Reverse Engineering Quantum Black Boxes

I was invited to solve a quantum circuit puzzle by Quantum Intuition (@explore_quantum). And, although the goal was simply to solve the puzzle, I felt that my solution was not intuitive; it worked, but it wasn’t clear to me why it worked. Therefore, I proceeded to reverse engineer the puzzle’s black box, after which my solution finally became intuitive.

By the way, I encourage everyone to check out the Quantum Intuition YouTube channel and subscribe.

the puzzle

This is what I found when I tapped on the link provided to me by Quantum Intuition: input states on the far left, a black box after the input states, and, on the far right, the probabilities of measuring 0 in the z basis. The goal of the puzzle was to “turn off” the three qubits by getting all of them to measure 0 with a probability of 1.

the solution

I recorded an 11-minute video explaining how I solved the puzzle, as well as how I made sense of the solution by reverse engineering…