Enhanced optomechanical interaction in an unbalanced Michelson-Sagnac interferometer

Quantum optomechanical systems enable the study of fundamental questions on the quantum nature of massive objects. For that a strong coupling between light and mechanical motion is required, which presents a challenge for massive objects. In particular, large interferometric sensors with low-frequency oscillators are difficult to bring into the quantum regime. Here we propose unbalancing the central beam splitter in the Michelson-Sagnac interferometer, which allows us to boost the optomechanical coupling strength compared with a balanced beam splitter. This unbalancing allows us to enhance the cooperative action of two types of optomechanical coupling present in the system: dissipative and dispersive. We analyze two different configurations, in which the optomechanical cavity is formed by the mirror for the laser pump field (power recycling) and by the mirror for the signal field (signal recycling). We show that the imbalance of the beam splitter allows us to dramatically increase the optical cooling of the test-mass motion. We also formulate the conditions for observing quantum radiation-pressure noise and ponderomotive squeezing. Our configuration could serve as the basis for more complex modifications of the interferometer that would utilize the enhanced coupling strength. This would allow us to efficiently reach the quantum state of large test masses, opening the way to studying the fundamental aspects of quantum mechanics and the experimental search for quantum gravity.