Quantum mechanics is the fundamental theory that describes the properties of subatomic particles, atoms, molecules, molecular assemblies and possibly beyond. Fundamentally, all matter—animate or inanimate—is quantum mechanical, being constituted of ions, atoms and/or molecules whose equilibrium properties are accurately determined by quantum theory. All living systems are made up of molecules, and fundamentally all molecules are described by quantum mechanics. As a result, it could be claimed that all of biology is quantum mechanical.
In quantum mechanics, all objects have wave-like properties, and when they interact, quantum coherence describes the correlations between the physical quantities describing such objects due to this wave-like nature.
However, Quantum effects are very delicate, and physicists have to work very hard to maintain it in labs. They cool their systems down to near absolute zero, carry out our experiments in vacuums, and try and isolate them from any external disturbance. That’s very different from the warm, messy, noisy environment of a living cell. And for many years, scientists operated on the idea that biology was merely a product of deterministic chemical reactions, and as such, unaffected by quantum effects.
What is meant by quantum biology is the involvement of phenomena that are normally confined to the quantum realm of atoms and molecules, such as coherence, tunnelling, entanglement or spin, because their effects are normally cancelled out at the macroscopic level due to decoherence and would thereby be thought highly improbable inside the warm, wet and disordered environments inside living cells.
However, this definition does not address the dynamical nature of biological processes, or the fact that a classical description of intermolecular dynamics seems often sufficient. Traditionally, however, the vast separation of scales between systems described by quantum mechanics and those studied in biology, as well as the seemingly different properties of inanimate and animate matter, has maintained some separation between the two bodies of knowledge.
Recently, developments in experimental techniques such as ultrafast spectroscopy, single-molecule spectroscopy, time-resolved microscopy and single-particle imaging have enabled us to study biological dynamics on the increasingly small length and time scales, revealing a variety
of processes necessary for the function of the living system that depends on a delicate interplay between quantum and classical physical effects.
Recently scientists are finding that Bird navigation, plant photosynthesis and the human sense of smell all represent ways living things appear to exploit the oddities of quantum physics. Quantum mechanics operates on the nanometre and sub-nanometre scales and is at the basis of fundamental life processes such as photosynthesis, respiration and vision. It has only been in the past few years that scientists have started figuring out how quantum mechanics is exploited by animals, plants and bacteria to give some of them a keen sense of smell and others a very efficient way to harness energy from the sun, said Lloyd, a professor of quantum mechanical engineering at the Massachusetts Institute of Technology.
Quantum biology, a young and increasingly popular science genre, seeks to understand whether quantum mechanics plays a role in biological processes. Quantum biology is the application of quantum theory to aspects of biology for which classical physics fails to give an
accurate description. quantum biology promises to give rise to design principles for biologically inspired quantum nanotechnologies, with the ability to perform efficiently at a fundamental level in noisy environments at room temperature and even make use of these ‘noisy environments’ to preserve or even enhance the quantum properties.
Quantum Effects exploited by living things
That means humans might be able to develop technology for more accurate sensors or far more efficient solar cells by mimicking the way living things use quantum physics, he said. “When you want to make an ultimate sensor, something that senses things at the limits allowed by the laws of physics — well, at that level, everything is quantum mechanical.”
In the case of human smell, it appears that receptors are triggered in part by “phonons” — tiny vibrational phenomena that provide the extra energy needed to create a signal that we recognize as a scent. “This process, which is called phonon-assisted tunnelling, is a purely quantum mechanical process,” Lloyd said. “It can’t be explained by ordinary classical models.
Meanwhile, birds like the European robin have an internal compass that helps them navigate, and it appears to make use of quantum entanglement — a linkage of two or more very small objects so that any change to one is immediately experienced by another, no matter how far apart they are. When a photon enters an animal’s eye, it hits a magnetic receptor which produces a pair of free-radicals. These free-radicals interact with each other, as well as a weak magnetic field, causing a spin that allows the animal to “see” the magnetic field and give it a kind of compass.
Lloyd said birds can sense the orientation of the Earth’s magnetic field, but can’t tell the difference between north or south, and become disoriented by oscillating magnetic fields like microwaves. Physics experiments show that certain entangled electrons are also very sensitive to the orientation of weak magnetic fields, and the birds’ behaviour suggests they are using that to navigate. Lloyd’s biological research, funded by the U.S. Defense Advanced Research Projects Agency, looks at how living things use quantum computation.
Parrots exploit quantum physics to produce their colourful feathers
New Zealand research published in the Royal Society Open Science journal in July 2018 found a parrot’s red feathers contain the same pigment molecules used to make yellow feathers — just arranged differently. Unlike other birds, parrots don’t rely on what they eat to colour their feathers red, orange and yellow. Instead, they get their warm hues from a particular group of pigments called “psittacofulvins”, said Monash University ornithologist Kaspar Delhey, who wasn’t involved in the study.
For the new study, the researchers, led by University of Otago physical chemist Jonathan Barnsley, looked at parrot tail feathers yellow and red patches using a technique called Raman spectroscopy, which takes advantage of the fact molecules vibrate when illuminated by laser light. “Each molecule can have a number of different vibrations which make up a signature ‘chord’ and we detect that,” Mr Barnsley said.got hold of a multicoloured from a yellow-naped amazon (Amazona auropalliata).
They found both yellow and red patches contained the same type of psittacofulvin pigment molecule — one that usually bestows a yellow hue. What differed, though, was how the molecules were arranged. In the yellow patch, pigment molecules appeared to be separated. But in the red, they were snuggled up close. So how does the closer arrangement of yellow pigment molecules turn a feather red? Here we enter the gnarly world of quantum physics. If you move molecules closer or further apart, what’s called the “energy gap” also changes. This gap affects what wavelengths of light are absorbed and reflected or transmitted — and changes the colour we see.
Instead of designing new expensive molecules, they might find new ways to “tune” and reorganise simpler, cheaper materials. “Nature starts with simple materials and, through interactions, results in some really complex materials,” Mr Barnsley said. I think we can learn a lot from that.”
Lloyd said he got into the area about 3½ years ago when someone in his lab found an article in the New York Times about researchers in Berkeley, Calif., who claimed green sulphur bacteria were performing a type of quantum calculation called a quantum search process while using photosynthesis to turn sunlight into energy. “We thought that was really hysterical,” he recalled. “It’s like, ‘Oh my God, that’s the most crackpot thing I’ve heard in my life!'” But after looking into it, he realized that although the bacteria weren’t performing quantum search, they were doing a different type of quantum computation.
When sunlight hits the part of the bacteria that collects sunlight, it creates a quantum particle of energy called an exciton. That exciton must travel through a complex that Lloyd likens to a gigantic forest in order to get to a place where it can be turned into chemical energy. “If you look at kind of a classical way of getting through some forest in the dark, surrounded by trees with no notion of what the direction is you’re supposed to go, then you just wander around at random … and you just get completely lost,” Lloyd said. Consequently, scientists were puzzled about how the exciton ever arrived at its destination.
The answer turns out to be a special kind of computation. If it uses quantum mechanics, it’s not limited to just taking just one path through the forest,” Lloyd said. “It can take all possible paths simultaneously. In quantum computing, this is what’s called a quantum walk.” As it turns out, plants use the same trick to achieve photosynthesis. Lloyd had spent his research career studying quantum computation and designing quantum computers. For humans, that’s a relatively new field. “It turns out,” Lloyd said, “that bacteria have been up to quantum computation for hundreds of millions of years.”
Scholes and his team decided to probe quantum behaviour in plants
When it comes to green living, nobody does it better than plants. When plants convert light into fuel through photosynthesis, not a single particle of light is wasted. If we could unlock plants’ secrets, we might be able to perfect the design of light harvesting in solar cells. Gregory Scholes, Princeton’s William S. Tod Professor of Chemistry, suspects that the key to plants’ efficiency stems from their ability to harness quantum physics, the unintuitive behaviors of very small particles. In 2010, he led a team that demonstrated quantum effects in marine algae.
But the finding was not without controversy. Quantum behaviors usually reveal themselves at extremely low temperatures isolated from real-world disturbances, raising questions as to whether these quantum states can survive the warm and wet conditions of life. So Scholes and his team decided to probe quantum behavior in one of the simplest known chemical reactions, the transfer of a hydrogen atom from one part of a molecule to another. If their experiments work out, the researchers could rewrite our understanding of how chemical reactions occur.
At the heart of quantum theory is the idea that matter can behave both like particles and like waves. If we fire particles at a barrier containing two slits, classical physics predicts that the particles will land in two piles, one behind either slit. In contrast, quantum mechanics predicts that each particle will act like spread-out waves and pass through both slits (see figure), where the intensity at any point can either add together or cancel itself out.
This quantum nature of matter, known as a superposition, has been predicted and observed since the 1920s, and has helped us comprehend the microscopic world. However, these quantum ideas have not yet shaped the understanding of chemical reactions. “That’s the big question we’re trying to answer,” Scholes said. “Can we harness quantum mechanics to work for us in chemistry?”
The researchers addressed the question by studying what happens when light hits a molecule that can undergo two separate hydrogen transfer reactions, one on the left and one on the right side of the molecule. If classical rules prevail, then each reaction will proceed one step at a time. If quantum rules are involved, both the left and the right reaction will occur in a quantum superposition.
To find out if this is happening, Scholes’ team set up an experiment to take snapshots of molecules during the reaction. With funding from the W.M. Keck Foundation, the researchers bombard the molecules with pulses of laser light, which place the normally chaotic molecules into the same quantum rhythm. “We use light to synchronize molecules so that they all dance to the same beat,” said graduate student Ben Xinzi Zhang.
Next, to see if the molecules are indeed in a superposition, researchers use a second laser pulse. It monitors the state of the molecule using rapid bursts that light up the positions of the atoms like a strobe light illuminating a dancer.
The researchers then look for patterns from reaction pathways that add and cancel, just like in the double-slit experiment. “The signatures will tell us how these two sides are interacting,” said Kyra Schwarz, a postdoctoral research associate on the tea
The results of the experiment are not yet in, but if quantum superposition plays a role in a reaction as ubiquitous as the transfer of hydrogen, it could underlie many processes in nature. The result could also reshape how chemical reactions are conceptualized and understood across disciplines, making it possible to leverage quantum properties to control and create new reactions.
Scholes will also lead a new Energy Frontier Research Center, announced in July 2018 and funded by the U.S. Department of Energy, to study how light harvesting and solar photochemistry can make new molecules and fuels. The center includes faculty members at Princeton as well as national laboratories and leading universities.
Diagram of FMO complex. Light excites electrons in an antenna. The excitation then transfers through various proteins in the FMO complex to the reaction center to further photosynthesis.
Organisms that undergo photosynthesis absorb light energy through the process of electron excitation in antennae. These antennae vary among organisms. For example, bacteria use ring-like antennae, while plants use chlorophyll pigments to absorb photons. Photosynthesis creates Frenkel excitons, which provide a separation of charge that cells convert into usable chemical energy. The energy collected in reaction sites must be transferred quickly before it is lost to fluorescence or thermal vibrational motion.
Various structures, such as the FMO complex in green sulfur bacteria, are responsible for transferring energy from antennae to a reaction site. FT electron spectroscopy studies of electron absorption and transfer show an efficiency of above 99%, which cannot be explained by classical mechanical models like the diffusion model. Instead, as early as 1938, scientists theorized that quantum coherence was the mechanism for excitation energy transfer. Studies suggest that, through evolution, nature has developed a way of protecting quantum coherence to enhance the efficiency of photosynthesis. Latest research suggests, while quantum coherence dominates in the short-term, a classical description is most accurate to describe long-term behavior of the excitons.
Another process in photosynthesis that has almost 100% efficiency is charge transfer, again suggesting that quantum mechanical phenomena are at play. The authors, Don DeVault and Britton Chase, postulated that these characteristics of electron transfer are indicative of quantum tunneling, whereby electrons penetrate a potential barrier despite possessing less energy than is classically necessary
From Michigan State University: “Nature provides roadmap to potential breakthroughs in solar energy technology”
Sunlight, although abundant, is a low-density energy source. To collect meaningful amounts of energy you need larger amounts of space. However, the most effective materials in use today for solar energy conversion, such as Ruthenium, are some of the rarest metals on Earth. Future solar technologies must be able to scale up with more efficient and cheaper methods of energy conversion.
MSU Foundation Professor James McCusker, Department of Chemistry, believes that the future of solar energy lies in abundant, scalable materials designed to mimic and improve upon the energy conversion systems found in nature. Light-absorbing compounds in common synthetic methods for artificial photosynthesis make use of excited molecular states produced after a molecule absorbs energy from sunlight. The absorption of light energy exists long enough to be used in chemical reactions that rely on the ability to move electrons from one place to another. One possible solution is to find more commonly available materials that can achieve the same result.
“The problem with switching (from rare Earth metals) to something Earth-abundant like iron — where the scalability problem disappears — is that the processes that allow you to convert the absorbed sunlight into chemical energy are fundamentally different in these more widely available materials,” McCusker said. The excited state produced by absorbing light energy in an iron-based compound, for example, decays too quickly to enable its use in a similar manner.
In a groundbreaking new study in Nature reported in June 2020 , McCusker reveals a novel process that allows molecules to tell scientists how they should be modified to better absorb and convert solar energy. The method uses a molecular property known as quantum coherence where different aspects of a molecule are synchronous, like when your car’s turn signal blinks in unison with that of the car in front of you. Scientists believe that quantum coherence may play a role in natural photosynthesis. By hitting a molecule with a burst of light lasting less than one-tenth of one trillionth of a second, McCusker and his students could observe the interconnection between the molecule’s excited state and its structure, allowing them to visualize how the atoms of the molecule were moving during the conversion of solar to chemical energy.
“Once we had a picture of how this process occurred, the team used that information to synthetically modify the molecule in such a way as to slow the rate of the process down,” McCusker said. “This is an important goal that must be achieved if these types of chromophores — a molecule that absorbs particular wavelengths of visible light and are responsible for a material’s color — are to find their way into solar energy technologies.”
“The research demonstrates that we can use this coherence phenomenon to teach us what sorts of things we might need to incorporate into the molecular structure of a chromophore that uses more earth-abundant materials to enable us to use the energy stored in the molecule upon absorption of light for a wide range of energy conversion applications.”vFor McCusker, this breakthrough will hopefully speed up development of new technologies, “eliminating a lot of the trial and error that goes into scientific endeavors by telling us right out of the gate what kind of system we need to design.”
Physicists make quantum leap in understanding life’s nanoscale machinery
Researchers at the University of Queensland’s Precision Sensing Initiative and the Australian Centre for Engineered Quantum Systems have applied quantum physics to single molecule sensing for the first time. Their new approach harnesses light to track and analyse biomolecules and is significant because where previous technologies used light to examine individual biomolecules and risked damaging specimens, theirs does not. In fact this new technique not only achieves state-of-the-art sensitivity, but also does this with a light intensity lower than has been previously possible by a factor of 10,000.
This sensing technique enables tracking of biomolecules around 3.5 nanometres in size, and the monitoring of surface–molecule interactions over extended periods of time. It is the first technique of its kind that uses sufficiently low intensities to detect single unlabelled biomolecules without modifying their behaviour, growth or viability. Project researcher Dr Lars Madsen said the project applied techniques used to detect gravitational waves from black holes in outer space to the nanoscale – super small – world of molecular biology. “The techniques required to detect extremely faint signals produced by distant black holes were developed over decades,” Dr Madsen said.
This enables the study of the nanoscale machinery of life in its native state, like the motor molecules that unravel DNA and produce energy within cells. This work compliments the 2017 Nobel Prize in Chemistry, awarded for cryogenic electron microscopy of the structure of single molecules, but offers the prospect of extending those advances into living biological systems.
Australian Research Council Future Fellow Professor Warwick Bowen said the research – reported in Nature Photonics demonstrated how quantum technologies could revolutionise the study of life’s “nanoscale machinery, or biological motor molecules”. “Motor molecules encode our genetic material, create the energy our cells use to function, and distribute nutrients at a sub-cellular level,” Professor Bowen said.
“Our research translates this technological development over to the biosciences and offers the possibility of a new biomedical diagnostics technique capable of detecting the presence of even a single cancer marker molecule. It is funded by the United States Air Force Office of Scientific Research, which aims to use the technique to help understand stress on pilot behaviour at the sub-cellular level. ” Researchers from five countries – Australia, New Zealand, Denmark, France and Pakistan – were involved in the project.