Bad Qubits — Learn Quantum by Doing

Multiple choice

Postulate 4 states that a closed quantum system evolves by a unitary operator U with U^dagger U = I. What key property does this guarantee for the state?

It preserves inner products and norms, so a normalized state stays normalized and probabilities still sum to one
It makes the evolution irreversible, with no inverse operator
It collapses the state onto an eigenstate of the Hamiltonian
It steadily decreases the norm, so probability leaks away over time