Cryptochrome
A protein in a robin's eye that may let it see Earth's magnetic field.
Suspected to be the molecular sensor that lets night-migrating songbirds, sea turtles, and possibly other animals navigate by Earth's geomagnetic field. Activated by blue light, cryptochrome forms a transient pair of unpaired electrons (a radical pair) whose spin state is exquisitely sensitive to weak magnetic fields.
When blue light strikes the flavin cofactor inside cryptochrome, an electron tunnels along a chain of tryptophan amino acids. The result is a 'radical pair' — two molecules each holding one unpaired electron. The two electron spins start in a singlet state but oscillate between singlet and triplet at a rate that depends sensitively on Earth's ~50 µT magnetic field. The downstream chemistry depends on which spin state the radical pair is in when it relaxes — providing, in principle, a magnetic-direction signal to the bird's nervous system.
In plain terms
Imagine you had a tiny pair of magnets in your eye, and the tiny magnets twirled when you faced north or south. The way they twirl changes how blue light gets through your eye, and your brain learns to read 'north' the way you read shapes. Birds may actually do this.
Also called: bird-compass protein, Cry4.
Why it matters
- If confirmed, it's the only known case of biology exploiting quantum coherence at room temperature for a sense.
- Cry4 in European robins shows the right magnetic sensitivity in vitro.
- The mechanism predicts that radio-frequency noise should disrupt navigation — and experiments with caged robins confirm exactly that.
- Connects condensed-matter physics, organic chemistry, neuroscience, and animal behavior.
Where it lives
- Eyes of European robins (Erithacus rubecula)
- Sea turtle navigation systems (suggested but unconfirmed)
- Drosophila magnetic-orientation behavior
Key numbers
- Activation light: ~450 nm (Blue)
- Required coherence time: ~1–100 µs
- Earth's field strength: ~50 µT (Sensor must read changes of ~µT)
Surprising facts
- European robins held in cages with weak radio-frequency magnetic fields lose their ability to orient — exactly as the radical-pair theory predicts.
- The Earth's field at the bird's eye is about a million times weaker than a refrigerator magnet, yet may produce a measurable spin signal.
- The same protein family that lets your skin sense daylight (in mammals, cryptochromes regulate circadian rhythms) appears, in birds, to also serve as a quantum compass.
- If the mechanism is correct, the radical pair must remain quantum-coherent for tens of microseconds in a wet, warm environment — far longer than physicists once believed possible.
Open questions
- Radio-frequency field disrupts bird navigation: European robins placed in cages exposed to oscillating magnetic fields at frequencies around 1.4 MHz — far too weak to exert any classical magnetic force on the bird — lose their ability to orient in Earth's field. The effect is precisely what the radical-pair model predicts: the RF field resonantly scrambles the quantum spin oscillations that encode directional information. No other known compass mechanism predicts this frequency-specific disruption.
- Microsecond quantum coherence in a warm protein: Computational work from the Hammes-Schiffer and Subotnik groups (2026) and earlier Oxford spectroscopy demonstrated that radical pairs in cryptochrome-like molecules can survive as coherent spin states for microseconds in aqueous environments at room temperature. This far exceeds what quantum physicists expected for open quantum systems in biology, and the mechanism by which the protein environment protects — rather than destroys — the quantum spin state is not fully understood.
A short history
Klaus Schulten proposed the radical-pair compass model in 1978. Cryptochrome was first identified in plants in 1993, and as a candidate avian magnetoreceptor by Ritz, Adem, and Schulten in 2000. In 2021, Xu, Hore and colleagues demonstrated in vitro that European robin Cry4 has the magnetic sensitivity required by the model — the strongest direct evidence to date.
Sources
- Ritz, Adem, Schulten, Biophys. J. (2000) — radical-pair model (https://doi.org/10.1016/S0006-3495(00)76629-X)
- Xu et al., Nature (2021) — magnetic sensitivity of robin Cry4 (https://www.nature.com/articles/s41586-021-03618-9)
- Hore & Mouritsen, Annu. Rev. Biophys. (2016) — review (https://doi.org/10.1146/annurev-biophys-032116-094545)