Fig. 1: Experimental setup for magnetometry of cultured neurons using a superconducting flux qubit.

a Experimental setup. The flux qubit (blue) works as a sensitive magnetometer. The quantum state of the qubit is read out by a direct current superconducting quantum interference device (DC SQUID) with a room temperature readout circuit (sky blue). The flux qubit is excited by applying a microwave tone to an on-chip microwave (MW) line (purple). Cultured neurons (green) on a parylene-C film are attached to the qubit chip (orange arrow). An in-plane magnetic field B_∥ (pink) is applied to polarize the electron spins in neurons, while a perpendicular magnetic field B_⊥ (blue) is applied to control the operation flux of the qubit. b Stereomicroscope image of the qubit chip. The neurons cultured on a parylene-C film are indicated by the white arrow. Scale bar: 1 mm. Inset: Phase-contrast image of neurons on a parylene-C film. Scale bar: 100 μm. c The principle of magnetometry using a flux qubit. The qubit spectrum with (without) magnetic flux generated by the electron spins in neurons is shown by the blue (pink) curve. The magnetometry is performed by monitoring the change in the qubit resonance frequency at a fixed operation flux (sky blue). The green dotted rectangle corresponds to measured regions in Fig. 2a. It is worth mentioning that the direction of the shift depends on experimental details (e.g., direction of the in-plane field) and can be reverse. d, e Brightfield image of neurons cultured in medium containing d 0.2 μM (control) and e 50 μM of Fe³⁺ stained by nuclear fast red (cell soma; pink) and Prussian Blue with 3,3’-diaminobenzidine (Fe³⁺; dark brown). The white arrows in e indicate examples of the neurons with high Fe³⁺ concentration. Scale bars: 100 μm.