Two kicks, one parity
Phase kickback, iterated. Put q0 and q1 into equal superposition, park q2 in |−⟩, then run CX(q0→q2) and CX(q1→q2). Each CX writes its control bit into the ancilla — but the ancilla is in |−⟩, so each write comes back as a sign on the control register instead. Predict the pattern before you build it: the amplitude of every basis state picks up (−1)^(q0⊕q1), and the ancilla's own |1⟩ half carries its |−⟩ minus. All eight bars sit at 12.5%; the grader checks the signs.
Stuck on the concept? Read Interference: Where the Power Comes From.
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.