  The radical pair reaction underlies the magnetic field sensitivity of
chemical reactions and is suggested to play an important role in both chemistry
and biology. Current experimental evidence is based on ensemble measurements,
however, the ability to probe the radical pair reaction at the single molecule
level would provide valuable information concerning its role in important
biological processes. Here, we propose a scheme to detect the charge
recombination rate in a radical pair reaction under ambient conditions by using
single nitrogen-vacancy center spin in diamond. We demonstrate theoretically
that it is possible to detect the effect of the geomagnetic field on the
radical pair reaction and propose the present scheme as a possible hybrid model
chemical compass.
