Bad Qubits — Learn Quantum by Doing

Multiple choice

For a system S coupled to a bath B, the total Hamiltonian is written as H = H_S (x) 1_B + 1_S (x) H_B + H_SB. What happens to the system S if the interaction term H_SB equals zero?

S immediately becomes maximally entangled with B
S and B never interact, and S remains a closed system evolving unitarily
S can no longer be assigned any Hamiltonian of its own
S relaxes to the thermal state of the bath