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March 28, 2017

Quantum Biology: Making Waves in the Natural World
Could quantum effects explain the mechanisms behind smell, photosynthesis and bird navigation?
by Carinne Piekema
February 20, 2013
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Simon Benjamin
University of Oxford
The evocative smell of fresh lavender in spring. A ray of sunlight on a leaf, fuelling the plant’s growth. The flight of migrating birds…

Uncovering the secrets behind such poetic natural processes has long been the task of biologists. But now things seem to be changing. Physicists are trying to muscle in on biologists’ territory, claiming—controversially—that at their heart, some of these key processes are quantum. These physicists are at the vanguard of a new discipline, "quantum biology." But how far will the domain of quantum biology stretch? Is nature just one big quantum computer?

Quantum mechanics is the branch of physics that governs the behavior of particles in the subatomic realm. Today, it forms the basis for our understanding of the foundations of chemistry and ultimately controls the way that atoms bond to form molecules. In this sense, there’s no dispute that quantum factors underlie the chemical reactions at work within biological systems. But proponents of quantum biology are making a far bolder claim: that nature enhances its efficiency by exploiting the same weird quantum effects that physicists hope will one day power super-fast quantum computers.

The idea that biological systems may be implementing subtle quantum effects, which are notoriously difficult to generate in the lab, surprises even the physicists who work in the field. "In our experiments, we have to control the system to an to an exquisite level: many of our best results require temperatures that we only see in deep space," says Simon Benjamin, a quantum physicist who shares his time between Oxford University, UK and the Centre for Quantum Technologies, in Singapore. "How can it be that the things we are struggling with in the laboratory could actually be present to any extent in the warm and wet environment of a living system?"

One of the earliest champions of quantum biology was biophysicist Luca Turin, from the Fleming Biological Research Sciences Centre in Vari, Greece. In 1996, Turin kicked up a stink when he proposed a model that explains olfaction—our sense of smell—in quantum terms. Despite being an everyday phenomenon, little is understood about why certain receptors in our noses respond to some smells and not others. In general, for biological receptors, shape is everything. Molecules of a certain shape are able to bind with particular receptors, triggering them via what’s known as the "lock-and-key" mechanism. This is true for antibodies, hormones, enzymes and even many neurotransmitters, so it seems perfectly reasonable to assume that molecules with similar shapes would bind to the same receptors in the nose, generating identical smell sensations. Indeed, this idea forms the basis of the conventional, non-quantum theory of smell.

But there is a major problem with the standard notion that a molecule’s shape determines its smell, notes Turin. "There doesn’t seem to be a correlation between molecular structure and its smell," he says. And Turin should know. For many years, he has been pursuing a side-interest in perfumery—even co-writing a guide to perfumes and starting up a fragrance molecule design company. "In the long history of perfumery there has never been a chemist who can predict the smell by looking at a molecule," Turin says.

Good Vibrations

You don’t even have to have a particularly well-trained sense of smell to notice this, Turin adds. Anyone who has turned their nose up at the whiff of rotten eggs could attest to the fact that molecules that include sulphur and hydrogen atoms bonded together smell equally foul—even if these molecules have very different shapes. Recognising this fact led Turin to propose that we recognise smells on the basis of the characteristic vibration frequencies of the bonds in molecules, rather than on their shape.

Turin was not the first to posit that there may be a connection between olfaction and molecular vibrations. In the 1920s, an instrument called an "infrared spectroscope" was devised that could identify chemicals based on the wavelengths of light they absorb, which in turn depend on the frequencies with which their bonds vibrate. At that time, chemists became fascinated by the notion that the nose might itself be a naturally-occurring spectroscope. But the idea fell out of favor because nobody could figure out how nature could have created a spectroscope-like mechanism from nanoscale proteins small enough to fit in your nose.

Making Scents of Lavender
Quantum processes could enable flowers to absorb the sun’s energy
efficiently and help us to recognise their distinctive fragrance too.
And that’s when smell turns quantum. Specifically, quantum mechanics allows a process called tunnelling that does not occur in classical (non-quantum) physics. Tunnelling enables particles to travel across a high-energy barrier, such as that presented by a whole molecule standing in its path, even if the particle does not seem to have enough energy to jump over this ’wall’ under normal circumstances. Tunnelling has also been exploited in studying molecular vibrations, in a technique called "electron tunnelling spectroscopy," which too inspired Turin. In this case, conditions are tuned so that electrons only tunnel across molecules that vibrate at specific frequencies. Crucially, this form of spectroscopy can occur on a much smaller scale. Could quantum tunnelling be the mechanism that provides the nose with a biological spectroscope? (Listen to Luca Turin talking about the "Secret Quantum Science of Scents" on the FQXi podcast.)

It sounded plausible, but the vibrational theory of the nose was about to suffer a blow. One way to test the idea would be to see if humans can smell the difference between two molecules with similar shapes, but different vibrational frequencies. This can be achieved by replacing hydrogen atoms in molecules with its isotope deuterium ("heavy hydrogen"), preserving the molecule’s overall shape, but changing its vibrational frequency. In 2004, Leslie Vosshall, a biochemist at The Rockefeller University in New York carried out the test using acetophenone molecules—but found a negative result. The quantum theory of smell, it seemed, had been ruled out.

But Turin was not ready to give up yet. Over the next few years, he refined these experiments, arguing that the negative result could be put down to the fact that humans have a notoriously poor sense of smell. In the meantime, Turin was heartened to see that the field of quantum biology was receiving an independent boost, from an unexpected quarter: photosynthesis. And this time, the evidence was so significant, it couldn’t easily be sniffed at.

Lapping Up Light

Photosynthesis is the process by which plants and some algae lap up light from the sun and convert it into useable chemical energy. Key to this process is the transfer of energy from particles of light, or photons, to electrons. Even in extremely low light conditions at the bottom of oceans, this process is extremely efficient. "For one photon absorbed, one electron almost always comes out, so that is almost 100% efficiency," says Alex Chin, a physicist who researches photosynthetic light harvesting at the University of Cambridge, UK.

Alex Chin
University of Cambridge
The extreme efficiency seems surprising given that the signal needs to travel all the way down the leaf, to its "reaction centre," which in itself costs energy. In 2007, chemist Greg Engel, then at the University of California, Berkeley, and colleagues ran a series of experiments that suggested that the process might actually make use of a quantum property known as superposition—the ability to be in two or more places at the same time. "When people were able to zoom in on what was happening in these tiny time windows, they saw that actually energy doesn’t just hop from molecule to molecule," explains Chin. "It actually spreads in a wave-like manner, thus evolving according to the laws of quantum mechanics."

Thanks to superposition, the energy is able to explore all the different pathways to the reaction centre in one go and pick the most efficient route. Imagine you’ve lost your house keys, for instance. Instead of trying to find your lost keys by combing out each room of your house, superposition would effectively allow you to search in all the rooms at the same time, helping you find the keys much more quickly.

Since then, further experiments have added support to the idea that quantum effects are at work in plants—although further investigation is needed for full confirmation. For Chin, the most important question to answer now is how superposition can last as long as it does in these messy biological systems. If physicists can understand that, they may be able to copy it to build quantum computers in the lab that could, for instance, search databases super-quickly. But quantum effects are fragile and are easily destroyed in the lab. "Trying to understand the architecture of photosynthesis might provide design principles to help us harness quantum mechanical properties for technological purposes," Chin says.

Sixth Sense

Quantum biology is also extending its tendrils beyond the plant world and into the animal kingdom. One tentative proposal put forward by Benjamin and colleagues is that exotic quantum states, which only exist momentarily in our best laboratories, could help certain migrating birds find their way to Africa to escape the winter in Northern Europe. "Birds have at least a sixth sense that humans don’t seem to have, which is the capacity to sense something about the Earth’s magnetic field," says Benjamin. But exactly how this avian compass works remains unknown.

Birds have at least a sixth
sense that humans don’t,
the capacity to sense the
Earth’s magnetic field.
- Simon Benjamin
In 2009, the Oxford-Singapore team created a quantum mechanical model for the avian compass using information about the migratory behaviour of the European robin, collected by a group of biologists in Frankfurt, Germany, some decades earlier. The biologists ran a series of experiments on robins caught en route from Scandinavia to Africa, via Frankfurt. The robins were placed in a special enclosure, with walls coated with a material that registers the marks created as the birds scratched those walls—indicating the direction that they wished to fly. The biologists checked whether their sense of direction was affected by the presence of an applied magnetic field. Notably, the robins could only head in the right direction under particular light conditions. This implied that the robins’ eyes were used not just for vision but also for navigation.

Based on their findings, the Frankfurt biologists came up with an explanation of how birds could detect the magnetic field, called the "radical pair" hypothesis, which Benjamin and colleagues have modified. The quantum physicists now believe that the bird might actually "see" a grid like pattern on its eyeballs. The idea is that when a molecule in the eye absorbs a photon from sun light, it gives an energetic nudge to a pair of electrons in the molecule. The electrons are entangled—that is, they are inextricably linked by a quantum property so that they influence each other, no matter how far apart they are separated. One of the electrons in the pair is dislodged and kicked off to a new location, but it remains linked to its partner, and each feels a slightly different level of magnetism, due to the Earth’s field. Before the transported electron relaxes back to its original state, a small electric dipole field is created, leaving a little trace on the bird’s vision. The orientation of the molecule with respect to the Earth’s magnetic field dictates how quickly the electron relaxes back and thus controls the strength of the superimposed image on the bird’s vision. (Listen to Benjamin and his colleague Erik Gauger talking about "Quantum Birds" on the FQXi podcast.)

Second Chance for Smell

The work on photosynthesis and birds, though still in its early stages, helped boost Turin’s belief that he was on the right track with his quantum theory of smell. By 2011, he and his colleagues had carried out a series of experiments with fruit flies—known to be much more discerning sniffers than humans—to see if they could pick out differences between molecules with different vibration frequencies. These experiments were a success: The fruit flies, it seemed, could tell the difference.

Spurred on by these positive results Turin decided to give human subjects another chance. Vosshall’s group had used quite small molecules in their test, but in a study published in the journal PLOS One in January 2013, Turin and colleagues replaced the hydrogen in larger molecules than those used by Vosshall’s team, like musks, with deuterium. They tested the molecules on 11 people and found that they could smell the difference. Turin argues that this provides positive evidence for the vibration model.

Luca Turin
Fleming Biological Research Sciences Centre
Tim Jacob, a smell researcher in the School of Biosciences at the University of Cardiff, UK, says that Turin’s work "should be taken seriously." "The vibration theory does indeed explain things about smell that shape doesn’t," says Jacob. But he adds that our experience of smell goes beyond the mechanism that occurs at the level of receptors and that the processing of the smell in the brain also plays a significant role in our reactions to specific molecules.

Vosshall, however, remains sceptical, noting that it’s difficult to rule out whether or not there were impurities in the compounds being smelled, which may have skewed the results. "This debate on the vibration theory will never end," she says. "The very important underlying question of why things smell the way they do will in my opinion only be solved by mechanistic atomic level study of the odorant receptors themselves."

The debate over quantum biology looks set to rage on. But what these three strands of research—encompassing smell, photosynthesis and the avian compass—show is that quantum mechanics may provide new ways of understanding biological phenomena. It also seems likely that other quantum-biological processes may be discovered in the future. As Turin puts it: "I don’t for a minute believe that this is all there is to it. This is only the beginning and a vast territory of biological hardware science will one day turn out to be quantum."

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