Xia, YiHuang, GuanhaoBeccari, AlbertoZicoschi, AlessioArabmoheghi, AmiraliEngelsen, Nils J.Kippenberg, Tobias J.2025-03-122025-03-122025-03-112025-02-2110.1103/PhysRevLett.134.0736022-s2.0-85218930318https://infoscience.epfl.ch/handle/20.500.14299/247737The motional sideband asymmetry of a mechanical oscillator interacting with a laser field can be observed when approaching the quantum ground state, where the zero-point energy of the mechanical oscillator becomes a sizable contribution to its motion. In the context of quantum optomechanics, it allows, in principle, calibration-free inference of the thermal equilibrium of a macroscopic mechanical resonator with its optical bath. At room temperature, this phenomenon has been observed in pioneering experiments using levitated nanoparticles. Measuring this effect with solid-state mechanical resonators has been compounded by thermal intermodulation noise, mirror frequency noise and low quantum cooperativity. Here, we sideband-cool a membrane-in-the-middle system close to the quantum ground state from room temperature and observe motional sideband asymmetry in a dual-homodyne measurement. Sideband thermometry yields a minimum phonon occupancy of n¯eff=9.5. Our work provides insights into nonlinear optomechanical dynamics at room temperature and facilitates accessible optomechanical quantum technologies without the need for complex feedback control and cryogenic cooling.falsetext::journal::journal article::research article