Chirality enhances the quantum Zeno effect in magnetoreception ✦
Radical pairs in cryptochrome are involved in magnetosensitive processes, including the proposed mechanism of avian magnetoreception. The molecule's chirality was thought to enhance the magnetic field effect through the chirality-induced spin selectivity (CISS) effect. The influence of CISS on the directional magnetic sensitivity was systematically investigated for two prototypical radical pair models: one generates spin polarization, while the other—analyzed for the first time—creates spin coherence. It was established that CISS-induced polarization significantly boosts sensitivity due to the emergence of triplet character in the initial state and the enhancement of the quantum Zeno effect, analogous to effects in triplet-born radical pairs with highly asymmetric recombination. In contrast, the spin coherence created by CISS does not lead to a substantial improvement in sensitivity. These results show that CISS by itself is not a universal amplifier of sensitivity or coherence, and its influence should be evaluated on a case-by-case basis, especially in connection with the quantum Zeno effect.
Context
Birds, both migratory and homing pigeons, use the Earth's magnetic field for navigation, but the exact mechanism remains a mystery. The leading hypothesis — the radical pair mechanism, implemented in the cryptochrome protein, is based on the entanglement of electron spins, first conceptualized by John Stewart Bell. However, the weak sensitivity to the magnetic field requires the search for natural amplifiers, such as molecular chirality, which can influence spin dynamics through an unusual quantum superposition.
Methods
The authors applied the Nakajima–Zwanzig formalism to model the spin dynamics of FAD•−/W•+ and FADH•/O2•− radical pairs in cryptochrome. Two descriptions of CISS were introduced: the spin polarization model (CISP) and the spin coherence model (CISC). The anisotropy of the recombination yield was calculated for 300 magnetic field orientations while varying rate constants and the degree of chirality χ.
Results
In the CISP model, a significant increase in the anisotropy ΔΦb is observed at asymmetric rates (kb ≫ kf), characteristic of the quantum Zeno effect, first realized in ion traps by the group of David Wineland. For FADH•/O2•−, maximum sensitivity is achieved at χ = π/2 and kb ~ 10³ µs⁻¹, and spin relaxation (γ = 1 µs⁻¹) reduces the effect only by half. Meanwhile, the CISC model does not provide substantial enhancement, and sometimes even weakens sensitivity. With an increase in the number of hyperfine interactions, the enhancement persists, though its magnitude decreases.
Implications
The results indicate that CISS in cryptochrome is not a universal amplifier but acts only upon generating polarization, enhancing the quantum Zeno effect. This changes the understanding of the role of quantum effects in biology and paves the way for a targeted search for similar mechanisms in other systems.
Future development
Future research may test the predictions on synthetic donor-acceptor triads, where stepwise electron transfer is possible, and explore the role of intermediate states. It is also important to experimentally confirm the presence of CISS in cryptochrome.
Impact
The work touches upon quantum biology, spintronics, and the development of molecular quantum sensors.
Next steps
Experimental verification of spin polarization generation under CISS in photolyases and cryptochromes, as well as modeling of systems with multiple electron transfers.
Key open problems
Connection to the fundamental problem of quantum decoherence in biological systems at finite temperatures and the role of chirality in symmetry breaking.
🎯 The Zeno effect is named after the philosopher Zeno, who argued that an arrow would never reach its target if its path were divided into infinitely many stages — similarly, frequent quantum measurements 'freeze' the system's evolution.
Key numbers
- Earth's magnetic field: ~50 µT
- radical pair lifetime: 1 µs
- recombination rate constant for enhancement: ~10³ µs⁻¹
- number of field orientations in calculations: 300
- effective relaxation rate: 1 µs⁻¹