A biological quantum sense
Migratory birds, such as the European Robin (Erithacus rubecula), perform incredible feats of navigation across thousands of kilometers. Evidence suggests they achieve this by perceiving the Earth's magnetic field, which is around 100 times weaker than a refrigerator magnet. The leading explanation is a quantum process called the radical pair mechanism, which happens inside proteins in the birds' retinas.
The main protein is cryptochrome 4 (CRY4), a light-sensitive molecule. When a photon of blue light from the sky strikes a CRY4 molecule, it causes an electron to jump from one part of the protein to another. This creates two molecules that each have an unpaired electron; this is known as a radical pair. The two unpaired electrons are quantum-mechanically entangled, and their quantum property of "spin" is linked. The spins of the two electrons oscillate between two states: a "singlet" state (spins opposite) and a "triplet" state (spins parallel). The Earth’s weak magnetic field influences the rate of this oscillation. This change affects the chemical products of the radical pair's reaction, creating a signal that is sent to the bird's brain. This process effectively allows the bird to "see" the magnetic field lines as a pattern superimposed on its normal vision, providing a quantum compass for navigation.
Photosynthesis at quantum speed
Photosynthesis converts sunlight into chemical energy with nearly 100% efficiency in its initial steps. This near-perfect process relies on quantum mechanics to transfer energy from light-harvesting antenna molecules to a reaction center where the energy is converted. The transfer happens so quickly that almost no energy is lost as heat.
A main structure studied in this process is the Fenna-Matthews-Olson (FMO) complex, found in green sulfur bacteria. The FMO complex is a protein containing bacteriochlorophyll molecules that absorb photons. When a photon strikes, it creates an excited state, or "exciton." This exciton must find its way to the reaction center. Instead of trying one path at a time, the exciton uses quantum superposition to exist in multiple states at once, effectively exploring all possible pathways simultaneously. This allows it to instantly identify and take the most efficient route to the reaction center. This wave-like behavior, known as quantum coherence, has been observed to last for at least 300 femtoseconds at physiological temperatures, long enough to ensure efficient energy transfer. While the functional role of long-lived coherence is debated, the underlying quantum effects are central to the system's speed and efficiency.
Other biological processes also show evidence of quantum phenomena. Enzymes, the catalysts of life, use quantum tunneling to speed up reactions. In certain reactions, particles like protons can pass through an energy barrier instead of needing enough energy to go over it, a feat impossible in classical physics.