Back action evasion in optical lever detection

The optical lever is a centuries old precision measurement technique and is widely used in applications ranging from consumer products and industrial sensors to precision force microscopes for scientific research. However, despite the long history, its quantum limits have yet to be explored. Here, I will talk about the physics of the back-action evasion that can be used to beat standard quantum limit (SQL) in optical lever detection. We developed a simple qualitive ray optics picture and a complementary quantitative multi-mode Gaussian optics description of back-action evasion. We perform a proof-of-principle demonstration of the back-action evasion mechanism in the classical regime, in our optical lever system consisting of a laser reflecting off a node of a high mechanical quality factor, vibrating silicon nitride membrane or string. To evade back-action, we developed a lens system where extra tilting of the reflected light caused by beam-pointing-noise-induced optical torques is cancelled at the quadrant photodetector. We achieve an effective optomechanical cooperativity of about 100 at 4 K in this cavity-less system. We foresee that the demonstration of quantum back action evasion and beating of SQL is possible in the near future as we improve with higher quality devices and better laser power handling. The protocol will be beneficial for future super high precision quantum sensing applications.