Studying phonon coherence with a quantum sensor

Area of Science:

• Quantum technologies
• Nanomechanical systems
• Superconducting circuits

Background:

• Nanomechanical oscillators are crucial for quantum technologies, especially when coupled with superconducting qubits for quantum error correction.
• Mechanical decoherence, particularly from two-level system (TLS) defects, limits the performance of these systems.
• TLS defects have been traditionally studied using classical methods, limiting quantum regime insights.

Purpose of the Study:

• To utilize a superconducting qubit as a quantum sensor for high-resolution phonon number-resolved measurements.
• To investigate mechanical dissipation and dephasing in variable-sized coherent states within a phononic crystal cavity.
• To elucidate the role of TLS defects in quantum decoherence of mechanical oscillators.

Main Methods:

• Employing a superconducting qubit as a quantum sensor.
• Performing phonon number-resolved measurements on a piezoelectrically coupled phononic crystal cavity.
• Utilizing a numerical model to simulate TLS interactions and decoherence.

Main Results:

• Observed nonexponential relaxation dynamics in mechanical coherent states.
• Detected a state size-dependent reduction in the dephasing rate, attributed to TLS.
• Successfully reproduced dissipation signatures using a numerical model with a small ensemble of rapidly dephasing TLS.

Conclusions:

• TLS defects significantly contribute to phonon decoherence in the quantum regime.
• The study provides a detailed examination of TLS-induced decoherence mechanisms.
• Findings offer critical insights for mitigating decoherence in quantum hardware.