A two-level atom has ground state ∣1⟩ and excited state ∣2⟩ separated by ℏω0. It is driven on resonance (ω=ω0) by a classical field producing Rabi frequency ΩR. The system starts at t=0 in ∣1⟩ with amplitudes c1(0)=1, c2(0)=0. Find the transition probability P1→2(t)=∣c2(t)∣2.