Abstract：Cavity QED with ultra-cold atoms is applied as a useful tool to investigate fundamental problems in quantum physics. In our experiment, we detected the collective motion of atomic ensemble trapped in 1D optical lattice in a high finesse cavity，we were able to approach to the standard quantum limit and work on the quantum backaction due to the measurement. I shall also talk about the new research filed we are heading to, i.e., a macro mechanical object is cooled down to its standard quantum limit by laser cooling. This would allow us to learn more about the quantum-classical correspondence and the possibility of quantum machines.
