Appears in collection : PRACQSYS 2018: Principles and Applications of Control in Quantum Systems

Weak measurement (WM) can amplify otherwise invisible small signals and hence promise great potentials for many precision measurements. I will describe a spectroscopy method based on weak measurement which enables resonance linewidth below the coherence-lifetime-limit. We experimentally demonstrated resonance linewidth about 0.1Hz and magnetometer sensitivity at the fT/Hz0.5 level in a room temperature vapor cell. Then, I show that weak measurement can be also used to enhance spin squeezing, a process that allows metrology with precision below the standard quantum limit. A theoretical proposal will be described, and experiment progress of spin squeezing of 1011 atoms will also be reported.