  The ability to perform noninvasive, non-contact measurements of electric
signals produced by action potentials is essential in biomedicine. A key method
to do this is to remotely sense signals by the magnetic field they induce.
Existing methods for magnetic field sensing of mammalian tissue, used in
techniques such as magnetoencephalography of the brain, require cryogenically
cooled superconducting detectors. These have many disadvantages in terms of
high cost, flexibility and limited portability as well as poor spatial and
temporal resolution. In this work we demonstrate an alternative technique for
detecting magnetic fields generated by the current from action potentials in
living tissue using nitrogen vacancy centres in diamond. With 50pT/$\sqrt{Hz}$
sensitivity, we show the first measurements of sensing from mammalian tissue
with a diamond sensor using mouse muscle optogenetically activated with blue
light. We show these measurements can be performed in an ordinary, unshielded
lab environment and that the signal can be easily recovered by digital signal
processing techniques.
