Achieving sufficiently low residual excited-state populations remains a key challenge in superconducting quantum circuits, particularly for protocols operating close to noise limits or requiring repeated qubit initialization. Existing protocols primarily address this challenge through sophisticated control, engineered dissipation, or feedback mechanisms. Here, we demonstrate an alternative approach in which a superconducting qubit is reset using a physically distinct, intrinsically colder phononic bath. Specifically, we interface a transmon with a high-overtone bulk acoustic resonator (HBAR), enabling cooling of the qubit into GHz-frequency modes. Using this approach, we achieve a mean residual excited-state population of the qubit below ${10}^{-4}$ and an upper error bar below $3\times {10}^{-4}$ with 95% confidence, representing an improvement of 1 to 2 orders of magnitude compared with typical reset schemes. These results highlight the potential of phononic baths as a resource for high-fidelity qubit initialization in superconducting circuits.

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• Entropy
• Quantum control
• Quantum information with solid state qubits
• Quantum protocols
• Quantum sensing
• Superconducting qubits
• Acoustic techniques
• Resonance techniques