Quantum Cognition: The possibility of processing with nuclear spins in the brain (1508.05929v2)

Abstract: 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", $\text{Ca}_9 (\text{PO}_4)_6$, 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.

• The paper demonstrates that phosphorus nuclear spins can act as qubits to facilitate quantum processing within the brain.
• It details how phosphate ions and Posner molecules preserve spin coherence, providing a plausible biochemical basis for neural quantum computation.
• It outlines experimental approaches, including NMR and cryoTEM, to validate the role of quantum effects in cognitive processes.

Quantum Cognition: Potential Mechanisms of Nuclear Spin Processing in the Brain

The paper explores the intriguing proposition that quantum processing occurs in the human brain, centered around nuclear spins and posits a plausible biochemical basis for this phenomenon. Specifically, the manuscript suggests the phosphorus nuclear spin as a viable candidate for the qubit in quantum cognition. Phosphate ions, prevalent in various biological molecules, are proposed as qubit transporters capable of maintaining spin coherence times to facilitate neural processing. Meanwhile, the Posner molecule, a calcium phosphate compound, is identified as a potential contender for quantum memory—preserving this coherence over extended periods.

Mechanism of Quantum Processing

The central hypothesis revolves around utilizing the phosphorus nuclear spin (I = 1/2) for computational purposes. This particular spin state affords relatively long coherence times, even in the brain's aqueous environment. The phosphate ions function as transporters, but for sustained coherence, they require a host like the Posner molecule—$\text{Ca}_9 (\text{PO}_4)_6$. This molecule's structure purportedly shields qubits from environmental decoherence over substantially long durations.

A critical aspect of this mechanism is the entanglement required for quantum computing. The paper contends that the enzyme-catalyzed dissolution of pyrophosphate (PPi) into phosphate ions might entangle neural qubits. Following this, entangled qubit pairs can be incorporated into Posner molecules, which continue to shield them during transportation and storage.

Quantum processing necessitates entangling these Posner molecules further while preserving their coherence. When embedded into presynaptic neurons, possibly through vesicle endocytosis by VGLUT—a glutamate transporter which also functions as a phosphate transporter—these Posner molecules contribute to neural quantum processing. Within these neurons, quantum measurements could presumably occur via reactions between Posner pairs that alter neuron firing rates through calcium ion release, thus influencing neurotransmitter release and synaptic activity.

Implications and Future Directions

The theoretical framework outlined initiates several provocative implications for neuroscience. It bridges quantum mechanics with cognitive processes, potentially redefining interpretations of consciousness, memory, and neural computation. Beyond the theoretical intrigue, several experimental avenues present themselves. For instance, experiments focused on dynamic light scattering and cryoTEM could further elucidate Posner molecule prevalence and behavior in biological fluids. Moreover, NMR could investigate the coherence times of nuclear spins within these molecules or examine isotope effects on coherence and behavior.

From a neuroscience and psychiatric perspective, such a framework could underpin novel therapeutic approaches, potentially influencing current practices such as trans-cranial magneto-stimulation. Understanding interactions between spin manipulation and mental states might offer innovative treatments for neurological disorders.

While the theoretical articulation necessitates extensive empirical validation, the potential for posing—and answering—new questions regarding the brain's quantum capabilities merits thorough exploration. The paper's framework offers fertile ground for future theoretical and empirical inquiries into the field of quantum cognition.