A. Neumaier said:
This is strange since the region under discussion contains a beam, not a vacuum. Since at least sometimes it contains particles, the state of the region cannot be the vacuum state.
In the Heisenberg picture the statistical operator is time-independent. I describe only the particles, and according to your assumption at ##t=0## there are no particles, i.e., the initial statistical operator is the vacuum.
A. Neumaier said:
But the statistical operator is not an observable. it transforms under time translations not like observables in quantum theory but with a wrong sign. So you cannot apply Born's rule to its matrix elements.
Of course, the time evolution of the stat. op. is different from the time evolution of observable operators. It's more or less arbitrary how you put the time-dependence on the stat. op. and the observable operators. That's the choice of the "picture of time evolution". What's observable are the matrix elements of the stat. op. wrt. the eigenvectors of the observed observable.
The states transform with ##\hat{C}(t,t_0)## according to
$$\hat{\rho}(0)=\hat{C}(t,t_0) \hat{\rho}(t_0) \hat{C}^{\dagger}(t,t_0),$$
where ##\hat{C}## obeys the EoM.
$$\mathrm{i} \partial_t \hat{C}(t,t_0)=\hat{H}_1(t) \hat{C}(t,t_0).$$
The (formal) solution is
$$\hat{C}(t,t_0)=\mathcal{T}_c \exp [-\mathrm{i} \int_{t_0}^{t} \mathrm{d} t' \hat{H}(t')].$$
Where ##\mathcal{T}_c## is the "causal time-ordering operator", ordering operator products with increasing time arguments from right to left.
The observable operators transform with ##\hat{A}(t,t_0)##,
$$\hat{O}(t)=\hat{A}(t,t_0) \hat{O}(t_0) \hat{A}^{\dagger}(t,t_0),$$
where
$$-\mathrm{i} \partial_t \hat{A}(t,t_0) = \hat{H}_2(t) \hat{A}(t,t_0)$$
with the formal solution
$$\hat{A}(t,t_0) = \mathcal{T}_c \exp[+\mathrm{i} \int_{t_0}^t \mathrm{d} t' \hat{H}_2(t')].$$
Finally
$$\hat{H}=\hat{H}_1+\hat{H}_2$$
is the Hamiltonian of the system.
In the Heisenberg picture ##\hat{H}_1=0## and ##\hat{H}_2=\hat{H}##, while in the Schrödinger picture it's ##\hat{H}_1=\hat{H}## and ##\hat{H}_2=0##. The "interaction picture" is the split ##\hat{H}_1=\hat{H}_I## (the "interaction part") and ##\hat{H}_2=\hat{H}_0## (the "free part", i.e., the part in the Hamilton density with is quadratic in the field operators and their derivatives).
A. Neumaier said:
I know how textbook detection theory works, but formulas there are derived there in a piecemeal fashion. I asked for a more fundamental description.
The derivation in the quantum optics books does not start from the state of the lab, which is the fundamental description in qunatum field theory. (If the lab were a single block of ice, you'd also start with the state of the block of ice, and then make approximations to get the standard thermodynamic results.)
You cannot describe the lab quantum mechanically, and fortunately that's not necessary. The description in the quantum optics textbooks is pretty sufficient to explain the high-precision results of quantum-optical experiments.
A. Neumaier said:
Though only incompletely known, we know a large amount about the state of the lab, namely (slightly averaged) values of the effective fields at essentially all points of the lab. This is enough to make the relevant deductions.
Indeed. If this were not the case, I doubt that mankind would have ever discovered any "fundamental laws" of nature ;-).
A. Neumaier said:
What is ''it''? Do you mean the energy density replaces the statistical operator? How can that be, given their opposite behavior under time translations? Moreover, unlike the statistical operator, the energy density is not normalizable.
Now, the energy density is not the statistical operator but representing an observable. In classical as well as quantum theory the intensity is physically nothing else than the (time-averaged) energy density of the em. field. Of course the corresponding distribution can be normalized to 1 since the total field energy is always finite.
A. Neumaier said:
I try my best. I'm a bit surprised that we have to discuss again the very basics of QT.