title: Multimode optomechanical system in the quantum regime.
abstract: 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.
authors.affiliation: Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.
authors.forename: William Hvidtfelt Padkær
authors.initials: WH
authors.lastname: Nielsen
authors.affiliation: Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.
authors.forename: Yeghishe
authors.initials: Y
authors.lastname: Tsaturyan
authors.affiliation: Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.
authors.forename: Christoffer Bo
authors.initials: CB
authors.lastname: Møller
authors.affiliation: Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.
authors.forename: Eugene S
authors.initials: ES
authors.lastname: Polzik
authors.affiliation: Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark aschlies@nbi.ku.dk.
authors.forename: Albert
authors.initials: A
authors.lastname: Schliesser
keywords: multimode; optomechanics; quantum correlations