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The building blocks of Nature, namely atoms and elementary particles, aredescribed by quantum mechanics. This fundamental theory is the ground on whichphysicists have built their major mathematical models [1]. Today, the uniquefeatures of quantum objects have led to the advent of promising quantumtechnologies [2, 3]. However, the macroscopic world is manifestly classical,and the nature of the quantum-to-classical crossover remains one of the mostchallenging open question of Science to date. In this respect, moving objectsplay a specific role [4, 5]. Pioneering experiments over the last few yearshave begun exploring quantum behaviour of micron-sized mechanicalsystems,either by passively cooling single GHz modes, or by adapting lasercooling techniques developed in atomic physics to cool specific modes far belowthe temperature of their surroundings [6-11]. Here instead we describe a verydifferent approach, passive cooling of a micromechanical system down to 500microK, reducing the average number of quanta in the fundamental vibrationalmode at 15 MHz to just 0.3 (with even lower values expected for higherharmonics); the challenge being to be still able to detect the motion withoutdisturbing the system noticeably. With such an approach higher harmonics andthe surrounding environment are also cooled, leading to potentially much longermechanical coherence times, and enabling experiments questioning mechanicalwave-function collapse [12], potentially from the gravitational background [13,14], and quantum thermodynamics [15]. Beyond the average behaviour, here wealso report on the fluctuations of the fundamental vibrational mode of thedevice in-equilibrium with the cryostat. These reveal a surprisingly complexinterplay with the local environment and allow characteristics of two distinctthermodynamic baths to be probed.

A macroscopic object passively cooled into its quantum ground state of motion: beyond single-mode cooling