Multimode optomechanical system in the quantum regime

Significance Optomechanics is the field of research studying the interaction of light and mechanical motion of mesoscopic objects. Recently, the quantum mechanical character of this interaction has been of particular interest. So far, experimental research, especially in the quantum regime, has focused on canonical systems with only one optical and mechanical degree of freedom—or mode—, respectively. In this work, we introduce a simple and robust optomechanical system featuring many, highly coherent mechanical modes. We evidence and investigate strong quantum correlations in this system, generated by the presence of this multitude of mechanical modes. This represents a key step toward multimode quantum optomechanics, which offers richer dynamics, new quantum phenomena, and a more accurate representation of real-world mechanical sensors. We realize a simple and robust optomechanical system with a multitude of long-lived (Q > 107) mechanical modes in a phononic-bandgap shielded membrane resonator. An optical mode of a compact Fabry–Perot resonator detects these modes’ motion with a measurement rate (96 kHz) that exceeds the mechanical decoherence rates already at moderate cryogenic temperatures (10 K). Reaching this quantum regime entails, inter alia, quantum measurement backaction exceeding thermal forces and thus strong optomechanical quantum correlations. In particular, we observe ponderomotive squeezing of the output light mediated by a multitude of mechanical resonator modes, with quantum noise suppression up to −2.4 dB (−3.6 dB if corrected for detection losses) and bandwidths ≲90 kHz. The multimode nature of the membrane and Fabry–Perot resonators will allow multimode entanglement involving electromagnetic, mechanical, and spin degrees of freedom.