strange ways?

A buggy quantum circuit rarely crashes. It runs and makes a histogram. The histogram is just wrong, and sometimes it even looks believable. Four kinds of bug cause most of the pain:

Control/target swaps. A two-qubit gate has its operands, the qubits it acts on, in the wrong order. CX [0,1] means "q0 controls, q1 flips." Write [1,0] by mistake and you built a different circuit.
Phase bugs. These are errors in the sign or angle attached to an amplitude. Chances are the squares of amplitudes. So a wrong sign changes nothing in a direct measurement. But it changes everything once interference happens.
Early measurement. This means measuring a qubit before a gate that needed its superposition. It quietly swaps the quantum branches for an ordinary coin flip.
Bit-order mix-ups. This means misreading which printed digit is which qubit. QPU137 is little-endian: q0 is the right-most digit, the one worth the least. Qiskit uses the same rule.

The simulator gives you one tool that real hardware never will. You can look at the amplitudes themselves before any measurement. Use it. Looking at the state tells apart "my state is wrong" from "my state is right, but I'm misreading the samples."

It is like a car mechanic who can open the hood while the engine runs. Unlike a car, a real quantum chip won't let you peek without changing what's inside. Only the simulator can.