title: Unconditional quantum-noise suppression via measurement-based quantum feedback.
abstract: We demonstrate unconditional quantum-noise suppression in a collective spin system via feedback control based on quantum nondemolition measurement. We perform shot-noise limited collective spin measurements on an ensemble of 3.7×10(5) laser-cooled (171)Yb atoms in their spin-1/2 ground states. Correlation between two sequential quantum nondemolition measurements indicates -0.80(-0.12)(+0.11) dB quantum-noise suppression in a conditional manner. Our feedback control successfully converts the conditional quantum-noise suppression into the unconditional one without significant loss of the noise reduction level.
authors.affiliation: Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
authors.forename: Ryotaro
authors.initials: R
authors.lastname: Inoue
authors.forename: Shin-Ichi-Ro
authors.initials: SR
authors.lastname: Tanaka
authors.forename: Ryo
authors.initials: R
authors.lastname: Namiki
authors.forename: Takahiro
authors.initials: T
authors.lastname: Sagawa
authors.forename: Yoshiro
authors.initials: Y
authors.lastname: Takahashi