  High-accuracy microwave sensing is widely demanded in various fields, ranging
from cosmology to microwave quantum technology. Quantum receivers based on
inorganic solid-state spin systems are promising candidates for such purpose
because of the stability and compatibility, but their best sensitivity is
currently limited to a few pT/$\sqrt{\rm{Hz}}$. Here, by utilising an enhanced
readout scheme with the state-of-the-art solid-state maser technology, we
develop a robust microwave quantum receiver functioned by organic molecular
spins at ambient conditions. Owing to the maser amplification, the sensitivity
of the receiver achieves 6.14 $\pm$ 0.17 fT/$\sqrt{\rm{Hz}}$ which exceeds
three orders of magnitude than that of the inorganic solid-state quantum
receivers. The heterodyne detection without additional local oscillators
improves bandwidth of the receiver and allows frequency detection. The scheme
can be extended to other solid-state spin systems without complicated control
pulses and thus enables practical applications such as electron spin resonance
spectroscopy, dark matter searches, and astronomical observations.
