Bad Qubits — Learn Quantum by Doing

Multiple choice

Why does plain VQE find only the ground state, and what must be changed to reach excited states like E_1, E_2?

The ground state is the only eigenstate of H, so excited states require adding extra qubits to create new eigenstates
The variational principle bounds the energy from below by E_0, so minimizing always slides toward E_0; reaching excited states requires enforcing orthogonality to lower states
Plain VQE maximizes the energy, so excited states are found simply by minimizing it instead
VQE can only prepare real-amplitude states, so excited states need a complex-valued ansatz