The link between quantum physics and life has long been debated, perhaps most famously by Erwin Schrödinger in his 1942 book “What is life” (insert hyperlink). However, research into quantum biology is still very much in its infancy – this is despite the fact that applications of quantum theory are readily used in many different fields, ranging from explaining chemical reactions and how the sun works, to MRI machines and computers. One of the main reasons for this is that the quantum world is inherently counter-intuitive and bears little resemblance to the intuitively sensible macroscopic Newtonian universe. Indeed, while some attempts to integrate quantum theories into a “Grand Unified Theory” of everything have met with some success, there are still big holes in our understanding. For this reason quantum physics has largely remained the domain of physicists and theoretical mathematicians, which has been compounded because studying the “quantum world” generally requires highly controlled conditions as quantum effects rapidly disappear when they come into contact with the chaotic macroscopic world (called “decoherence”), and therefore generally require the use of some very expensive equipment. In contrast, biological research is generally much cheaper and easier to do, and has been incredibly successful in explaining the top level biochemistry of life, but as any chemist will tell you, to truly understand the underlying chemistry does require quantum physics to explain how chemistry actually works.

It has been said that the brain is not simply a computer system and as Roger Penrose has proposed, it may utilise  quantum principles to enable it to process information and generate awareness [31]. Quantum theories of the mind have led to a whole new field of science – “quantum neurophysics” [32], which mirrors the idea that life is anchored in the quantum world  [33]. Interestingly, it is now becoming clear that bacteria can transfer electrons both between the same species and with other species in a form of symbiosis via “bacterial nanowires”. In effect, these are biological conductors; they can transfer energy. Significantly, this “conductance” appears to have been solved in at least two ways by nature: one is more similar to classical metallic conductance based on free electron theory, while the other seems to depend on quantum effects – such as “tunnelling” [34]. It is thus very likely that if life is using quantum principles today, they were probably important in its origins.

Quantum literally means “how much”, but is today used to describe the minimum unit of energy or matter. It was Planck who realised that there was a minimal “quantum of action”, in effect, there is a minimum change that can be measured in nature, which became known as Planck’s constant, or h, which equals 6.6 x 10-34 Js (joules per second). The implications from this were profound, not least of which were that any measurement of nature is based on quantum effects, and that the size and shape of things is also determined by Planck’s constant. It also means that there is always motion within matter; at the molecular level, the shape of things is determined by an average and motion is therefore “fuzzy”, and it is impossible to assign both momentum and position of a particle. It also means that the so called “energy barriers” normally encountered in most physical/biological/chemical system may not be barriers at all. This describes one of the most fascinating principles of the quantum world, “tunnelling”. The phenomenon of “tunnelling” explains how objects can permeate energy barriers without the necessary energy because they can exist as probability waves; the likelihood of this can be predicted by the Schrödinger equation. This basically tells us that the ability to do this depends on their energy and mass, and the width of the barrier. It is actually quite likely for very small particles like electrons and protons, but extremely unlikely for large objects such as humans. Thus increasing temperature can enhance the effect as it can impart more energy, although as we will discuss later, it also can inhibit it.

To explain quantum tunnelling, a signature effect predicted by quantum mechanics, one of the basic concepts underlying the quantum world is that of wave-particle duality; De Broglie showed that just as a photon can behave both as a wave and a particle, all particles could have a “wave function” ascribed to them – matter-waves. This was pivotal, as electrons could thus also behave as waves. The wave function also displays something called “phase”, in effect quantum particles behave as a rotating cloud, and thus can be influenced by magnetic fields; they have “spin”. Spin explains Pauli’s exclusion principle and why atoms, or planets, don’t collapse in on themselves and matter feels “hard”. However, tunnelling also depends on “quantum coherence” such that an electron, proton, atom, or a group of atoms, exist in “quantum superposition” - in effect, it or they exist as a collection of all possible states. Another facet of this is “entanglement”, or as Einstein put it, “spooky action at a distance” – which describes the ability of two entangled particles to “know” the state of the other when one is observed, regardless of distance – instantaneously. This is known as “non-locality”, as encompassed by Bell’s theorem; this is a profound departure from classical physics. Bell’s inequality has now been tested repeatedly, and the most recent experiment does strongly suggest that quantum entanglement is entirely real [37]. From the quantum point of view, once entangled, two particles have to be regarded as the same entity, irrespective of distance. Thus entanglement is not only key to understanding reality, but is key to many current and future technologies, including quantum computing [38].

However, when particles are observed, they appear in one particular state and thus display classical properties we associate with the everyday world. Thus to exist in a non-classical quantum state, they need to be isolated from external interference from the environment; as soon as this system interacts with it, it becomes “decoherent” and they would appear to behave as particles rather than probability waves; effectively they are being “observed” (a sort of Schrödinger’s cat condition). The more particles involved, the quicker the quantum state collapses – as maintaining a coherent state becomes increasingly difficult with increasing size due to interaction with the environment. This is why a tennis ball, although it can be technically be assigned a wave length, is always observed as a tennis ball; calculating its de Broglie wavelength, which is obtained by dividing the Planck constant by the ball’s momentum, is around 10-34 m, while that of an electron, with a rest mass energy of 0.511 MeV, at 1eV, is 1.2 nm. It is also why a cat does not exist in superposition; it is intimately coupled to its environment. The important message here is that microscopically, coherence is possible, not only for single entities, such as electrons, but also for larger groups of atoms – indicating that they can behave as one entity. But this state is rapidly lost via interaction with the wider environment; this is explainable thermodynamically, because most “environments” contain vast number of molecules that display randomness. This is why we view the world macroscopically. For a basic introduction to quantum physics, a good starting point is the 30-second quantum theory book, edited by Brian Clegg [39], or for a more detailed over-view, the free to down load text book: “Motion mountain – adventures in physics, volume IV, the quantum of change”, edition 28.1, 2016, by Christoph Schiller (http://www.motionmountain.net/) is a good, but more in-depth reference.

The possibility that electrons could move along enzymes by rising to a higher energy level, a bit like they might do in metals, was first suggested by Szent-Györgyi in 1941 [35], but it was DeVault and Chance in 1966 who proposed it could be due to quantum tunnelling [36]. In fact it is now becoming clear that components of living systems use quantum principles, for instance, by absorbing light energy and transferring it across a series of molecules – a fundamental quantum process, where quantum entanglement can be viewed as a form of quantum superposition [40]. One of the factors that can influence coherence is the environmental temperature, as this indicates the energy of a particle, and thus its ability to interact with other components. The higher its energy, the more likely it is to disrupt it. For many years, it was thus thought that life was simply too “warm and wet” for coherence to occur. As it now turns out, this is far from being correct, as life has actually tuned itself to use thermal vibrations to “pump” coherence, rather than disrupt it, which results in a phenomenon known as “quantum beating”. This effect has been detected in bacterial light harvesting complexes and essentially represents a coherent superposition of electronic states, analogous to a nuclear wavepacket in the vibrational regimen. In essence, the energy in light can be harvested very efficiently and transferred using wavelike resonance. There is thus a “goldilocks zone” to optimise efficiency; in effect, just the right amount of “noise” can result in enhanced “coherence” and “tunnelling” due to stimulating particular vibrational modes in proteins – so called exciton-vibrational coupling (vibronic coupling) [41-44]. A good example of this is the material that makes up the cytoskeleton of a cell, tubulin, which contains chromophoric aromatics molecules such as tryptophan, and thus may play a role in coherent energy transfer; key in this maybe their free pi electrons [45, 46]. Today long-range electron tunnelling is thought to occur in many proteins, and seems to be enhanced by particular molecules, such as the aromatics – which occur more frequently in oxidoreductases, which are key components of respiratory chains [47]. Crucially, it is now thought that electron tunnelling plays an important role in how mitochondria produce energy [48, 49] and ensures a tight coupling between electron flow and protonation via a process known as “redox tuning” [50]. In effect, electron tunnelling appears to be a pivotal component of mitochondrial function. Certainly, it seems likely that quantum tunnelling and entanglement were essential for the beginnings of life, especially in relation to photosynthesis, allowing a greater spectrum of photons to be gathered and more efficient transfer of electrons [40, 51]. Indeed, many biomolecules may have been selected for their “quantum criticality”, and thus behave somewhere between an insulator and a conductor, so also potentially acting as charge carriers [52]. Practical examples include quantum effects used in bird navigation [53], an explanation of how photosynthesis works [41], and possibly, even olfaction [54]. The recent discovery that lysozyme appears to demonstrate a “Fröhlich condensate” [55], when combined with concept that strong electro-magnetic fields generated by mitochondria could generate “water order”, and thus protect against decoherence [56], is perhaps further evidence. In fact, emerging mathematical models suggest that quantum coherence can be maintained for significant periods of time, orders of magnitude longer in complex biological systems than in simple quantum systems at room temperature – in effect the system can hover in the “Poised Realm” between the pure quantum and incoherent classical worlds [57].