The two-qubit QFT, missing its last step
This is a two-qubit quantum Fourier transform applied to the input state (|00⟩ + |01⟩)/√2 — the H on q0 is the input, the rest is the transform. The gates are all correct and the output is still wrong: the probabilities land on the right values but the wrong labels. Add the one gate that fixes it, so the histogram reads 50% on |00⟩, 25% on |01⟩, nothing on |10⟩ and 25% on |11⟩.
Stuck on the concept? Read Interference: Where the Power Comes From.
Select a gate to see what it does, then tap a wire — or Tab to a wire and press Enter — to place it.
Generate a fresh fix the bug drill at this difficulty. Every generated puzzle is verified solvable by execution before you see it — the generator runs its own solution through the same simulator and grader that will judge yours.