Equation Icon  Hˆ ( ) --PDC-= ω1ˆa†1ˆa1 + ω2ˆa†2ˆa2 + i χˆa†1ˆa†2e− 2iω0t − χ ∗ˆa1 ˆa2e2iω0t , (71 ) ℏ