Noise Spectroscopy in Superconducting Qubits via a High-Quality Cavity Sensor

Qubit performance is fundamentally constrained by environmental noise, which induces energy relaxation and dephasing, leading to reduced coherence times and lower gate fidelities. Noise spectroscopy plays a crucial role in characterizing this noise and serves as the first step toward understanding and mitigating its impact on quantum systems.

3D superconducting cavities with ultrahigh quality factors [1] are emerging not only as a platform for quantum computation but also as ultra-sensitive detectors for quantum systems. We demonstrate how a long-lived superconducting cavity can function as a sensitive probe for detecting and characterizing high-frequency noise in a chip-based superconducting qubit. Our method is based on dressed dephasing, a process where transmon frequency noise is converted into a photon loss channel in a dispersively coupled cavity [2]. We use this method to place a lower bound on noise at the transmon-cavity detuning frequency in our system. This approach provides a novel diagnostic tool for superconducting qubit noise, supporting continued progress in extending coherence times.

 

[1] Milul, O. et al. Superconducting Cavity Qubit with Tens of Milliseconds Single-Photon Coherence Time. PRX Quantum 4 (2023).

[2] Slichter, D. H. et al. Measurement-induced qubit state mixing in circuit QED from Up-converted dephasing noise. Physical Review Letters 109 (2012).