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. Conventional qubit-noise spectroscopy methods use the qubit itself as a noise sensor, but the qubit’s short coherence time limits sensitivity and confines measurements to low-frequency noise.

We introduce a method for measuring qubit frequency noise using a high-quality three-dimensional superconducting cavity [1]. The method exploits dressed dephasing, a process in which qubit frequency fluctuations are converted into photon loss within a coupled cavity [2]. By preparing a single photon in the cavity and post-selecting on repeated qubit measurement results, we isolate qubit noise-induced photon loss from other decay processes. Artificially generated frequency noise is used to calibrate and validate the scheme, allowing us to bound the intrinsic dressed-dephasing rate at the transmon–cavity detuning frequency to below Γ < 1/300 ms⁻¹. This approach extends noise detection to higher frequencies and rare events, providing a new pathway for probing subtle decoherence mechanisms that may ultimately limit the performance of superconducting qubits.

 

[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).