Quantum Coherence in Biological Systems

" In this PhD thesis I investigate the occurrence of quantum coherences and their consequences in biological systems. I consider both finite (spin) and infinite (vibrations) degrees of freedom.

Chapter 1 gives a general introduction to quantum biology. I summarize key features of quantum effects and point out how they could matter in biological systems.

Chapter 2 deals with the avian compass, where spin coherences play a fundamental role. The experimental evidence on how weak oscillating fields disrupt a bird’s ability to navigate is summarized. Detailed calculations show that the experimental evidence can only be explained by long lived coherence of the electron spin.

In chapter 3 I investigate entanglement and thus coherence in infinite degrees of freedoms, i.e. vibrations in coupled harmonic oscillators. Two entanglement measures show critical behavior at the quantum phase transition from a linear chain to a zig-zag configuration of a harmonic lattice.

The methods developed for the chain of coupled harmonic oscillators will be applied in chapter 4 to the electronic degree of freedom in DNA. I model the electron clouds of nucleic acids in DNA as a chain of coupled quantum harmonic oscillators with dipole-dipole interaction between nearest neighbours resulting in a van der Waals type bonding. Crucial parameters in my model are the distances between the acids and the coupling between them, which I estimate from numerical simulations. I show that for realistic parameters nearest neighbour entanglement is present even at room temperature. I find that the strength of the single base von Neumann entropy depends on the neighbouring sites, thus questioning the notion of treating the quantum state of single bases as independent units. I derive an analytical expression for the binding energy of the coupled chain in terms of entanglement and show the connection between entanglement and correlation energy, a quantity commonly used in quantum chemistry.

Chapter 5 deals with general aspects of classical information processing using quantum channels. Biological information processing takes place at the challenging regime where quantum meets classical physics. The majority of information in a cell is classical information which has the advantage of being reliable and easy to store. The quantum aspects enter when information is processed. Any interaction in a cell relies on chemical reactions, which are dominated by quantum aspects of electron shells, i.e. quantum mechanics controls the flow of information. I will give examples of biological information processing and introduce the concepts of classical-quantum (cq) states in biology. This formalism is able to keep track of the combined classical-quantum aspects of information processing. In more detail I will study information processing in DNA. The impact of quantum noise on the classical information processing is investigated in detail for copying genetic information. For certain parameter values the model of copying genetic information allows for non-random mutations. This is compared to biological evidence on adaptive mutations.

Chapter 6 gives the conclusion and the outlook."

Last modified on 15-Mar-16

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