The paper presents the realization of a single-phonon nonlinear regime in a solid-state mechanical system, demonstrating the operation of a mechanical qubit. The system, composed of a high-overtone bulk acoustic wave resonator (HBAR) coupled to a transmon superconducting circuit, exhibits a strong phonon anharmonicity that exceeds the decoherence rate by a factor of 6.8. This allows for the initialization, readout, and direct single-qubit gates on the mechanical qubit. The authors show that the mechanical mode remains predominantly mechanical in nature, with the phonon component constituting 89.3% of the dressed state at a specific detuning. They also perform Rabi oscillations, T1 and T2 measurements, and Wigner tomography to demonstrate the full control over the mechanical qubit. The ability to tune the anharmonicity continuously and quickly, combined with the long lifetime and compactness of mechanical systems, makes this platform a promising candidate for quantum simulations, sensing, and information processing.The paper presents the realization of a single-phonon nonlinear regime in a solid-state mechanical system, demonstrating the operation of a mechanical qubit. The system, composed of a high-overtone bulk acoustic wave resonator (HBAR) coupled to a transmon superconducting circuit, exhibits a strong phonon anharmonicity that exceeds the decoherence rate by a factor of 6.8. This allows for the initialization, readout, and direct single-qubit gates on the mechanical qubit. The authors show that the mechanical mode remains predominantly mechanical in nature, with the phonon component constituting 89.3% of the dressed state at a specific detuning. They also perform Rabi oscillations, T1 and T2 measurements, and Wigner tomography to demonstrate the full control over the mechanical qubit. The ability to tune the anharmonicity continuously and quickly, combined with the long lifetime and compactness of mechanical systems, makes this platform a promising candidate for quantum simulations, sensing, and information processing.