Improving SQL Measurements Using Cross Correlation Measurements

Metrology experiments can be limited by the noise produced by the laserpower or frequency fluctuation. Typically, active power stabilization schemes consisting of an in-loop sensor and a feedback control loop are employed. Those schemes are fundamentally limited by shot noise coupling at the in-loop sensor. Here we demonstrate the effectiveness of cross correlation measurements to further characterize the shot noise present in the system with the goal of removing shot noise from our measurement. This will allow a fuller understanding of the noise present in our system which has previously been shown to beat the standard quantum limit: a limit coming from balance between unwanted backaction and precision of optical measurements on interferometric devices.