  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.
