Read the phase bit
The three pre-placed gates prepare Φ− = (|00⟩ − |11⟩)/√2, which the histogram cannot tell apart from the Φ+ you built in the Bell challenge: both read 50/50 on |00⟩ and |11⟩. The minus sign is real physics, and there is a measurement that extracts it. Append gates — keeping the preparation — so the circuit ends on |01⟩ with certainty: q0 reads 1 exactly because the phase was −, and q1 reads 0 exactly because the parity was "equal". Par is 5.
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 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.