The possibility that quantum processing with nuclear spins might be operative in the brain is proposed and then explored. Phosphorus is identified as the unique biological element with a nuclear spin that can serve as a qubit for such putative quantum processing – a neural qubit – while the phosphate ion is the only possible qubit-transporter. We identify the “Posner molecule”, Ca9(PO4)6, as the unique molecule that can protect the neural qubits on very long times and thereby serve as a (working) quantum-memory. A central requirement for quantum-processing is quantum entanglement. It is argued that the enzyme catalyzed chemical reaction which breaks a pyrophosphate ion into two phosphate ions can quantum entangle pairs of qubits. Posner molecules, formed by binding such phosphate pairs with extracellular calcium ions, will inherit the nuclear spin entanglement. A mechanism for transporting Posner molecules into presynaptic neurons during a “kiss and run” exocytosis, which releases neurotransmitters into the synaptic cleft, is proposed. Quantum measurements can occur when a pair of Posner molecules chemically bind and subsequently melt, releasing a shower of intra-cellular calcium ions that can trigger further neurotransmitter release and enhance the probability of post-synaptic neuron firing. Multiple entangled Posner molecules, triggering non-local quantum correlations of neuron firing rates, would provide the key mechanism for neural quantum processing. Implications, both in vitro and in vivo, are briefly mentioned.
Researcher Matthew Fisher’s interest in neural nuclear spin processing was stimulated by a paper that explored the effects of different isotopes of lithium on rats. Li naturally occurs in the ratio 92.6% Li-7 and 7.4% Li-6. Somewhat quizzically, mothers given the Li-7 isotope were less stimulated and ignored their pups while the Li-6 moms were maternalistic and nursed more. The interesting part for us here, is that while Li-7 has spin 3/2 and a short Tcoh of just a few seconds, Li-6 has an “honorary” spin 1/2 due to its electric dipole moment and a nice 5 minute long Tcoh
Fisher intends to re-explore the older Li isotope work and further refine the mechanisms and any potential shortfalls of the Posner conception. As alluded to above, electron spin itself is still a relatively obscure concept in biology save for a few niche revelations on things like chemical compasses or other radical pair-pair inspired biologics. Yet free radicals (unpaired electron spins) have a magnetic moment 1,000 or so times larger than that of a proton. Their presence alone could be a significant factor in things like phosphorus nuclear spin decoherence
pair of entangled Posner molecules in (a). The purple dashed lines represent singlet entangled phosphorus nuclear spins. Acomplex of highly entangled Posner molecules in (b). With two pairs of entangled Posner molecules, labelled(a; a0) and(b; b0) as in panel (c),a chemical binding between one member in each pair – the black box connecting(a; b)- can change the probability of a subsequent bindingof the other members of the pair,(a0; b0). If the Posner molecules chemically bind after being transported into two presynaptic neurons asdepicted in (d), they will be susceptible to melting, releasing their calcium into the cytoplasm enhancing neurotransmitter release, therebystimulating (quantum) entangled postsynaptic neuron firingrelease, thereby stimulating (quantum) entangled postsynaptic neuron firing
SOURCES – Arxiv, Physorg