  Enhancement of the sensitivities of optical magnetometers, atomic clocks and
atom interferometers and other quantum metrology devices requires introducing
new physical processes to improve on their present achievements. Many body
collective correlations among the atoms, spins or, in general, quantum systems
may prove to be a suitable method. As these correlations introduce interference
terms in the intensity of the scattering amplitudes, they may enhance the
signal as $N(N-1)$ for N correlated quantum systems. These correlations enhance
the signal to noise ratio by a factor of $N^2$ and contribute to better
sensitivity in quantum metrology. Moreover atomic correlation may provide
quantum noise limit, Heisenberg limit. In the present communication excitation
exchange induced by photons in a cavity between two atoms is calculated and
clearly exhibits correlation and collective effects. A novel operator is
introduced that expresses photon-induced excitation exchange that takes in
account energy conservation, $V_{ij}=\hat{a}^\dag\sigma_i\sigma_j^\dag\hat{a}$,
$\sigma_i=\left|g\right\rangle_{i}\left\langle e\right|_{i}$ is lowering
operator of $i$-th atom, and $\hat{a}^\dag,\hat{a}$ are photon creation and
annihilation operators. Here $i$ and $j$ stand for two atoms. This operator
describes real or virtual photon assisted dipole-dipole interaction. Moreover,
it conserves the total number of excitations in the joint em field and the
quantum system. Experimental challenges are suggested.
