I am going through Gardiner & Zoller's 'Quantum Noise', and following the derivation of the Langevin equations in terms of input/output operators. Let the bath Hamiltonian be:
$$H_B=\hbar\int_{-\infty}^\infty d\omega\omega b^\dagger(\omega)b(\omega)$$
with interaction term:
$$H_I=\int_{-\infty}^\infty d\omega k(\omega)[b^\dagger(\omega)c-c^\dagger b(\omega)]$$
for some system operator $c$ and function $k(\omega)$ which we later take to be constant (first Markov approximation). Suppose we wish to derive the equations of motion for arbitrary system operator $a$. We then define the input operator to be (for some initial time $t_0$):
$$b_{in}(t)=\frac{1}{\sqrt{2\pi}}\int d\omega e^{-i\omega (t-t_0)}b_{t_0}(\omega)$$
I understand that this gives us a nice final form for the Langevin equation:
$$\dot{a}=-\frac{i}{\hbar}[a,H_S]-[a,c^\dagger]\left[ \frac{\gamma}{2}c+\sqrt{\gamma}b_{in}(t) \right]+\left[ \frac{\gamma}{2}c^\dagger+\sqrt{\gamma}b_{in}^\dagger \right](t)[a,c]$$
However, given a situation we then tend to swap $b_{in}$ for whatever input we have to the system. For example if the system is a two level atom being illuminated by a laser, we let $b_{in}$ represent the input laser field. It isn't clear to me at all from the definition why we can do that.
Similarly we define the output operator: $$b_{out}(t)=\frac{1}{\sqrt{2\pi}}\int d\omega e^{-i\omega(t-t_1)}b_{t_1}(\omega)$$ and claim that in general, we measure the output field $b_{out}$. Why is that, and what in particular do we have to measure to be viewing this field?
