The link between quantum physics and life has long been debated, perhaps most famously by Erwin Schrödinger in his 1942 book “What is life”. 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 the Nobel prize winning scientist, Roger Penrose has proposed somewhat controversially, it may utilise quantum principles to enable it to process information and generate awareness (Hameroff and Penrose 2014) . Quantum theories of the mind have led to a whole new field of science – “quantum neurophysics” (Tarlaci and Pregnolato 2016), which mirrors the idea that life is anchored in the quantum world (Al-Khalili and McFadden 2014). This of course suggests that it has always been using it. 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” (Lovley and Malvankar 2015). It is thus possible that if life is using quantum principles today, they were probably important in its origins. As another Nobel Prize winning scientist was purported to have said, Albert Szent-Gyögyi, “life is nothing but an electron looking for a place to rest”  (Lane 2015). This, of course, is highly significant, as electrons display wave-particle duality – “electron waves”.

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 energy 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” and ‘superposition’. 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, 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 (Schmied et al. 2016) . 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 (Gribbin 2013) .

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 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 (1.2-10^-9 m). 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 (Ball et al. 2014) , 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.

Unlike quantum mechanics itself, which has been repeatedly verified by experimental observation in physics laboratories, quantum biology is still very much more theoretical, but some evidence does appear to be accruing.

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 (Szent-Gyorgyi 1941), but it was DeVault and Chance in 1966 who proposed it could be due to quantum tunnelling (DeVault and Chance 1966). Certainly , it continues to be suggested that components of living systems could be using 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 (Tamulis and Grigalavicius 2014). Certainly, it is possible that quantum tunnelling and entanglement were important for the beginnings of life, especially in relation to photosynthesis, and could have allowed a greater spectrum of photons to be gathered and more efficient transfer of electrons (Tamulis and Grigalavicius 2014; Trixler 2013). Indeed, it seems that many biomolecules may have been selected for during evolution for their “quantum criticality”, and thus behave somewhere between an insulator and a conductor, so also potentially acting as charge carriers (Vattay 2015).

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.  However, theories are continuing to indicate that it may well be possible that 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”. Critically, it does seem that this effect has been detected in bacterial light harvesting complexes, which could represent 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. Hence, there could be 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) (Engel et al. 2007; Fassioli et al. 2014; Lim et al. 2015; Weber et al. 1995). It is thus interesting that emerging mathematical models suggest that quantum coherence can be maintained for significant periods of time in complex biological systems, potentially orders of magnitude longer than in simple quantum systems at room temperature, by hovering in the “Poised Realm” between the pure quantum and incoherent classical worlds (Vattay, Kauffman, and Niiranen 2014).

A potential 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 (Craddock et al. 2014; Craddock, Priel, and Tuszynski 2014). Today it is thought that long-range electron tunnelling could occur in many proteins. This may or may not be enhanced by aromatic molecules that are often found in oxidoreductases, which are key components of respiratory chains (Winkler and Gray 2014). Indeed, it has been suggested for some time that electron tunnelling may play an important role in how mitochondria produce energy (Hayashi and Stuchebrukhov 2011; Moser et al. 2006), which could ensure a tight coupling between electron flow and protonation via a process known as “redox tuning” (de Vries et al. 2015). There are also theories that strong electro-magnetic fields generated by mitochondria could generate “water order”, and thus protect against decoherence (Pokorny, Pokorny, and Kobilkova 2013).

Like the proposed possible role of quantum effects in bioenergetics suggested above, it is also now being suggested that quantum effects could be important in bird navigation (Zhang 2015), as well as olfaction (Gane et al. 2013). However, there does appear to be actual evidence for significant quantum effects in photosynthesis (Engel et al. 2007), and possibly, in enzyme function involving proton tunnelling, which is investigated by using kinetic isotope effects (KIE) comparing hydrogen, deuterium and tritium transfer (Layfield and Hammes-Schiffer 2014). Data has also been published suggesting that lysozyme appears to demonstrate a “Fröhlich condensate” – in effect, this is where energy in the form terahertz (THz) photons, rather than simply heating up the protein and being dissipated as heat, actually induces a form of quantum molecular resonance similar to a Bose-Einstein condensate (Lundholm et al. 2015).  Fröhlich’s ideas, which he first proposed in 1968 (Frohlich 1968), have been pivotal in providing a theoretical back bone to the concept of quantum effects in biology.