The transform that turned the wrong way
This circuit prepares (|00⟩ + i|01⟩)/√2 — the H and S at the front — and then runs what was meant to be the two-qubit QFT, bit-reversal SWAP included. The builder copied the controlled phase from a table of the INVERSE transform: the T-gate block in the middle rotates by −π/2 where the forward QFT wants +π/2. Every probability is right except two: 50% sits on |01⟩ and it belongs on |11⟩. One appended gate can fix it — without touching the T gates at all. Target: 25% on |00⟩, 25% on |10⟩, 50% 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.