  A single spin quantum sensor can quantitatively detect and image fluctuating
electromagnetic fields via their effect on the sensor spin's relaxation time,
thus revealing important information about the target solid-state or molecular
structures. However, the sensitivity and spatial resolution of spin relaxometry
are often limited by the distance between the sensor and target. Here, we
propose an alternative approach that leverages an auxiliary reporter spin in
conjunction with a single spin sensor, a diamond nitrogen vacancy (NV) center.
We show that this approach can realize a 10^4 measurement speed improvement for
realistic working conditions and we experimentally verify the proposed method
using a single shallow NV center. Our work opens up a broad path of inquiry
into a range of possible spin systems that can serve as relaxation sensors
without the need for optical initialization and readout capabilities.
