The hard problem of consciousness is the question how subjective experience arises from brain matter. I suggest exploring the possibility that quantum physics could be part of the answer. The simultaneous unity and complexity of subjective experience is difficult to understand from a classical physics perspective. In contrast, quantum entanglement is naturally both complex and unified. Moreover the concept of matter is much more subtle in quantum physics compared to classical physics, and quantum computing shows that quantum effects can be useful for information processing. Building on recent progress in quantum technology and neuroscience, I propose a concrete hypothesis as a basis for further investigation, namely that subjective experience is related to the dynamics of a complex entangled state of spins, which is continuously generated and updated through the exchange of photons. Spins in condensed matter systems at room or body temperature can have coherence times in the relevant range for subjective experience (milliseconds to seconds). Photons are well suited for distributing entanglement over macroscopic distances. Neurons emit photons, reactive oxygen species in the mitochondria being likely sources. Opsins, light-sensitive proteins that are plausible single-photon detectors, exist in the brain and are evolutionarily conserved, suggesting that they serve a function. We have recently shown by detailed numerical modeling that axons can plausibly act as photonic waveguides. The oxygen molecule, which has non-zero electronic spin and emits photons, might serve as an interface between photons and spins. The achievable photon rates seem to be more than sufficient to support the bandwidth of subjective experience. The proposed hypothesis raises many interesting experimental and theoretical questions in neuroscience, quantum physics, evolutionary biology, psychophysics, and philosophy.
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