Surpassing the Standard Quantum Limit using an Optical Spring

Quantum mechanics places noise limits and sensitivity restrictions on physical measurements. The balance between unwanted backaction and precision of optical measurements impose a standard quantum limit (SQL) on interferometric devices. In order to surpass this limit, this back action effect must be evaded or one must devise a way to cancel unwanted noise. Many proof of principle experiments have been tested, but only until recently has a sub-SQL technique been successfully demonstrated. We show that by exploiting quantum correlations created by an optical spring, we measure a total sensitivity below the SQL by 2.8 dB, corresponding to a reduction in the noise power by 72 % below the limit. This result further pushes the noise limit below the SQL, and at frequencies ranges more applicable to LIGO. Additionally, this technique is directly applicable to Advanced LIGO, which could observe similar effects with a detuned signal recycling cavity.