Ultra-sensitive microwave magnetometry with organic-molecular sensors

Ultra-sensitive microwave sensing is widely demanded in various fields, ranging from cosmology to microwave quantum technology. Quantum magnetometers based on inorganic solid-state spin systems are promising for this because of their stability and biocompatibility, but the sensitivity is currently limited to a few pT/Hz. Here, by utilizing an enhanced readout scheme with state-of-the-art solid-state maser technology, we develop a robust microwave quantum magnetometer based on spins in organic molecules at ambient conditions. Owing to the maser amplification, the sensitivity of the magnetometer reaches 6.1(2) fT/Hz, which is three orders of magnitude better than that of the inorganic solid-state quantum magnetometers. Heterodyne detection without additional local oscillators improves the bandwidth of the sensors and allows determination of the field frequency. The scheme can be extended to other solid-state spin systems without complicated control pulses and thus enables applications, such as electron spin resonance spectroscopy, dark matter searches, and astronomical observations.