Cooling of micromechanical resonators towards the quantum
mechanical ground state in their centre-of-mass motion has
advanced rapidly in recent years1’8. This work is an important
step towards the creation of ‘Schrödinger cats’, quantum
superpositions of macroscopic observables, and the study
of their destruction by decoherence. Here we report optical
trapping of glass microspheres in vacuum with high oscillation
frequencies, and cooling of the centre-of-mass motion from
room temperature to a minimum temperature of about 1.5 mK.
This new system eliminates the physical contact inherent
to clamped cantilevers, and can allow ground-state cooling
from room temperature9’15. More importantly, the optical
trap can be switched off, allowing a microsphere to undergo
free-fall in vacuum after cooling15. This is ideal for studying
the gravitational state reduction16’19, a manifestation of the
apparent conflict between general relativity and quantum
mechanics16,20. A cooled optically trapped object in vacuum can
also be used to search for non-Newtonian gravity forces at
small scales21, measure the impact of a single air molecule14
and even produce Schrödinger cats of living organisms9.