We developed an on-chip electron spin resonance (ESR) system to investigate the impact of magnetic flux noise on the coherence of superconducting qubits. The ESR setup consisted of a superconducting magnet, a magnetic circuit, and a rectangular copper cavity. A superconducting on-chip LC resonator was embedded inside the cavity, in which radiative losses of the resonator were suppressed, to detect paramagnetic spins on a surface of a substrate. The resonator was coupled to external circuits through input and output antennae with coupling strengths (κ1 and κ2) determined by electromagnetic simulation. The ESR setup was cooled to temperatures between 15 mK to 700 mK using a dilution refrigerator.
To calibrate the ESR setup, a standard reagent called DPPH was placed on the resonator. ESR signals were detected by observing changes in the Q factor of the resonator as a function of in-plane flux bias, indicating coupling between the LC resonant mode and the DPPH paramagnetic spin ensemble. The resonance frequency was measured at ∼8.15 GHz, consistent with the design, while the Q factor was degraded to ∼4,000 due to quasiparticle dissipation in the superconducting film under a larger flux bias. To improve sensitivity in the ESR spectroscopy with DPPH, we adjusted the coupling strengths (κ1/2π and κ2/2π) to be 6.8 kHz and 5.2 kHz, respectively. As a result, a significant reduction in the Q factor was observed at ∼290 mT, corresponding to the ESR spectrum of the DPPH free-radical Zeeman resonance.
In our presentation, we discuss the architecture of our setup including the LC resonator and the cavity design with electromagnetic simulation, experimental characterization of the resonator, and results of the ESR spectrum from the DPPH.