Speeaker: Gregory Scholes (University of Toronto)

Title: Coherently wired light-harvesting in a photosynthetic marine alga at ambient temperature

The photosynthetic machinery of plants, algae, and bacteria has diversified and evolved over billions of years. The initial reactions involve absorption of light by molecules in specialized light-harvesting antenna proteins followed by remarkably efficient funneling of that electronic excitation energy within and between proteins to a reaction center. Isolated antenna proteins have proven to be important model systems enabling researchers to learn how excitation energy is transmitted by resonance energy transfer, and recent work has discovered a role of quantum-coherence in energy funneling for some antenna proteins at temperatures as high as 180K. Quantum-coherence means that light-absorbing molecules in the protein capture and funnel energy according to quantum-mechanical probability laws instead of classical laws. The subject has stimulated cross-disciplinary interest because it was previously thought that long-range quantum-coherence between molecules could not be sustained in complex biological systems, even at low temperature. Here we report observations of quantum coherence at ambient temperature in the energy funnel of the phycocyanin 645 (PC645) antenna protein isolated from the marine cryprophyte alga Chroomonas CCMP270. Electronic excitations interfere in a way to 'wire' distant molecules together in the PC645 photosynthetic antenna protein. We find that quantum-coherence is maintained for long enough that it may feasibly be employed within live cryptophyte marine algae to increase the spatial cross-section for light-harvesting. This work leads to new, more demanding, questions such as This opens up questions such as 'do quantum effects offer an evolutionary advantage in biology, and if so, how'?

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Speaker: Ross McKenzie (University of Queensland)

Title: Electronic excited states in optically active biomolecules: functional quantum systems with a tuneable environment interaction

Optically active molecules (chromophores) are crucial to the function of wide range of biomolecules. Examples, include the green flourescent protein, porphyrins associated with photosynthesis, and retinal associated with vision. The electronic states of the chromophores can be viewed as discrete quantum systems which are interacting with an environment composed of the surrounding protein and water. The interaction of the chromophore with its environment may be modelled quantum mechanically by an independent boson model which describes a two-level quantum system interacting with a bath of harmonic oscillators. Femtosecond laser spectroscopy experiments give a parametrisation of the spectral density describing the system-environment interaction for a wide range of chromophores and proteins. This spectral density completely determines the quantum dynamics and decoherence of electronic excited states. We have recently proposed and analysed several continuum dielectric models of the environment[1]. Our results provide a framework to understand experimental measurements and molecular dynamics simulations, including the relative importance of the contributions of the protein, the water bound to the surface of the protein, and the bulk water to decoherence. Our results show that because biomolecules function in a ``hot and wet'' environment, quantum coherence will generally not be significant for processes occuring slower than a picosend, the timescale for the dielectric relaxation of water. The ``collapse'' of the quantum state of the chromophore due to continuous measurement of its state by the environment occurs on the timescale of 10's femtoseconds.

[1] J. Gilmore and R.H. McKenzie, J. Phys. Chem. A 112, 2162 (2008).

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Speaker: Lian-Ao Wu (University of the Basque Country)

Title: Looking into the relation between quantum phase transition and entanglement via density functional theory

Density functional theory (DFT) is shown to provide a novel conceptual and computational framework for entanglement in interacting many-body quantum systems. DFT can, in particular, shed light on the intriguing relationship between quantum phase transitions and entanglement. We use DFT concepts to express entanglement measures in terms of the first or second derivative of the ground state energy. We illustrate the versatility of the DFT approach via a variety of analytically solvable models. As a further application we discuss entanglement and quantum phase transitions in the case of mean field approximations for realistic models of many-body systems.

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Speaker: Michael Spanner (SIMS/NRC)

Title: Decoherence and the quantum-to-classical transition of a symmetry breaking coherent control scenario in an optical lattice

Coauthors: Ignacio Franco

An experimentally accessible way to study the quantum-to-classical (hbar ? 0) transition of a symmetry breaking coherent control scenario, in both isolated systems and in the presence of tunable amounts of decoherence, is proposed. The setup exploits the experimental control over the depth of the potential wells in optical lattices to define an effective hbar that, in principle, can be experimentally manipulated. Simulations of the transition show that the symmetry breaking effect survives in the classical limit and hence that matter interference effects are not required for the emergence of coherent laser control. Even when the average photoinduced momentum (i.e. the net degree of symmetry breaking) approaches smoothly its classical limit, the probability distribution of the observable does not, having an extremely fine oscillatory structure superimposed on the classical background that has little effect on the average. This fine structure due to quantum coherences is extremely fragile to environmental decoherence in the small hbar limit and a very small amount of decoherence is required to ensure the classical limit. We conclude that the detrimental effects induced by interaction with environmental degrees of freedom in this coherent laser control scheme are due to a decay of the temporal correlations in the system's dynamics, and not due to a decay of matter interference effects due to decoherence.