While in the days of Darwin and Mendel the life sciences were mainly focusing on botany or zoology, modern biology, pharmacology, and medicine are deeply rooted in a growing understanding of molecular interactions and organic information processing.
Quantum physics, on the other hand, was initially centered on microscopic phenomena with photons, electrons, and atoms. But objects of increasing complexity have attracted a growing scientific interest, and since the size scales of both physics and the life sciences have approached each other, it is now very natural to ask: what is the role of quantum physics in and for biology?
Erwin Schrödinger, most famous for his wave equation for nonrelativistic quantum mechanics, already ventured across the disciplines in his lecture series “What is life?” (Schrödinger, 1944). He anticipated a molecular basis for human heredity, which was later confirmed to be the DNA molecule (Watson and Crick, 1953). Since the early days of quantum physics, its influence on biology has always been present in a reductionist sense: quantum physics and electrodynamics shape all molecules and thus determine molecular recognition, the workings of proteins, and DNA. Also van der Waals forces, discrete molecular orbitals, and the stability of matter: all this is quantum physics and a natural basis for life and everything we see.
But even 100 years after its development, quantum physics is still a conceptually challenging model of nature: it is often acclaimed to be the most precisely verified theory of nature and yet its common interpretation stands in discrepancy to our classical, i.e., prequantum, world-view, and our natural ideas about reality or space-time. Is there a transition between quantum physics and our everyday world? And how will the life sciences then fit into the picture—with objects covering anything from molecules up to elephants, mammoth trees, or the human brain?
Still half a century ago, the topic had some rather skeptical reviews (Longuet-Higgins, 1962). But experimental advances have raised a new awareness and several recent reviews (e.g., Abbot et al., 2008) sketch a more optimistic picture that may be overoptimistic in some aspects.
The number of proven facts is still rather small. Many hypotheses that are formulated today may be found to have lacked either visionary power or truth by tomorrow. We will therefore start on well-established physical grounds and recapitulate some typical quantum phenomena. We will then elucidate the issue of decoherence and dephasing, which are believed to be central in the transition between the quantum and the classical world. They are often regarded to be the limiting factors if we want to observe quantum effects on the macroscopic scale of life. Next, we give an overview over modern theories and experiments at the interface between quantum physics and biology. The final section will be devoted to open speculations, some of which still face less supporting experimental evidence than theoretical counterarguments.
Experimental studies at the interface between quantum physics and the life sciences have so far been focused on two different questions: (1) can genuine quantum phenomena be realized with biomolecules?
Photon antibunching in proteins (Sanchez-Mosteiro et al., 2004), the quantum delocalization of biodyes in matter-wave interferometry (Hackermüller et al., 2003) and the implementation of elementary quantum algorithms in nucleotides (Jones and Mosca, 1999) are some recent examples.
These experiments are optimized for revealing fundamental physics, such as quantum statistics, delocalization, and entanglement. But they all also show that quantum phenomena are best observed in near-perfect isolation from the environment or at ultralow temperatures, in order to avoid the detrimental influence of decoherence and dephasing. They are thus not representative for life as such. (2) Are nontrivial quantum phenomena relevant for life?
Nontrivial quantum phenomena are here defined by the presence of long-ranged, long-lived, or multiparticle quantum coherences, the explicit use of quantum entanglement, the relevance of single photons, or single spins triggering macroscopic phenomena.
Photosynthesis, the process of vision, the sense of smell, or the magnetic orientation of migrant birds are currently hot topics in this context. In many of these cases the discussion still circles around the best interpretation of recent experimental and theoretical findings.