Versatile High-Sensitivity EPR Using Superconducting Spiral Microresonators

Electron paramagnetic resonance (EPR) spectroscopy is a broadly used technique to study paramagnetic centers in diverse fields ranging from biology to quantum technologies. The availability of well-established commercial instrumentation, including features such as rapid sample exchange, has been a key enabler for EPR to be applied widely across disciplines. Here, a three-order-of-magnitude increase is presented in the spin number sensitivity of the commonly used X-band pulsed EPR while retaining full compatibility with conventional instrumentation and typical sample conditions. This approach employs planar spiral-shaped microresonators with 7 nL mode volumes fabricated from yttrium barium copper oxide (YBCO) high-temperature superconductor. A wide range of microwave coupling is achieved by a single microresonator inside a conventional EPR tube, loaded into an EPR cavity. The performance of the spiral microresonators is demonstrated through a suite of pulsed EPR experiments on standard samples, including dipolar and hyperfine spectroscopies. By placing a sample within a microfluidic microstructure fabricated to match the mode profile of the microresonator, a high-fidelity spin control is obtained with a spin-number sensitivity of 10⁷ spins/G/ $\sqrt{\text{Hz}}$ . The approach significantly advances the applicability of superconducting microresonators as versatile and readily applicable tools for high sensitivity EPR.