  A key challenge for neuroscience is noninvasive, label-free sensing of action
potential (AP) dynamics in whole organisms with single-neuron resolution. Here,
we present a new approach to this problem: using nitrogen-vacancy (NV) quantum
defects in diamond to measure the time-dependent magnetic fields produced by
single-neuron APs. Our technique has a unique combination of features: (i) it
is noninvasive, as the light that probes the NV sensors stays within the
biocompatible diamond chip and does not enter the organism, enabling activity
monitoring over extended periods; (ii) it is label-free and should be widely
applicable to most organisms; (iii) it provides high spatial and temporal
resolution, allowing precise measurement of the AP waveforms and conduction
velocities of individual neurons; (iv) it directly determines AP propagation
direction through the inherent sensitivity of NVs to the associated AP magnetic
field vector; (v) it is applicable to neurons located within optically opaque
tissue or whole organisms, through which magnetic fields pass largely
unperturbed; and (vi) it is easy-to-use, scalable, and can be integrated with
existing techniques such as wide-field and superresolution imaging. We
demonstrate our method using excised single neurons from two invertebrate
species, marine worm and squid; and then by single-neuron AP magnetic sensing
exterior to whole, live, opaque marine worms for extended periods with no
adverse effect. The results lay the groundwork for real-time, noninvasive 3D
magnetic mapping of functional neuronal networks, ultimately with synapse-scale
(~10 nm) resolution and circuit-scale (~1 cm) field-of-view.

Quantum Physics;Mesoscale and Nanoscale Physics;Atomic Physics;Neurons and Cognition