Spin Squeezing In Two-level Atomic Systems

The study of the atomic spin squeezing is very significant in depressing the quan-tum noise and generating the multipartite entanglement. In this paper, we investigate the atomic spin squeezing under the interaction of a two-level atomic ensemble with a single-mode cavity field via Langevin theory. We consider three different cases for the input light field, i.e., a coherent input field, a squeezed input field, and a thermal field. The results are presented as follows.1. We can use the nonlinear interaction of the coherent light field with the atomic ensemble to generate the atomic spin squeezing. We find the atomic spin squeezing exists in the case of the small atomic decay rate compared to the cavity decay rate, and the atomic spin squeezing exists on the certain system-parameter region. To generate the atomic spin squeezing, a strong correlation between atoms is needed. We calculate the atomic correlation to explain the relevant parameters effects on atomic spin squeezing and we will be able to understand the physical mechanism more clearly.2. The squeezing feature of the squeezed light field would be transferred to the atomic ensemble, which can generate better atomic spin squeezing than the first case. The atomic spin squeezing is symmetric with respect to the zero point of the atomic detuning. If the cavity detuning and atomic detuning are equal to zero, we can obtain the optimized atomic spin squeezing. As the cooperation parameter increases, the atomic spin squeezing degree presents stronger. If the squeezing factor becomes large, the atomic spin squeezing degree becomes strong and the squeezing range becomes narrow. If the atomic decay rate becomes small, the atomic spin squeezing strength becomes strong and the range becomes narrow.3. We can use the nonlinear interaction of the thermal field with the atomic ensemble to generate the atomic spin squeezing. As the average thermal photon number is small, the atomic spin squeezing exists. However, with the increase of the average thermal photon number, the atomic spin squeezing is destroyed and the atomic spin squeezing degree becomes weak and the squeezing range become narrow until the atomic spin squeezing disappears.