Communications of the ACM
Are Human Brains Quantum Computers?
Researchers in the Quantum Brain Project (QuBrain) want to confirm whether the brain is a quantum computer.
Credit: David Ope/Quantum Magazine
One of Philosophy's perennial questions relates to the existence of free will. Since the brain is considered deterministic — the notion that all events are completely determined by previously existing causes — how can we have genuine free will?
The standard response is that free will is an illusion, but scientists on the Quantum Brain Project (QuBrain) think the brain is a quantum computer, and thus non-deterministic.
If true, and the three-year, $1.2-million QuBrain project aims to prove it, then not only will the philosophers have to rethink free will, but neuroscientists, brain-like deep network builders, and quantum computer inventors will also have to rethink both their theories and their architectures.
"What is intriguing about this work is that it is as much scientifically fundamental as it is philosophically stimulating," said Alexej Jerschow, a professor of chemistry at New York University who is working on QuBrain with professors Matthew Fisher, Matt Helgeson, Aaron Ettenberg at the University of California, Santa Barbara (UCSB), as well as with research scientist Tobias Fromme at the Technical University of Munich (Germany).
Fisher acknowledged, "The question of free will is a tricky one; we all feel like we have it despite determinism. It is also possible that human self-consciousness is only possible with quantum processing. If we find that quantum mechanics is in operation cognitively, then it could be a necessary component of consciousness." However, he explained, the goal of the QuBrain project "is to look at the pieces that would have to be put together to make quantum cognition possible. If found to be true, then we could start building a synthetic quantum brain. If not, it may end up that humans are just clever robots, with all our actions deterministic."
While the "conventional wisdom," as Massachusetts Institute of Technology theoretical physicist Senthil Todadri puts it, stacks the cards against the success of the QuBrain project, even its critics concede that its worthwhile to put the matter to the test.
"Conventional wisdom would hold that quantum cognition is impossible in the environment of the brain, but it is always good to question accepted things," Todadri says. "It seems that Fisher has identified one possible loophole in the conventional wisdom, and it is good that he is attempting to confirm experimentally that this possible loophole exists. If it does, the loophole either has to be fixed within the conventional wisdom, or shown that its possibility is in fact true, which would have widespread implications."
The initial goal of the QuBrain project, according to Fisher, is to reverse-engineer the brain's possible use of quantum states in cognition; in particular, to investigate the nuclear spin (a quantum property) of phosphate atoms in the brain. "In my reverse-engineering approach, phosphate is the only good candidate in the body for qubits. Phosphate is very active in biology in general and very important to the human nervous system, especially in our mitochondria organelle. In every cell, phosphate enables the making of ATP [adenosine triphosphate, the energy carrier of cells], and phosphate also stores and releases calcium phosphate, which has a particularly important role in the function of synapses [which play a role in memory]."
The project aims to determine whether the brain operates like a quantum computer, with neural qubits suspended in states of superposition and entangled with phosphate atoms in the brain. Entanglement—when the quantum states of particles remain identical even when a large distance separates the particles—could coordinate processing in many different parts of the brain simultaneously. as Fisher explains in his paper Quantum Cognition: The possibility of processing with nuclear spins in the brain.
The QuBrain team will attempt to verify its hypotheses of quantum cognition using methods including in vitro (test tube) and in vivo (in animals, here rats) testing, and quantum measurement via nuclear magnetic resonance (NMR. Their hope is to demonstrate unequivocally how quantum cognition takes place in the brain by directly emulating it in the lab.
According to Fisher, phosphorus is a unique atom whose nuclear spin could encode quantum states, thus acting as neural qubits. Such phosphate atoms, bonded with calcium in Posner molecules (clusters of nine calcium atoms and six phosphorous atoms), can "protect" coherent neural qubits from collapsing into decoherence (non-quantum states).
Jerschow explains, "Posner clusters form an environment where phosphorus nuclear spins would be well isolated from the environment and in particular from fluctuations in magnetic fields, as introduced by nearby other spins—for example, by proton spins in the surrounding water. Therefore, these nuclear spin states can be expected to be particularly long-lived. What will be interesting to study is how the information would be encoded and read out from storage."
The QuBrain project will focus on the storage aspect first, Jerschow says. "We want to know whether in principle it is possible to store information for long periods of time in Posner molecules (or in similar constructs). This information will be a central component and requirement in quantum processing. Even if we ultimately fail to prove the specific mechanism of quantum processing, we will at least provide fundamental studies towards the viability of these mechanisms."
Most quantum computers under development use the spin of electrons, rather than atoms, to encode their quantum states, seriously hampering their work, according to Fisher. Since the state of qubits involves a delicate superposition of 1s and 0s on the same particle, the more stable the qubit, the better, because superposition allows qubits to perform operations simultaneously that conventional computers must do serially.
Unfortunately, the sensitivity of electrons makes their spins etherial, their qubit's coherent superposition collapsing (a process called decoherence) at the slightest thermal or electromagnetic disturbance. Electron coherence today typically lasts only microseconds, even when they are cooled to near Absolute Zero. According to Fisher, however, phosphate atoms have the size and requisite shielding to serve as neural qubits that could hold their quantum superposition states at body temperature for minutes, hours, days, weeks or even longer (similar to the ability of lasers to maintain their quantum coherence over long periods of time).
"Electron spins are strongly coupled to electric fields, but nuclear spins are much more stable compared to electrons, because they are relatively insensitive to electric fields. Also, electron spins can cancel each other out, whereas nuclear spins cannot. You can think of the spin of atoms as physical rotation, whereas the spin of an electron is more of an electrical component of its electromagnetic field," said Fisher. "In the body, phosphorus atoms, in particular, have the requisite nuclear spin to serve as a biochemical qubits in the brain."
The QuBrain researchers aim to prove that Posner molecules use their spherically shaped calcium cages to perform long-lasting quantum processing in the brain. Their quest will be to determine and whether Posner molecules have the ability to shield the coherence of the nuclear spins of neural qubits, and when entangled, whether they could enable the storage of quantum information and the simultaneous synchronization of quantum information processing throughout the brain. In particular, the scientists will explore the potential for non-local quantum information processing enabled by pair-binding and entanglement of Posner molecules.
To test possible quantum processing in the mitochondrial matrix of brain cells (which hold together the DNA, ribosomes, soluble enzymes, small organic molecules, nucleotide cofactors, and inorganic ions), Jerschow said QuBrain scientists "will utilize both isolated mitochondria and labeled mitochondria within cultured cells to probe for correlative responses to physiological stimuli. Since we know which biochemical processes are required to form entangled entities within our hypothesis, we are able to test whether correlative mitochondrial and cellular behavior is sensitive to inhibition of these."
Fisher also hopes to explain how memory, particularly very long-term memories, work, as well as exploring the biology of the brain, its mental health, and perhaps even "what it is to be human," said Fisher.
R. Colin Johnson is a Kyoto Prize Fellow who has worked as a technology journalist for two decades.
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