royalsocietypublishing | Biological systems are dynamical, constantly exchanging energy and matter with the environment in order to maintain the non-equilibrium state synonymous with living. Developments in observational techniques have allowed us to study biological dynamics on increasingly small scales. Such studies have revealed evidence of quantum mechanical effects, which cannot be accounted for by classical physics, in a range of biological processes. Quantum biology is the study of such processes, and here we provide an outline of the current state of the field, as well as insights into future directions.
1. Introduction
Quantum mechanics is the fundamental theory that describes the properties of subatomic particles, atoms, molecules, molecular assemblies and possibly beyond. 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. 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.
In photosynthesis, respiration and vision, the models that have been developed in the past are fundamentally quantum mechanical. They describe energy transfer and electron transfer in a framework based on surface hopping. The dynamics described by these models are often ‘exponential’ and follow from the application of Fermi’s Golden Rule [1,2]. As a consequence of averaging the rate of transfer over a large and quasi-continuous distribution of final states the calculated dynamics no longer display coherences and interference phenomena. In photosynthetic reaction centres and light-harvesting complexes, oscillatory phenomena were observed in numerous studies performed in the 1990s and were typically ascribed to the formation of vibrational or mixed electronic–vibrational wavepackets. The reported detection of the remarkably long-lived (660 fs and longer) electronic quantum coherence during excitation energy transfer in a photosynthetic system revived interest in the role of ‘non-trivial’ quantum mechanics to explain the fundamental life processes of living organisms [3]. However, the idea that quantum phenomena—like coherence—may play a functional role in macroscopic living systems is not new. In 1932, 10 years after quantum physicist Niels Bohr was awarded the Nobel Prize for his work on the atomic structure, he delivered a lecture entitled ‘Light and Life’ at the International Congress on Light Therapy in Copenhagen [4]. This raised the question of whether quantum theory could contribute to a scientific understanding of living systems. In attendance was an intrigued Max Delbrück, a young physicist who later helped to establish the field of molecular biology and won a Nobel Prize in 1969 for his discoveries in genetics [5].
All living systems are made up of molecules, and fundamentally all molecules are described by quantum mechanics. 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 [6], single molecule spectroscopy [7–11], time-resolved microscopy [12–14] and single particle imaging [15–18] have enabled us to study biological dynamics on increasingly small length and time scales, revealing a variety of processes necessary for the function of the living system that depend on a delicate interplay between quantum and classical physical effects.
Quantum biology is the application of quantum theory to aspects of biology for which classical physics fails to give an accurate description. In spite of this simple definition, there remains debate over the aims and role of the field in the scientific community. This article offers a perspective on where quantum biology stands today, and identifies potential avenues for further progress in the field.
2. What is quantum biology?
Biology, in its current paradigm, has had wide success in applying classical models to living systems. In most cases, subtle quantum effects on (inter)molecular scales do not play a determining role in overall biological function. Here, ‘function’ is a broad concept. For example: How do vision and photosynthesis work on a molecular level and on an ultrafast time scale? How does DNA, with stacked nucleotides separated by about 0.3 nm, deal with UV photons? How does an enzyme catalyse an essential biochemical reaction? How does our brain with neurons organized on a sub-nanometre scale deal with such an amazing amount of information? How do DNA replication and expression work? All these biological functions should, of course, be considered in the context of evolutionary fitness. The differences between a classical approximation and a quantum-mechanical model are generally thought to be negligible in these cases, even though at the basis every process is entirely governed by the laws of quantum mechanics. What happens at the ill-defined border between the quantum and classical regimes? More importantly, are there essential biological functions that ‘appear’ classical but in reality are not? The role of quantum biology is precisely to expose and unravel this connection.
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. As a result, it could be claimed that all of biology is quantum mechanical. 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. Quantum biology should, therefore, be defined in terms of the physical ‘correctness’ of the models used and the consistency in the explanatory capabilities of classical versus quantum mechanical models of a particular biological process.
As we investigate biological systems on nanoscales and larger, we find that there exist processes in biological organisms, detailed in this article, for which it is currently thought that a quantum mechanical description is necessary to fully characterize the behaviour of the relevant subsystem. While quantum effects are difficult to observe on macroscopic time and length scales, processes necessary for the overall function and therefore survival of the organism seem to rely on dynamical quantum-mechanical effects at the intermolecular scale. It is precisely the interplay between these time and length scales that quantum biology investigates with the aim to build a consistent physical picture.
Grand hopes for quantum biology may include a contribution to a definition and understanding of life, or to an understanding of the brain and consciousness. However, these problems are as old as science itself, and a better approach is to ask whether quantum biology can contribute to a framework in which we can repose these questions in such a way as to get new answers. The study of biological processes operating efficiently at the boundary between the realms of quantum and classical physics is already contributing to improved physical descriptions of this quantum-to-classical transition.
More immediately, 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 [19,20]. Through engineering such systems, it may be possible to test and quantify the extent to which quantum effects can enhance processes and functions found in biology, and ultimately answer whether these quantum effects may have been purposefully selected in the design of the systems. Importantly, however, quantum bioinspired technologies can also be intrinsically useful independently from the organisms that inspired them.