Earth’s magnetosphere is a guiding beacon for a variety of species capable of sensing its presence.

Physicists have now discovered two types of sensors in animals that can detect magnetic fields close to the quantum limit, information that could improve our own design of magnetometer devices.

In multiple ways, such as iron-rich cells responding to the field’s pull, or a bias in photoreceptor chemistry at the back of eyes, magnetoreception has emerged through evolutionary history as a means of directing life around the globe.

Curious to know how biological solutions compare with advances in magnetometer technology, University of Crete physicists Iannis Kominis and Efthimis Gkoudinakis evaluated the energy resolution limit of three adaptations, finding at least two of them come within a whisker of the quantum limits of magnetic field detection.

Armed with little more than a suitably magnetized slither of iron, humans have navigated the unknown under the direction of Earth’s compass points for thousands of years.

Today, our ability to put an exact number on the strength of a faint or highly confined magnetic field demands a detailed and thorough understanding of the quantum nature of electromagnetism, which not only improves sensitivity but allows us to predict the physical limits of any measurement.

Fundamental to calculating the push and pull of a magnetic field is the ability to gauge the energy contained within. As our ability to measure magnetism becomes increasingly precise, quantum uncertainty increasingly takes over, as if the Universe isn’t quite sure of anything as we continue to focus on its details.

Adding to the challenge is the tendency for quantum-level systems to entangle with their environment, blurring the lines even further on the energy mitigated by a magnetic field.

The energy resolution limit (ERL) is a mix of parameters representing the economics of a quantum system within a sensor’s grasp, which includes an estimate of its uncertainty, the size of the sensed region, and the time or bandwidth over which a measure is made.

The end result is a unit of energy over time, equivalent to a quantum unit known as Planck’s constant, which both allows engineers to compare existing technologies for their level of precision while also evaluating the ability of any potential system to reach, or even exceed what is considered a limit.

To Kominis and Gkoudinakis, calculations of a sensor’s ERL provide the perfect opportunity to hold biological magnetoreception to the quantum standards and see how they fare against our best attempts.

Currently, there are several generalized means by which living things are thought to detect Earth’s magnetic field, referred to as induction, radical pair, and magnetite mechanisms. A fourth, combining magnetite with radical-pair approaches, was also considered.

Induction mechanisms turn the energy within a magnetic field into electrical energy in a biological system, setting off a series of changes that ultimately affect behavior. For example, in 2019 researchers proposed Earth’s magnetic field might create a subtle difference in voltage detectable by hair cells inside a pigeon’s ear canals, affecting its balance.

The radical-pair mechanism involves correlations between unpaired electrons attached to different molecules.

Under a magnetic field, the balance in this pairing will vary enough to affect the nature of chemical reactions, triggering a cascade of biological effects determined by the magnetic field’s orientation.

Magnetite-based magnetoreception is a far more straightforward approach. Tiny crystals of iron-based compounds in an organism’s cells are thought to react to magnetic fields with a force large enough to be detectable, forcing microbial cells to orientate themselves or triggering animals into sensing their north and south from their east and west.

While research in the field is ongoing, and still largely speculative, each mechanism has the potential to be highly sensitive, potentially revealing novel ways we might detect faint or confined signs of magnetic fields.

Calculations made by Kominis and Gkoudinakis find that induction mechanisms don’t come close to a quantum level of sensitivity. Yet measures that employ radical pairing just might come as close to quantum limits as our own tech.

Not only might it point in new directions for innovation, but the findings could inform future experiments into the diverse ways life on Earth has evolved to be guided by the invisible cage of magnetism overhead.

This research was published in PRX Life 3.