Introduction {#introduction .unnumbered}
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Extraordinary progress has been made in the last half-dozen years [@Review; @Kipp] towards the goal of cooling a small mechanical resonator down to its quantum ground state and hence to realise quantum behavior in a macroscopic system. Implementations include cavity cooling of micromirrors on cantilevers [@Metzger; @Arcizet; @Gigan; @Regal]; dielectric membranes in Fabry-Perot cavities [@membrane]; radial and whispering gallery modes of optical microcavities [@Schliess] and nano-electromechanical systems [@NEMS]. Indeed the realizations span 12 orders of magnitude in size [@Kipp], up to and including the LIGO gravity wave experiments. In 2011 two separate experiments [@Teufel; @Chan] achieved sideband cooling of micromechanical and nanomechanical oscillators to the quantum ground state. In Ref. [@Asymm], spectral signatures (in the form of asymmetric displacement noise spectra) of quantum ground state cooling were further investigated. Corresponding advances in the theory of optomechanical cooling have also been made [@Brag; @Paternostro; @Marquardt; @Wilson].

Over the last year or so, a promising new paradigm has been attracting much interest: several groups [@Isart; @Zoller; @Barker; @Ritsch; @Isart2; @Opto1] have now investigated schemes for optomechanical cooling of levitated dielectric particles, including nanospheres, microspheres and even viruses. The important advantage is the elimination of the mechanical support, a 