The physical term quantum means the smallest unit of a physical quantity the system is able to possess. The quantum world is the microworld of elementary particles, which are the fundamental building blocks of matter. The brain is made up of physical matter like all other living and nonliving systems. The ultimate pursuit for brain science is to give an explanation of how matter that comprises the physical structure of the brain gives rise to its functions, in particular higher cognition and consciousness. That the brain does give rise to consciousness is a key assumption of modern neuroscience and we will take it as a given, otherwise we would be compelled to seek these answers in the realm of religion or metaphysics. A search for that link has occupied numerous philosophers and scientists for at least two millennia . In recent years some scientists have begun to probe the brain at the quantum level of physical description, facing considerable opposition from the traditionally inclined academics in both physics and neuroscience.
Perhaps the most bizarre quantum feature is the effect called superposition that implies that quantum particles can exist in multiple spatial locations or states being described by a mathematical superposition of pure state wave functions simultaneously. Such quantum superposition states can end when out of each multiplicity of the possible states the system selects one definite state or spatial location. Because quantum systems are described mathematically by a quantum wave function, and because quantum systems switch states they occupy very rapidly, the transition from quantum to classical states is often termed wave-function collapse (or sometimes state reduction). A number of experiments in the early 20th century demonstrated that quantum superpositions persisted until they were observed or measured by an experimentalist (observer). If a machine measured a quantum system, the results appeared to remain in superposition within the machine until actually viewed by experimenters. Therefore, the prevalent view in physics at that time (expressed within the famous Copenhagen interpretation) was that conscious observation led to a collapse of the wave function. To illustrate this paradox and the apparent absurdity of the notion, Erwin Schrodinger in 1935 described his celebrated thought experiment known as Schrodinger's cat. In this example, a cat is placed in a box with a vial of poison. Outside the box, a quantum event (e.g., passage/not passage of a single photon through a half-silvered mirror) is causally connected to the release of the poison inside the box. Since the photon is a quantum object in a superposition state, it both passes and does not pass through the mirror. Hence the poison is both triggered and not triggered. Therefore, by quantum logic, the cat must be both dead and alive until the box is opened and the cat observed. (Analogous to quantum logic, superposition of mental events is commonplace, further suggesting that the mind is a quantum system. Cognitively, we are simultaneously prepared for the cat being alive and prepared for the cat being dead until we open the box.) At the moment the box is opened, the system chooses either to reveal a dead cat or a live cat. Therefore, consciousness essentially selects reality. The precise choice in any given quantum collapse experiment was believed to be probabilistic, an idea Einstein found unsettling by proclaiming in a famous statement that: "God does not play dice with the universe."
Today the generally accepted view is that any interaction of a quantum superposition state with the classical environment causes decoherence. Due to these difficulties many physicists maintain that quantum theory is incomplete and that other approaches to the problem of collapse of the quantum wave function need to be found. One suggestion, called the multiple-worlds hypothesis, was put forward by Hugh Everett  and it holds that each collapse event is a branching of reality into parallel manifolds, so for example, a dead cat in this universe corresponds to a live cat in a newly formed parallel universe. If so, there must exist an infinity of parallel worlds, a bizarre notion to many. David Bohm's theory of quantum reality  avoids collapse altogether, while still other views hold out for an objective factor causing wave-function collapse. These latter ones are called objective reduction (OR) theories. For example, Ghirardi et al.  predicted that OR would occur at a critical number (on the order of 1017) of superpositioned particles.
Complementarity and entanglement are quantum-level concepts with potential explanatory power with regard to some properties of consciousness . A number of prominent figures in quantum physics, including Planck, Bohr, Schrodinger, and Pauli, argued for the irreconsilability of physical determinism and conscious free-will (for reviews see [4, 31, 60]). These early physicists sometimes used terms such as entanglement, superposition, collapse, and complementarity metaphorically without defining precisely how they should be applied to specific situations in cognition. Later, approaches were proposed that described neurophysiological and/or neuropsychological processes in some detail (e.g., [54, 62, 70, 72]. In his 1999 paper, Stapp addresses potential causal interactions, raising possibility that: "conscious intentions of a human being can influence the activities of his brain". Stapp further argues that the probabilities for eigenstates after collapse can be mentally influenced and that conscious mental events are assumed to correspond to quantum collapses of superposition states at the level of macroscopic brain activity.
Quantum field theory has been used in a preliminary way to describe memory. Ricciardi and Umezawa  emphasized many-particle systems and vacuum states of quantum fields as potential memory storage devices (see also [27, 63]). This type of memory would not be accessible to consciousness without external stimuli activating a neuronal assembly, however. The activation of coherent neuronal assemblies enables a conscious recollection of the content encoded in the vacuum state. Pessa and Vitiello  speculate that dissipation, chaos and quantum noise generate an arrow of time for the system. This would not be a plausible mechanism for long-term memory, however, because the model gives rise to a temporally limited memory .
Up until John Eccles' death in 1997, Beck and Eccles applied the principles of quantum mechanics to vesicular release at the synaptic cleft, hypothesizing that quantum indeterminacy was a factor in the all-or-none quantal release of neurotransmitter (see posthumous account in Beck and Eccles ). Chemical synapses depend upon vesicular release of transmitters from the presynaptic terminal, and this is triggered by a nerve impulse reaching the axon terminal. Although the biochemistry of vesicular docking and exocyto-sis is reasonably well understood, the trigger mechanism can also be viewed in a statistical way. In the latter case, either stochastic, thermodynamics or quantum mechanics should apply. Due to the nanometer size range of most proteins or macromolecules in the presynaptic terminal, quantum processes would be expected to prevail over thermal processes and purely stochastic release is less attractive as a correlate of consciousness. Beck and Eccles built their quantum concept of a release trigger on quasiparticle tunneling, which results in a probability of exocytosis in the range between 0 and 0.7, comparable to experimental observations. Beck and Eccles relied on theory worked out by Marcus  and Jortner , who similarly modeled quantum-based electron transfer between biomolecules.
More recently, Penrose [43, 44, 45] has claimed that the underlying reality itself, namely the fundamental space-time geometry, actually bifurcates during the superposition process. This is similar to the multiple-worlds view except the separations are unstable and hence they rapidly reduce to a single, undivided reality. Classical noncomputability is a key feature of conscious processes, which may also elevate our mental processes above that of mechanistic determinism that appears grossly inadequate. In Penrose's books "The Emperor's New Mind" and "Shadows of the Mind", he claims that the phenomenon of quantum collapse can explain the features of consciousness since the spontaneous wave function collapse is what distinguishes our thought processes from the behavior of completely deterministic classical computers (for a review, see ). According to Penrose, consciousness involves a time-ordered series of quantum-state reductions corresponding to individual thoughts. Although such ideas are controversial, the fact that quantum theory is being applied successfully to a new kind of computing (called quantum computing) where the collapse of multiple quantum possibilities to definite classical states is the key element lends credence to quantum approaches to consciousness. Applications of quantum physics to new modes of computation are currently being hotly pursued in the hope of finding a more powerful technology where the possibility of manipulating quantum states gives rise to the ultimate miniaturization of computer chips that would ultimately represent individual atoms or particles. While in classical computation, elementary units of information are the discrete bits (1 or 0), the basic units of quantum computation are quantum superposition states called qubits, where both 1 and 0 are represented simultaneously with arbitrary relative amplitudes. While qubits interact (or compute) with each other, they then reduce or collapse to a particular set of measurable states. Quantum computers would offer enormous potential advantages for certain applications, and prototype devices have already been constructed. Hence, comparisons involving the brain, mind, and quantum computers are logically linked and worth further investigation.
Other quantum properties of microscopic physical systems offer possible explanations of various aspects of consciousness. Because of a physical property called quantum coherence, individual particles lose their separate identity and become part of a common unit described by one wave function, as is the case with lasers where they produce optical coherence. Hameroff , Vitiello , Jibu and Yasui  and others have suggested this type of quantum coherence as an explanation for the unitary nature of self and the binding property in conscious experience. In nonlocal quantum entanglement, particles once unified in a common quantum state remain physically connected at a distance . When one particle is measured, its quantum entangled partner particle reacts instantaneously, regardless of its location. This quantum interaction-over-distance has been proposed to provide a basis for associative memory, as well as an explanation of emotional connections between conscious individuals .
If validated, these speculations would indicate that biological evolution has taken advantage of quantum processes, one use being quantum computation in the brain. Indeed, modern biochemistry can easily identify molecules in the brain that operate at least partially in a quantum manner at the subneuronal level. Examples include various receptor proteins, enzymes, membrane lipids, presynaptic vesicle structures, gap junctions, neurotransmitter molecules, calcium ions, DNA, RNA, and microtubules and other protein filaments. The key question still remains: At what level of organization do quantum effects cease to exist, or become thermalized in a noisy system like the brain? In other words, where can we place the quantum/classical boundary? Conservative scientists argue that quantum effects are destroyed already at the level of individual molecules and ions in a thermal environment. On the other hand, advocates of quantum consciousness theories see more highly organized and spatially extended quantum states, for example, involving a number of different microtubules in the same neuron or even in several neurons forming a coherent cluster. Penrose and Hameroff  have put forth a highly original model of consciousness based on quantum computation in microtubules within the brain's neurons. This and other quantum models elucidate a number of enigmatic features of consciousness; however, a few hurdles remain in establishing their likelihood. Some of these difficulties are identified when designing prototype quantum computers. One such obstacle is that quantum computers will require a high degree of isolation from decoherence effects of the local environment, or alternatively some kind of fault-tolerant architecture that permits delicate quantum computing in the presence of realistic levels of decoherence . The brain operates at body temperature, its mass comprises 60 percent water, and is electromagneti-cally, chemically and mechanically noisy, all of which would seem to severely shorten the time allowed for quantum computation. Long-lasting, large-scale quantum states are deemed to be impossible in the brain because a single ion, photon, or thermal vibration can cause decoherence and hence random reduction to classical states. On the other hand, proponents of a quantum approach to consciousness point to a number of physical mechanisms in the brain that may lengthen the time of quantum coherence and provide necessary quantum isolation. Firstly, microtubules may be able to perform quantum computations at room temperature because basic maintenance of microtubules is energy dependent, resulting in energy being continuously pumped in and out. This situation is analogous to that of lasers, which work according to quantum optical principles at room temperature [21, 39]. Secondly, the water of hydration surrounding microtubules appears to be in an ordered state, which decreases noise . Thirdly, topological error correction (in a manner similar to that of the fault-tolerant architecture described above) may protect delicate quantum states .
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