The sensitivities of optical magnetometers, atomic clocks and atom interferometers and other quantum metrology devices, whose signals are linear in the number N of active atoms, have practically arrived at their theoretical limit. Further enhancement of the sensitivities of such devices requires the introduction of new physical processes to improve on their present achievements. Many-body collective correlation among the atoms, spins or, in general, quantum systems may prove to be a suitable method. As such 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 a quantum noise limit, the Heisenberg limit. In the present letter a novel operator is introduced that expresses photon-induced excitation exchange that takes into account energy conservation, , where is the lowering operator of the i-th atom, and are photon creation and annihilation operators. Here i and j stand for two different atoms. This operator describes real or virtual photon-assisted excitation exchange between two atoms. Moreover, it conserves the total number of excitations in the joint electromagnetic field and the quantum system. A photon-induced excitation exchange between two atoms is calculated and clearly exhibits correlation and collective effects.

光学磁力计、原子钟和原子干涉仪以及其他量子计量设备的灵敏度，其信号与活性原子数N呈线性关系，实际上已达到其理论极限。进一步提高此类设备的灵敏度需要引入新的物理过程来改进它们目前的成就。原子、自旋或通常的量子系统之间的多体集体关联可能被证明是一种合适的方法。由于这种相关性在散射幅度的强度中引入了干涉项，因此对于N个相关的量子系统，它们可以将信号增强为N ( N  - 1) 。这些相关性将信噪比提高了一个因子N ²并有助于提高量子计量的灵敏度。此外，原子相关性可以提供量子噪声极限，即海森堡极限。在本信中介绍了一个新的算子，它表示考虑能量守恒的光子诱导的激发交换，其中是第i 个原子的降低算子，是光子产生和湮灭算子。这里i和j代表两个不同的原子。该算子描述了两个原子之间的真实或虚拟光子辅助激发交换。此外，它保留了联合电磁场和量子系统中的激发总数。计算了两个原子之间的光子诱导激发交换，并清楚地表现出相关性和集体效应。