Question

Alice and Bob share a state a|++⟩+b|−−⟩, where the first qubit is Alice's and the second qubit is Bob's. Alice measures her qubit in the standard basis and sends the measurement outcome to Bob. If Bob wants his qubit to end up in the state a|0⟩+b|1⟩, what gates does he apply to his qubit in the following two cases? Write your answer in terms of X,Z,H,I using comma as a delimiter.