They're talking about propagating a density matrix instead of a state vector. If you're not familiar, a density matrix is an n x n matrix where n is the number

They're talking about propagating a density matrix instead of a state vector. If you're not familiar, a density matrix is an n x n matrix where n is the number of elements in the corresponding state vector, and contains information akin to a probability distribution over state vectors. However, actual quantum computers propagate state vectors, not density matrices, as far as I know. You'd need to run it a large number of times, or have a large ensemble of identical sets of qbits being propagated with the same algorithm to need a density matrix to describe it, I would have thought. So saying you'd need to simulate a density matrix to simulate a quantum computer confuses me. I know quantum physics but not quantum computing specifically, so I could be wrong - the grandparent comment sounds like they know what they're talking about otherwise.