State Preparation and the Rubik’s Cube
State preparation — the process of loading a known probability mass function or setting a quantum system to a certain known state — is a critical part of quantum algorithm development. It is also important in hybrid classical/quantum algorithms that perform some quantum calculation, which needs to start in a known state, measure the state, perform some classical calculation and then repeat this process as necessary, using different starting states for each iteration.
Is it difficult to prepare a certain quantum state? Quite so. I spoke with Ian Mason recently and he likened state preparation to reversing a Rubik’s cube. If I gave you a scrambled Rubik’s cube, you could learn how to solve and unscramble it. But if I gave you a solved cube and asked you to scramble it to a precisely known state, how difficult would that be?
