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Quantum Effects in Biological Systems?An Innsbruck research team, led by theoretical physicist Hans Briegel, study whether quantum phenomena such as entanglement can also exist in biological systems, and in which processes they could play a role. Today quantum mechanical phenomena can be studied at ultracold temperatures by using highly complex experimental setups. Through the use of extremely high vacuum, high-precision lasers and sophisticated cooling methods, matter can now be controlled at the level of individual atoms. This enables physicists to experimentally study genuine quantum effects such as entanglement, and to use their findings for potential applications, for instance a quantum computer. The question to what extent the findings of modern quantum physics may also be of interest for other scientific fields is addressed by scientists at the Institute for Theoretical Physics at the University of Innsbruck and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences. Theoretical physicist Hans Briegel and his research team are investigating where in biological systems non-trivial quantum effects, such as entanglement, may play a role. Unlike the conditions in a quantum optics laboratory, environmental conditions in biology seem to be rather unfavorable for observing quantum mechanical phenomena. “Biology is hot, wet and noisy,” says Hans Briegel. “We are talking about systems at room temperature and in a very noisy environment.“ Under such conditions, highly sensitive quantum phenomena are washed out rapidly and therefore they are usually neglected. Briegel and his colleagues now demonstrate, however, that it is not always justified to ignore quantum effects in biological systems. Motion and entanglementIn several publications the researchers have argued that quantum mechanical coherence and entanglement can also occur in biological systems. “Living organisms are persistently subject to metabolic processes, which supply energy and dissipate entropy,“ explains Briegel. “From a physics perspective, these are systems that operate far away from thermal equilibrium. This fact opens up new and unexpected possibilities also for the occurrence of new types of quantum effects.“Conformational changes of proteins are one example for a non-equilibrium process at the bio-molecular level. The shape of a bio-molecule plays an important role for its biological function; when a protein changes its shape, time-dependent intra-molecular interactions take place, reminiscent of interactions in a quantum computer. “Usually no interesting quantum effects are expected since the interactions are accompanied by a substantial amount of noise and the thermal state of the system is not entangled. However, the fact that the molecules move changes things,” says Hans Briegel. Even though no static entanglement survives in such an environment, the motion of bio-molecules can generate transient entanglement. The noisy environment thereby plays a constructive role by recurrently resetting the process. “Therefore, the statement that quantum interactions may always be neglected in a warm and noisy environment is shown to be incorrect. Non-equilibrium processes open indeed new possibilities, also for quantum physics, which have not been considered carefully enough,“ Briegel emphasizes. Chemical compass of migratory birdsThe Innsbruck researchers are not only interested in the mere existence of quantum phenomena in biological systems, but they also address the question to what extent quantum mechanics offers advantages for biology. Jianming Cai, Gian Giacomo Guerreschi and Hans Briegel use the radical-pair mechanism to exemplify their approach. This mechanism was proposed, more than 30 years ago, by bio-physicist Klaus Schulten, to be responsible for the marvelous ability of migratory birds to use the magnetic field of the Earth for orientation and navigation. Scientists suggest that magnetic receptors in the eye of birds, for example in European robins, are activated by photons involved the production of radical pairs. Depending on the orientation of the receptors towards the magnetic field of the Earth, different biochemical reactions are believed to take place.In a theoretical model, Briegel and his research team have investigated whether quantum mechanical entanglement of the electronic spin in a radical pair plays any role, e.g. to enhance the sensitivity to the magnetic field. “We have looked at two different molecules: pyrene-dimethylanilin (Py-DMA), for which experimental data in the field of spin chemistry exists, and a molecule in cryptochrome, which has been suggested to be a possible candidate for a chemical compass in birds,” explains Briegel. The results of the theoretical study are surprising: “In the case of Py-DMA, quantum mechanical entanglement causes a significantly higher sensitivity to the magnetic field. In the case of cryptochrome, however, we found no such connection,” says Briegel. “This illustrates the complexity of the issue. Whether or not entanglement plays a role for the orientation sense of European robins, may ultimately depend on which molecule is really responsible for the radical-pair mechanism in the eye of these migratory birds.“ Linking quantum information with biophysicsTo further elucidate this issue, Briegel and his colleagues suggest to apply methods and concepts of quantum information to experiments in spin chemistry. “In quantum information we have developed numerous protocols and control methods to study and characterize quantum phenomena,“ says Briegel. “These methods and protocols could in principle also be used for experiments with such molecules.“ Short radio- or microwave pulses, for example, could be used to investigate the hypothesis that entangled states between radicals in the radical pair mechanism do in fact exist. Results along these lines could even open up new avenues for experimenting with biological organisms, including plants and animals. However, Hans Briegel cautions against too high expectations: “Biology is complex and many processes play together, some of which may not even be known very well. Different from experiments in state-of-the-art quantum optics laboratories, proof of entanglement in living organisms may be a long way off.“ Yet, for the theorist it is still worthwhile to search for it. “We have shown in our studies that it is certainly not unreasonable to search for non-trivial quantum effects, which are otherwise only known e.g. from the study of quantum computers, also in biological systems,“ says Hans Briegel.The Innsbruck scientists are supported by the Austrian Science Fund (FWF). Publications:
Contact: Univ.-Prof. Hans Briegel Institute for Theoretical Physics (Institut für Theoretische Physik) University of Innsbruck phone: +43 512 507 6202 Hans.Briegel[at]uibk.ac.at Institute for Quantum Optics and Quantum Information Austrian Academy of Sciences phone: +43 512 507 4740 Hans.Briegel[at]oeaw.ac.at |