  The radical pair mechanism is one of the two main hypotheses to explain the
navigability of animals in weak magnetic fields, enabling e.g. birds to see the
Earth's magnetic field. It also plays an essential role in the field of spin
chemistry. Here, we show how quantum control can be used to either enhance or
reduce the performance of such a chemical compass, providing a new route to
further study the radical pair mechanism and its applications. We study the
role of quantum entanglement in this mechanism, and demonstrate intriguing
connections between radical-pair entanglement and the magnetic field
sensitivity of the compass. Beyond their immediate application to the radical
pair mechanism, these results also demonstrate how state-of-the-art quantum
technologies could potentially be used to probe and control biological
functions.
