Millikelvin cooling of an optically trapped microsphere in vacuum

摘要

The apparent conflict between general relativity and quantum mechanicsremains one of the unresolved mysteries of the physical world. According torecent theories, this conflict results in gravity-induced quantum statereduction of "Schr\"odinger cats", quantum superpositions of macroscopicobservables. In recent years, great progress has been made in coolingmicromechanical resonators towards their quantum mechanical ground state. Thiswork is an important step towards the creation of Schr\"odinger cats in thelaboratory, and the study of their destruction by decoherence. A direct test ofthe gravity-induced state reduction scenario may therefore be within reach.However, a recent analysis shows that for all systems reported to date, quantumsuperpositions are destroyed by environmental decoherence long beforegravitational state reduction takes effect. Here we report optical trapping ofglass microspheres in vacuum with high oscillation frequencies, and cooling ofthe center-of-mass motion from room temperature to a minimum temperature of 1.5mK. This new system eliminates the physical contact inherent to clampedcantilevers, and can allow ground-state cooling from room temperature. Aftercooling, the optical trap can be switched off, allowing a microsphere toundergo free-fall in vacuum. During free-fall, light scattering and othersources of environmental decoherence are absent, so this system is ideal forstudying gravitational state reduction. A cooled optically trapped object invacuum can also be used to search for non-Newtonian gravity forces at smallscales, measure the impact of a single air molecule, and even produceSchr\"odinger cats of living organisms.