European robins may maintain
quantum entanglement in their eyes a full 20 microseconds
longer than the best laboratory systems, say physicists investigating how birds may use quantum effects to "see" Earth's magnetic field.
Quantum entanglement is a state
where electrons are spatially separated, but able to affect one
another. It's been proposed that birds' eyes contain
Conclusive proof doesn't yet exist, but multiple lines of
evidence suggest it. Findings like this one underscore just how
sophisticated those compasses may be.
"How can a living system have evolved to protect a quantum state
as well -- no, better -- than we can do in the lab with these
exotic molecules?" asked quantum physicist Simon Benjamin of Oxford
University and the National University of Singapore, a co-author of
the new study. "That really is an amazing thing."
Many animals -- including not only birds, but some mammals,
fish, reptiles, even crustaceans and insects -- navigate by sensing
the direction of Earth's magnetic field. Physicist Klaus Schulten
of the University of Illinois at Urbana-Champaign proposed in the
late 1970s that bird navigation relied on some geomagnetically
sensitive, as-yet-unknown biochemical reaction taking place in
Research since then has revealed the existence of special
optical cells containing a protein called cryptochrome. When a
photon enters the eye, it hits cryptochrome, giving a boost of
energy to electrons that exist in a state of quantum
One of the electrons migrates a few nanometers away, where it
feels a slightly different magnetic field than its partner.
Depending on how the magnetic field alters the electron's spin,
different chemical reactions are produced. In theory, the products
of many such reactions across a bird's eye could create a picture
of Earth's magnetic field as a varying pattern of light and
However, these quantum states are notoriously fragile. Even in
laboratory systems, atoms are cooled to near-absolute-zero
temperatures to maintain entanglement for more than a few
thousandths of a second. Biological systems would seem too warm and
too wet to hold quantum states for long, yet that's exactly what
they appear to do.