Debugging Quantum Circuits
Most quantum circuit bugs are representation bugs: swapped control and target operands, phase errors that Z-basis histograms cannot show, measurement placed too early, or a misread bit order. Debug methodically — test the smallest deterministic input first, inspect amplitudes before measurement, and remember that more shots never fix a logic error.
Why do quantum circuits break in unfamiliar ways?
A buggy quantum circuit rarely crashes. It runs, produces a histogram, and the histogram is simply wrong — sometimes in ways that look plausible. Four bug families cause most of the pain:
- Control/target swaps — a two-qubit gate with its operands reversed.
CX [0,1]means "q0 controls, q1 flips"; write[1,0]by accident and you built a different circuit. - Phase bugs — errors in the sign or angle attached to an amplitude. Probabilities are the squares of amplitudes, so a wrong sign changes nothing in a direct measurement and everything once interference happens.
- Early measurement — measuring a qubit before a gate that needed its superposition, which quietly replaces coherent branches with a classical coin flip.
- Bit-order confusion — misreading which printed digit is which qubit. QPU137 is little-endian: q0 is the least significant, right-most digit, the same convention Qiskit uses.
The simulator gives you one weapon hardware never will: you can inspect the amplitudes themselves before any measurement. Use it. State inspection separates "my state is wrong" from "my state is right but I am misreading the samples".
You’ve read the opening of chapter 10 — 8 more sections follow, with worked examples and circuits you can run on the page. A free account unlocks every chapter of every course (paid plans aren’t live yet — early readers get everything free).