Kick the phase back
Put q0 into |+⟩ and q1 into |−⟩, then apply CX with q0 as the control and q1 as the target. Predict the result before you check it: the target comes out exactly as it went in, and the CONTROL is the qubit that changes. All four bars stay at 25%; this one is graded on amplitudes, and the target signs say the |1⟩ branch of q0 has picked up a minus.
Stuck on the concept? Read Gates & Circuits.
Heads up: this one is graded on amplitudes, not the histogram. A correct oracle leaves every bar exactly where it is — the two charts above will match long before you are done.
Select a gate to see what it does, then tap a wire — or Tab to a wire and press Enter — to place it.
Pick a gate, then tap a cell — or Tab into the grid and use the arrow keys and Enter — to place it.
Generate a fresh target state 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.