How biological microtubules may avoid decoherence

Full text: Entangled superpositions persisting for hundreds of milliseconds in protein assemblies such as microtubules (MTs) are proposed in biological functions, e.g. quantum computation relevant to consciousness in the Penrose-Hameroff 'Orch OR' model. Cylindrical polymers of the protein tubulin, MTs organize cell activities. The obvious question is how biological quantum states could avoid decoherence, e.g. in the brain at 37.6 degrees centigrade. Screening/sheelding: tubulin protein states/functions are governed by van der Waals London forces, quantum interactions among clouds of delocalizable electrons in nonpolar 'hydrophobic' intra-protein pockets screened from external van der Waals thermal interactions. Such pockets include amino acid resonance structures benzene and indole rings. (Anesthetic gases erase consciousness solely by interfering with London forces in hydrophobic pockets in various brain proteins). Hence tubulin states may act as superpositioned qubits also shielded at the MT level by counter-ion Debye plasma layers (due to charged C-termini tails on tubulin) and by water-ordering actin gels which embed MTs in a quasi-solid. Biological systems may also exploit thermodynamic gradients to give extremely low effective temperatures. Decoherence free subspaces: paradoxically, a system coupled strongly to its environment through certain degrees of freedom can effectively 'freeze' other degrees of freedom (quantum Zeno effect), enabling coherent superpositions and entanglement to persist. Metabolic energy supplied to MT collective dynamics (e.g. Froehlich coherence) can cause Bose-Einstein condenzation and counter decoherence as lasers avoid decoherence at room temperature. Topological quantum error correction: MT lattice structure reveals various helical winding paths through adjacent tubulins which follow the Fibonacci series. Propagation/interactions of quasi-particles along these paths may process information. As proposed by Kitaev (1997), various possible paths may superpose (by the Aharonov-Bohm effect) to act as decoherence-resistant qubits. Attempting to disprove a role for quantum states in consciousness, Tegmark (2000) calculated MT decoherences times of 10-13 sec, far too brief for neural activities. However Tegmark did not address Orch OR nor any previous proposal, but his own quantum MT model which he did indeed successfully disprove. Hagan et al (2002) recalculated MT decoherence times with Tegmark's formula but based on stipulations of the Orch OR model (10-6 smaller superposition separation distance, charge versus dipole, correct dielectric constant) and extended the calculated MT decoherence time to 10-5 to 10-4 sec. Shielding (counter-ions, actin gel) further extends calculated decoherence time to hundreds of milliseconds. Topological (Aharonov-Bohm) quantum error correction and laser-like Froehlich coherence/condenzation may extend MT decoherence time indefinitely. Ouyang and Awschalom (2003) showed that quantum spin transfer through biological benzene rings (identical to those in protein hydrophobic pockets) becomes more efficient at higher temperature, and Hackermueller et al. (2003) showed quantum wave behavior of biological porphyrin molecules. Evolution has had billions of years to solve the decoherence problem; quantum information technology may profit from emulating certain aspects of biology. (author)