Quantum-Based magnetic sensing: How can birds detect 10 nT magnetic fields?

Migratory birds and other animals can navigate thousands of miles to find their destinations. In this they are aided by a “sixth sense”, i.e. the ability to detect the geomagnetic field. Growing evidence indicates that the threshold of the biological magnetic sensor is on the order of 10 nT. This poses a major challenge for physicists since no mechanism can currently explain how such sensitivity can be realized in a concrete biomolecular system. Light-induced radical-pair reactions can be sensitive to earth-strength magnetic fields and thus may form the basis for the long-sought after magnetic sensors. If this hypothesis was proven, it would be a dramatic demonstration of the use of coherent quantum mechanics to establish biological functionality. We will review the current theoretical questions regarding this mechanism, such as how to design a particularly sensitive receptor system, the role of noise, symmetries and dealing with decoherence before turning to experimental evidence for this hypothesis. We will present a roadmap of key steps that will ultimately enable us to prove or disprove the now nearly twenty year old suggestion that cryptochromes may be the magnetoreceptor molecule and the even older suggestion of a radical-pair mechanism underlying magnetic sensing.