The scientific field of biochemistry — the study of chemical reactions and processes in biological organisms — backed up by physical chemistry — the application of standard physical concepts, such as motion, thermodynamics, and force, to chemical systems — is most of the time sufficient to gain a thorough understanding of the organizing mechanistic principles within living creatures.
However, some researchers think that quantum biology — the academic field where quantum physics (physics at subatomic scales) and biochemistry cross paths — might lend a hand with deepening our comprehension of the functioning of biological systems.
The system that we will dissect in this article is our brain.
The Open Question
In order to provide us with the necessary cerebral capability to engage in a conversation, to anticipate a dangerous situation, to learn a new language, to solve a math equation, to design and implement a business strategy, or to read a popular science book, many neurons in our brain team up to process input signals and to manifest a certain response.
What is as yet not entirely understood in the field of neuroscience — the scientific study of the structure and function of the nervous system — is how exactly neurons collaborate to give birth to this rich variety of cognitive brain functions.
For instance, when it comes to neural populations in the retina, Tatyana Sharpee et al. point out that “the organizing principles for how these neurons work together remain unclear.” Another case in point is a research study by Michel Hofman on the evolution of the brain in which he mentions that there is still a lack of knowledge around the nature of neural connectivity within the neocortex. A final example considers the brain in general: Christopher Lynn and Danielle Bassett argue that it continues to be a challenge to grasp “how the brain’s structural wiring supports cognitive functions.”
This article explores one possible answer to this unresolved question: Quantum-mechanical behaviour could help to explain the collective, cooperative behaviour of neurons that shapes the brain’s overall functioning.
A Deeper Look into the Brain
Our brain, which has an approximate storage capacity of a petabyte (i.e., 10¹⁵ bytes or the equivalent of 1.5 million CD-ROM discs), relies on roughly 100 billion neurons — with the assistance of glial cells — to fulfil its cognitive duties.
Although several types of neurons exist, a neuron generally consists of dendrites, the cell body (soma) and the axon (see Fig. 1). The dendrites consolidate the input signals from other neurons and carry them via the soma, where, among many other activities, energy is produced in the mitochondria and genetical material stored in the nucleus, to the axon.
The brain abounds with positively and negatively charged molecules called ions. Usually, a neuron is more negatively charged — the sum of the charges of all the negative ions outstrips that of the positive ones — than its environment; the difference in electrical charge is referred to as the membrane potential. For a neuron at rest, it measures around -65 millivolt (mV). That difference can either decrease (depolarization) or increase (hyperpolarization), which means that more positive or negative ions, respectively, momentarily dwell in the neuron with respect to the situation at rest.
When depolarization reaches the threshold of -55mV, voltage-gated sodium ion channels (pores) situated in the membrane of the initial segment of the axon (closest to the soma) open, allowing more positive ions to enter the neuron. That moment is when the neuron ‘fires’ and sets off a chain reaction of depolarization in the adjacent segments all the way towards the end of the axon (terminal).
This flow of change in the membrane potential travelling along the axon is designated as the action potential (a.k.a. spike or impulse) and is mediated by numerous ion channels. Once arrived at the terminal, the nerve signal must somehow traverse a region of connection (the synapse) between the terminal of the first neuron (the presynaptic neuron) and the dendrites of the following neuron (the postsynaptic neuron).
There are two kinds of synapses: electrical and chemical synapses. The former provides a physical bridge facilitating the almost instantaneous passage of the action potential. Chemical synapses, in contrast, do not dispose of such physical connection. Instead, the action potential is converted at the presynaptic terminal into a chemical signal by the release of chemical molecules, i.e., neurotransmitters, which then cross a gap — called the synaptic cleft — to finally attach to receptors on the postsynaptic dendrites which triggers the opening of ion channels, prompting a subsequent change in the postsynaptic neuron’s membrane potential (in this way, the chemical signal is converted back into an electrical one).
In a certain sense, neurotransmitters are the main instrument of communication between neurons (also, there are more chemical than electrical synapses), as they are accountable for passing along the electrical nerve impulses from one neuron to the next. Depending on the type of neurotransmitter released, presynaptic neurons can either continue to transmit the action potential unabatedly — these neurons are known as excitatory neurons — or slow down the signal — these neurons go by the term inhibitory neurons.
When looking more broadly at the structure of the brain, it has been established that dendrites, somata, shorter axons, and synapses are typically located in the grey matter of the brain, whilst the longer, insulated (myelinated) axons reside in the white matter. The latter axonal pathways ensure the propagation of action potentials across — and thus the communication between — various brain regions.
In terms of the emergence of functional brain states, it is the organization of an interconnected structural network of dynamic interactions between many neurons that gives rise to the brain’s complex cognitive functions and mental processes, including computation, information distribution, communication, learning, and memory.
But one question remains: What exactly does this underlying organizing mechanism responsible for such emergence look like?
Before we move towards higher organizational levels, first let us go into the opposite direction, i.e., the microscopic levels of reality.
Quantum Physics under the Lens
Quantum physics is the set of laws that explain the behaviour of subatomic particles, such as the electron and the quarks. It appears that at such small scales, the rules of the game are quite different relative to those in the macroscopic world (classical mechanics).
To begin with, consider the general statement that systems can contain a certain number of fundamental characteristics called states. For instance, the language of a classical computer consists of bits which come in two states: 0 and 1. Another example is the electron’s feature of spin which can be described by two elementary states: spin up and spin down.
In a quantum-mechanical world, one of these new kinds of behaviour is quantum superposition, which expresses the unique phenomenon whereby a new fundamental state can be created out of a linear combination of the other fundamental states. That is, while a bit in a classical computer can only adopt the value of 0 or 1 one at a time, a bit in a quantum computer — called a qubit — can take up a third elementary state which is a mixture of both 0 and 1.
An equivalent way of looking at this is saying that a qubit finds itself simultaneously in both these states. Similarly, the laws of quantum physics allow the electron’s spin to assume a new fundamental state which reflects an amalgamation of spin down and spin up.
When a qubit or a superposed electron is measured, then the act of measuring itself will destroy the state of superposition and the system will collapse into one — and only one — of the two original basic states. Put differently, the system will have decohered from a quantum into a classical state.
More generally, any slightest interference or disturbance of the system by their direct environment (think of either physical contact or radiation, such as heat) provokes almost immediately the destruction of quantum superposition (decoherence).
Quantum entanglement is a special case of superposition and it involves an unparalleled form of correlation between two or more superposed systems. That is to say, quantum entanglement between various systems represents a new fundamental, superposed state which describes the entangled system as a whole.
Moreover, it connects the individual systems in such a fashion that the quantum state of one system is inextricably linked to the quantum state of the other, irrespective of the physical distance between them. Applying this to the example of two quantum entangled electrons, this basically means that measuring the spin (the quantum state) in one of them — say, it is spin up — has the consequence that we instantaneously know what the spin of the other electron will be — it will be in a spin down state — without measuring it.
Bear in mind though, that, despite this non-local connection between the entangled electrons, no actual information is being interchanged or sent between them. This is what Albert Einstein intended with his phrase ‘spooky action at a distance’.
But how is quantum entanglement in any way relevant or useful? If we take quantum computing as an example, it turns out that entangled qubits exhibit a tremendous quantum advantage over ordinary, classical computers in terms of information processing capacity, for the reason that a quantum computer is able to carry out many calculations simultaneously as long as qubits can uphold their coherence and entanglement.
Here is where we arrive at the crux of this article: Some scientists claim that the brain has figured out how to harness quantum entanglement to perform its extraordinary cognitive computations.
The Quantum Case of Brain Interconnectivity
In a warm and lively biological environment in which a system, e.g., one particular molecule within a neuron, is constantly bombarded with inputs from its surroundings, it is predictably quite unlikely, according to several researchers, that any quantum effect has a noteworthy functional impact — any entangled or superposed system would almost immediately decohere — and would thus be considered irrelevant to our understanding of the functioning of the brain.
Be that as it may, quantum-mechanical effects have already been observed to fulfil a non-trivial role in living systems, including in the case of avian magnetoreception, energy transfer during photosynthesis, the detection phase of olfaction, and enzymatic reactions. Not only that, scientists, such as Susana Huelga and Martin Plenio, argue that interaction with the environment would even enhance or regenerate quantum correlations.
Turning to the brain, let us go over some of the scientific positions that give credit to quantum physics when it comes to illuminating the intricate workings of the brain.
Even though Christof Koch and Klaus Hepp claim that the ensemble of molecular activities associated with the action potential and the exchange of neurotransmitters would annihilate any delicate coherent quantum state, Paul Glimcher argues that inter-neuronal communication follows the tenets of quantum mechanics because its underlying, predominant working mechanism — membrane voltage — is the result of quantum-mechanical interactions at the atomic level.
To facilitate a proverbial ladder between the quantum and classical realm in support of such arguments, Peter Jedlicka suggests that microscopic quantum fluctuations or events could be amplified throughout various functional hierarchy levels in the brain, i.e., atoms, molecules, neurons, neural circuits, and brain regions, and therefore leave their mark on the overall neuronal dynamics.
Although such quantum vibrations usually cancel out at the macroscopic level due to decoherence, the reason for the alleged possibility of this amplification process is the observation that nerve impulses travel across neural structural networks in a nonlinear fashion. Nonlinearity refers to the idea that the effect is not proportional to the cause in a one-to-one manner; small variations in the initial conditions of a system can grow exponentially over time.
In other words, though quantum effects operative at the smallest of scales are most likely not directly macroscopically observable in the brain, they may exert an indirect influence on higher-level brain machinery and functions through a kind of nonlinear, magnifying ripple effect.
While Haim Sompolinsky stresses the point that the major sources of macroscopic fluctuations in the brain are related to thermal or chaotic dynamics and much less so to quantum physics, he does not entirely dismiss — together with the otherwise sceptical Christof Koch — Jedlicka’s line of thinking. As a matter of fact, Sompolinsky says that “to a degree [this expansion process] will affect the timing of spikes in neurons.”
In spite of the precarious state of coherence in live biological systems, Alipasha Vaziri and Martin Plenio put forward the argument that quantum coherence has nevertheless a role to play in the ion selection and transportation process within ion channels in bio-molecular systems generally.
Focussing specifically on the brain, such results go head-to-head with Max Tegmark’s calculations, which reveal that the duration of coherence within the brain is significantly shorter (between 10 and 20 orders of magnitude) than the time it takes for a neuron to convey an electrical signal (in the range of milliseconds) — Scott Hagan et al. rebut Tegmark’s results, however, and put the decoherence time much closer to the timespan of spiking neurons.
Nonetheless, Gustav Bernroider and Sisir Roy posit that the quantum entanglement of so-called informational states of potassium ions at the ion channel filter may well be protected from decoherence even during the gating time of the channel.
Johann Summhammer et al. furthermore report that quantum coherence may be indispensable to justify the observed transition rates of ions through their respective channels — as per Youngchan Kim et al., that rate is estimated at around 100 million ions per second — given that classical thermodynamics seems unable to account for them.
In fact, Vahid Salari et al. contend that even if decoherence occurs at very short timescales — their calculations deliver a timeframe of a trillionth of a second — the brief moment of coherence might still have some sway over the ion channel’s selectivity filter as well as the dynamics of action potentials.
Jim Al-Khalili and Johnjoe McFadden suggest that these findings might clarify the high speed of action potentials along the axon’s membrane, since this speed relies directly on the ion transportation rate. Formulated differently, both quantum entanglement and coherence support — and perhaps reinforce — the flow of nerve signals (and thus our cognitive computational capacities).
What is more, Al-Khalili and McFadden maintain that the information embedded in local neural firing patterns, which are underpinned by circulating electrical signals piloted by quantum coherent ion channels, is both mapped onto and synchronized by the brain’s pervading electromagnetic field — visible to us through various techniques, such as electroencephalography (EEG) — thereby effectively providing a link between quantum entanglement at the level of ion channels and our cognitive processes at higher functional levels, whose information is stored within that electromagnetic field.
Another position, introduced by Matthew Fisher, builds on the premise that quantum entangled phosphorus (P) nuclear spins present in the many neurons scattered across the brain form the basis of neural quantum processing in glutamatergic synapses.
The nuclear spin is an inherent property of atoms and alludes to the orientation of an intrinsic magnetic field within the atomic nucleus — the operation of magnetic resonance imaging (MRI) scanners, for instance, depends on hydrogen nuclear spins. Glutamatergic synapses are the main excitatory pathways in the brain and are populated by the neurotransmitter glutamate.
In this model, the nuclear spin is performing the function of a qubit in quantum computing and is accordingly referred to as the ‘neural qubit’. Also, quantum coherence could be preserved over long periods of time, as the nuclear spin is to some extent shielded from its environment.
Through enzyme-catalysed reactions, pyrophosphate splits, so the theory proposes, into two quantum entangled phosphate molecules — the entanglement exists because of the entangled nuclear spins of the two P atoms (there is one P atom for every phosphate molecule).
Interacting with calcium, the phosphate products each convert into larger compounds called Posner molecules (Ca₉(PO₄)₆), which sustain the entanglement. The P spins can retain their coherence for an even longer time now as a result of the additional shielding supplied by these molecules.
Once incorporated within neurons, the binding of Posner molecules can give rise to calcium-mediated presynaptic glutamate release and subsequent postsynaptic firing. Moreover, if two Posner molecules in one neuron bind and each is separately quantum entangled with a Posner molecule in another neuron, then the probability of binding of the two Posner molecules in the second neuron will increase — this also means that the probability of firing of this second neuron goes up.
In this way, quantum entangled Posner molecules dispersed throughout the brain impact non-locally nerve signal communication and might assist in explaining how postsynaptic neurons spike in a coordinated, synchronous manner.
In addition, there appears to exist some experimental groundwork, corroborating Fisher’s hypothesis that nuclear spin-driven quantum processing is at work in the brain. Research conducted on rats by Peter Stokes et al. demonstrates that lithium-6 stimulates their cognitive processes in contrast to lithium-7. On top of that, not only have these two isotopes — isotopes are stable variations of the original chemical element that only differ in their number of neutrons in the nucleus — different nuclear spins, but lithium-6 stays much longer in a coherent state (even up to 5 minutes) compared to lithium-7 (around 10 seconds).
Taken together, this means that these results indicate potential relationships between coherence time, the specific nuclear spin, and cognitive performance.
Such insights could find practical relevance, for instance, in the field of psychiatry. Regarding the treatment for the psychiatric condition of bipolar disorder, lithium is helpful although many questions remain unanswered as to how it succeeds in alleviating depression, minimizing episodes of mania, and stabilizing one’s general mood.
Interestingly, Fisher mentions that replacing calcium by lithium in the Posner molecules would lead to decoherence of the phosphorus nuclear spins, offering a possible explanation for the efficiency of the pharmaceutical lithium (which is for 92% composed of lithium-7) in tempering mania. Then again, Aaron Ettenberg et al. have recently shown that lithium-6 may be more effective in dealing with mania relative to the other isotope.
Notwithstanding the appealing theoretical framework, some researchers, such as Andy Stokely, call into question some of the assumptions laid out in Fisher’s proposal. Also, it still has to be clarified whether Posner molecules can actually be found in real body fluids. At the end of his paper, Fisher presents several experiments that could help to refute or bolster his hypothesis.
Switch On Your Inner Light
In this final theory that we will discuss in this article, Sourabh Kumar et al. investigate whether light produced inside our head could constitute an additional pathway for inter-neuronal communication and information sharing next to the well-known electrochemical nerve signal.
Scientists have observed that the decay (relaxation) of excited molecules involved in oxidative metabolic processes in neurons is responsible for sending out small wave packets of light — called biophotons — between near-infrared and near-ultraviolet frequencies (this includes for the most part the visible spectrum of electromagnetic radiation).
Although research studies have duly documented that cells may make use of biophotons to interact with each other and that light could alter the brain’s functional connectivity, what is not yet clear is which physical medium the photons would exploit to establish a communication network throughout the entire brain.
To that extent, Kumar et al. suggest that light could be propagated by means of myelinated axons, serving as optical waveguides to carry around the biophotons across different brain regions. More specifically, it is the compact sheath (the myelin sheath) around the axon that would act as the waveguide.
Konnie Hebeda et al. contribute with some experimental backing for this model, pointing out that light prefers to travel along the myelinated axons residing in the brain’s white matter. Rendong Tang and Jiapei Dai as well as Yan Sun et al. display further evidence of biophotonic transmission through axons.
To consider the brain as a light-driven quantum information processing instrument, Kumar et al. expect the biophotons to interact with the nuclear spins lodged within ions, molecules, or light-sensitive proteins (similar to cryptochrome in avian magnetoreception), leaving them in a quantum entangled state. Crucially, Parisa Zarkeshian et al. disclose that the decoherence time of nuclear spins in the brain can amount up to tens of milliseconds.
The authors Kumar et al. furthermore highlight that axon-axon synaptic junctions are of particular interest, as they could make up a vital physical link through which the biophotons extend their entangled network within the brain. To experimentally probe and advance their theory, Kumar et al. propose several possible testing setups in their paper.
Unravelling the Question
In accordance with the principles of quantum physics, the most coherent answer to the initial question “Do we have a quantum entangled brain?” is probably a simultaneous yes and no.
Quips aside, in order to get a better grip on whether quantum effects may indeed be relevant to describe what goes on inside our brain, gathering more direct experimental evidence will certainly be instrumental. Moreover, to expand the exploration at such microscopic scales, continuous technological innovations will allow for ever more rigorous testing.
For now, it is time to put our spinning brain to rest.