A resonator system for electron spin resonance (ESR) is disclosed. The resonator system comprises: a generally planar resonator layer defining an open-loop gapped by a non-conductive gap in the layer, and a microwave feed, positioned configured for transmitting microwave to the resonator layer such as to concentrate, with a quality factor of at least 100, a magnetic field within an effective volume of less than 1 nL above the layer.