Generating macroscopic quantum systems would allow new probes of quantum gravity and improve precision sensing capabilities, but cooling heavy objects to their motional ground state has proved intractable. We propose and evaluate an experiment which could cool milligram-scale objects to their motional ground state. By developing a scheme where the common tool of cavity-frequency locking becomes equivalent to cooling, which works by diamagnetically levitating one end mirror of a cavity, we show the movable end mirror can reach phonon occupancies $n<1$. We also show through simulations that our system can use the optical spring effect to greatly ease experimental constraints on vibrational isolation and feedback strength, by boosting the extremely low trapping frequency of diamagnetic traps. Furthermore, we characterize noise sources and analytically model the feedback loop to show that this experiment is experimentally possible with current techniques. Our setup provides a possible platform for experiments probing the interface of quantum mechanics and gravity, and significantly improved accelerometry.

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• Optomechanics
• Quantum feedback
• Quantum measurements
• Quantum metrology
• Quantum-to-classical transition
• Bragg structures
• Optical fibers
• Optical microcavities
• Cavity resonators
• Coherent control
• Cooling & trapping
• Laser techniques
• Stochastic differential equations