We have added the following sentences on p3 (and four new references, including the one to Gangl’s work, in the bibliography): “A number of experiments have already demonstrated how the light transmitted by a cavity can be used to track the motion of atoms trapped inside \cite{Hood98,Hood00,Pinkse00}, and in particular, a theoretical analysis of the information stored in the frequency spectrum has been given in \cite{Gangl00}, showing that atomic motion introduces sidebands either side of the pump frequency. In our case, Bloch oscillations at angular frequency $\omega_{B}$ generates sidebands separated from the pump frequency by $\pm \omega_{B}$ (and harmonics thereof in the strong coupling regime).Because the Bloch frequency is proportional to the applied force $\omega_{B}= F d/\hbar$, where $d=\lambda/2$ is thelattice period, a detection of the spectrum of the transmitted light gives $F$ directly.”
(2) It is very helpful to connect cavity output spectra and forces.