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Description
The current measurement scheme for a broadband quantum noise reduction in GW detectors relies on 300 m long and detuned filter cavities, that has been stunningly proven in Advanced LIGO and successfully commissioned in Advanced Virgo, for the run O4.
Nevertheless, they add infrastructural complexity and optical losses on top of an already large instrumental apparatus. Moreover, ET will need longer cavities to effectively achieve its unprecedented sensitivity. The present plan is to build a 1-km-long filter cavity for ET-HF, and two 5-km-long cavities for ET-LF.
Several proposals have been raised in the past years to avoid the presence of filter cavities, while attaining similar or better performances in terms of quantum noise suppression. One of the possible alternatives is represented by the conditional squeezing scheme, where a pair of EPR-entangled squeezed beams propagate in the interferometer, and one of them sees it as a filter cavity. Their parallel homodyne detection and the application of an optimal filter allow for broadband quantum noise reduction.
Despite several experimental demonstrations, the evidence for EPR effect in the audio frequency band of interest for GW detectors is still lacking, and this is the scientific goal of the ongoing experiment presented in this contribution. In particular, the implementation of an analytic simulator is highlighted here. Its purpose is to assess the real feasibility of EPR scheme in GW detectors, accounting for optical losses and decoherence effects.
