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Direct approach to realizing quantum filters for high-precision measurements

Quantum noise sets a fundamental limit to the sensitivity of high-precision measurements. Suppressing it can be achieved by using nonclassical states and quantum filters, which modify both the noise and signal response. We find an approach to realizing quantum filters directly from their frequency-domain transfer functions, utilizing techniques developed by the quantum control community. It not only allows us to construct quantum filters that defy intuition, but also opens a path towards the systematic design of optimal quantum measurement devices. As an illustration, we show an optical realization of an active unstable filter with anomalous dispersion, proposed for improving the quantum-limited sensitivity of gravitational-wave detectors.