Sub-kelvin optical cooling of a micromechanical resonator

Micromechanical resonators, when cooled down to near their ground state, can be used to explore quantum effects such as superposition and entanglement at a macroscopic scale(1-3). Previously, it has been proposed to use electronic feedback to cool a high frequency ( 10 MHz) resonator to near its ground state(4). In other work, a low frequency resonator was cooled from room temperature to 18 K by passive optical feedback(5). Additionally, active optical feedback of atomic force microscope cantilevers has been used to modify their response characteristics(6), and cooling to approximately 2 K has been measured(7). Here we demonstrate active optical feedback cooling to 135 +/- 15 mK of a micromechanical resonator integrated with a high-quality optical resonator. Additionally, we show that the scheme should be applicable at cryogenic base temperatures, allowing cooling to near the ground state that is required for quantum experiments - near 100 nK for a kHz oscillator.