Atomic quantum superposition state generation via optical probing

Light is a useful tool to probe the state of matter. Performing measurements on a light field, which has interacted with a system, changes the density operator of the system, and this back action has, e.g., been utilized to spin squeeze atoms. Here, we demonstrate that a similar measurement scheme allows preparation of N spin-1/2 atoms in a superposition of a state with most of the atoms in the spin up state and a state with most of the atoms in the spin down state [1]. The protocol exploits the strong coupling regime of cavity QED, achieved experimentally in [2], to reduce the decoherence effects of light field losses, and we use the stochastic master equation derived in [3] to analyze the performance of the setup. In addition to a continuous coherent state probe, we also investigate probing with a continuous beam of squeezed light. Probing with a slightly squeezed vacuum can improve the protocol. [1] A. E. B. Nielsen et al, arXiv:0812.4048. [2] F. Brennecke et al, Nature {\bf450}, 268 (2007); Y. Colombe et al, Nature {\bf450}, 272 (2007). [3] A. E. B. Nielsen et al, Phys. Rev. A {\bf77}, 052111 (2008).