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Quantum mechanics places noise limits and sensitivity restrictions onphysical measurements. The balance between unwanted backaction and theprecision of optical measurements impose a standard quantum limit (SQL) oninterferometric systems. In order to realize a sensitivity below the SQL, it isnecessary to leverage a back-action evading measurement technique, or elseexploit cancellations of any excess noise contributions at the detector. %Manyproof of principle experiments have been performed, but only recently has anexperiment achieved sensitivity below the SQL. In this work, we extend thatinitial demonstration and realize sub-SQL measurement sensitivity nearly twotimes better than previous measurements, and with architecture applicable tointerferometric gravitational wave detectors. In fact, this technique isdirectly applicable to Advanced LIGO, which could observe similar effects witha detuned signal recycling cavity. By exploiting quantum correlations createdby an optical spring, we measure a total sensitivity below the SQL by$\textbf{2.8}$ dB, corresponding to a reduction in the noise power by$\textbf{72}\pm\textbf{5.1}$ \% below the quantum limit. Through the use of adetuned optical spring, this noise reduction is tunable, allowing us to choosethe desired range of frequencies that fall below the SQL. This resultdemonstrates access to sensitivities well below the SQL at frequencies rangesapplicable to LIGO, with the potential to extend the reach of gravitationalwave detectors further into the universe.

Surpassing the Standard Quantum Limit using an Optical Spring