By using our recently reported method of coherent scattering into an optical cavity we cool the center of mass motion of a 140 nm diameter silica particle by more than 7 orders of magnitude to < 0.5 phonons along the cavity axis, corresponding to a temperature of 12 μK. The inferred optomechanical coupling rate of approx. 71 kHz places the system well into the regime of strong cooperativity (C ≈ 5). From the measured heating rate of approx. 21 kHz we estimate a coherence time of 7.6 μs – or 15 coherent oscillations – while the particle is optically trapped at a pressure of 1e−6 mbar. The free-fall coherence time is limited by gas scattering to 1.4 μs and would allow for an expansion of the wavepacket by approximately a factor of 3, from the ground state size of 3 pm to 10 pm. We expect that a combination of ultra-high vacuum with optical tweezer manipulation and free-fall dynamics will allow to further expand the spatio-temporal coherence of such nanoparticles by several orders of magnitude, thereby opening up new opportunities for macrosopic quantum experiments.
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