# Versatile High‐Sensitivity EPR Using Superconducting Spiral Microresonators

## TL;DR

A new superconducting microresonator design boosts EPR sensitivity by 1000 times while working with standard equipment.

## Contribution

A three-order-of-magnitude improvement in spin number sensitivity for X-band pulsed EPR using YBCO spiral microresonators.

## Key findings

- Planar spiral microresonators achieve 10⁷ spins/G/Hz sensitivity in pulsed EPR experiments.
- The microresonators are compatible with conventional EPR instrumentation and sample conditions.
- High-fidelity spin control is demonstrated using microfluidic structures matching the resonator mode profile.

## Abstract

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/Hz. The approach significantly advances the applicability of superconducting microresonators as versatile and readily applicable tools for high sensitivity EPR.

A significant sensitivity enhancement in conventional X‐band pulsed EPR is achieved using planar spiral microresonators fabricated from high‐temperature superconducting YBCO. Fully compatible with standard instrumentation, the approach enables high‐fidelity spin control under typical sample conditions. The demonstrated performance opens new possibilities for superconducting microresonators as versatile tools in high‐sensitivity EPR spectroscopy.