A nanomechanical oscillator is cooled into its quantum ground state using optical radiation pressure. The system consists of a patterned silicon nanobeam with co-localized acoustic and optical resonances, coupled via radiation pressure. Starting from a bath temperature of approximately 20 K, the 3.68 GHz mechanical mode is cooled to a phonon occupancy of 0.85 ± 0.04, indicating it is in the quantum ground state. This is achieved by using a laser to induce radiation pressure, which cools the mechanical motion. The mechanical oscillator is a simple mass-spring system used in various sensitive measurements, including detecting weak forces and small masses. In the quantum regime, the oscillator has a ground state with finite amplitude uncertainty. Previous experiments have shown that nanomechanical resonators can oscillate in their quantum ground state at very low temperatures. This work demonstrates the cooling of a mechanical oscillator to the quantum ground state using optical radiation pressure, achieving a much higher environmental temperature than previous experiments. The system uses a silicon microchip with an integrated optical and mechanical resonator, allowing for efficient back-action cooling and higher operating temperatures. The mechanical mode is cooled using a laser tuned to the red side of the optical cavity, resulting in optically-induced damping. The mechanical oscillator is then measured using a series of calibrated measurements, including noise power spectral density analysis. The results show that the mechanical mode is cooled to a phonon occupancy of 0.85 ± 0.04, indicating it is in the quantum ground state. The system is also used to measure the mechanical and optical properties of the system through calibrated measurements. The results demonstrate the effectiveness of optical cooling in achieving the quantum ground state for a mechanical oscillator. The work has implications for quantum metrology and the use of mechanical elements as quantum transducers or memory elements. The study also highlights the importance of thermal decoherence time in quantum protocols and devices. The results show that the mechanical oscillator can be cooled to the quantum ground state at room temperature, opening up new possibilities for quantum mechanical experiments. The work is supported by various funding agencies and acknowledges the contributions of several researchers. The study provides a detailed analysis of the optomechanical system, including the derivation of the transduced signal and the quantum mechanical derivation of the observed spectra. The results demonstrate the potential of optomechanical systems for quantum control and measurement.A nanomechanical oscillator is cooled into its quantum ground state using optical radiation pressure. The system consists of a patterned silicon nanobeam with co-localized acoustic and optical resonances, coupled via radiation pressure. Starting from a bath temperature of approximately 20 K, the 3.68 GHz mechanical mode is cooled to a phonon occupancy of 0.85 ± 0.04, indicating it is in the quantum ground state. This is achieved by using a laser to induce radiation pressure, which cools the mechanical motion. The mechanical oscillator is a simple mass-spring system used in various sensitive measurements, including detecting weak forces and small masses. In the quantum regime, the oscillator has a ground state with finite amplitude uncertainty. Previous experiments have shown that nanomechanical resonators can oscillate in their quantum ground state at very low temperatures. This work demonstrates the cooling of a mechanical oscillator to the quantum ground state using optical radiation pressure, achieving a much higher environmental temperature than previous experiments. The system uses a silicon microchip with an integrated optical and mechanical resonator, allowing for efficient back-action cooling and higher operating temperatures. The mechanical mode is cooled using a laser tuned to the red side of the optical cavity, resulting in optically-induced damping. The mechanical oscillator is then measured using a series of calibrated measurements, including noise power spectral density analysis. The results show that the mechanical mode is cooled to a phonon occupancy of 0.85 ± 0.04, indicating it is in the quantum ground state. The system is also used to measure the mechanical and optical properties of the system through calibrated measurements. The results demonstrate the effectiveness of optical cooling in achieving the quantum ground state for a mechanical oscillator. The work has implications for quantum metrology and the use of mechanical elements as quantum transducers or memory elements. The study also highlights the importance of thermal decoherence time in quantum protocols and devices. The results show that the mechanical oscillator can be cooled to the quantum ground state at room temperature, opening up new possibilities for quantum mechanical experiments. The work is supported by various funding agencies and acknowledges the contributions of several researchers. The study provides a detailed analysis of the optomechanical system, including the derivation of the transduced signal and the quantum mechanical derivation of the observed spectra. The results demonstrate the potential of optomechanical systems for quantum control and measurement.