  We demonstrate magnetometry of cultured neurons on a polymeric film using a
superconducting flux qubit that works as a sensitive magnetometer in a
microscale area. The neurons are cultured in Fe$^{3+}$ rich medium to increase
magnetization signal generated by the electron spins originating from the ions.
The magnetometry is performed by insulating the qubit device from the laden
neurons with the polymeric film while keeping the distance between them around
several micrometers. By changing temperature (12.5 - 200 mK) and a magnetic
field (2.5 - 12.5 mT), we observe a clear magnetization signal from the neurons
that is well above the control magnetometry of the polymeric film itself. From
electron spin resonance (ESR) spectrum measured at 10 K, the magnetization
signal is identified to originate from electron spins of iron ions in neurons.
This technique to detect a bio-spin system can be extended to achieve ESR
spectroscopy at the single-cell level, which will give the spectroscopic
fingerprint of cells.
