Summary 2nd Black Forest Focus on Soft Matter
2nd Black Forest Focus on Soft Matter
Quantum Efficiency: From Biology to Material Science
October 22-24, 2009
The School of Soft Matter Research of the Freiburg Institute for Advanced Studies (FRIAS), the Department of Physics of the Albert Ludwig University Freiburg, and the Fraunhofer Institute for Solar Energy Systems (ISE) organized an exploratory workshop at Titisee, a resort in the midst of the Black Forest, aiming at a concerted effort of physicists, chemists, and engineers to investigate the potential of quantum mechanics for efficient energy harvesting and conversion.
The welcome address by Hermann Grabert, director of the FRIAS School of Soft Matter Research, opening the workshop after lunch on Thursday, October 22nd, briefly reviewed the role of quantum mechanics in the technological revolution of the last century and alluded to the advent of a 2nd quantum technological revolution based on quantum coherence and entanglement. The address ended with the crucial question for the workshop theme: Can nontrivial quantum effects survive in complex biological and artificial nanoscale systems at room temperature?
Graham Fleming from the University of California at Berkeley then gave the first scientific talk presenting his groundbreaking studies of exciton motion in light harvesting complexes by means of ultrafast two-dimensional electronic spectroscopy. These experiments indicate that excitons are delocalized over a number of chromophore sites and move coherently for a few hundred femtoseconds. The question whether quantum coherence plays an essential role in explaining the high efficiency of bio-photosynthetic organisms and whether nontrivial quantum effects can also be exploited to improve artificial light harvesting devices was the motivation behind this workshop. Graham Fleming pointed out that biological light harvesting complexes have unique features currently not attainable with artificial structures. In particular, they are characterized by weak electron-phonon interaction as exemplified by a small Stokes shift. These features are likely to be essential for the survival of quantum coherence in a complex environment.
The presentation of these experimental facts was followed by talks from two theoreticians. Ross McKenzie (University of Queensland in Brisbane, Australia) and Michael Thorwart, a Junior Fellow at the Freiburg Institute for Advanced Studies (FRIAS), who suggested concepts why quantum coherence can survive in a wet room temperature environment. Ross McKenzie focussed on the spin-boson model as a paradigm describing effects of an environment on a quantum degree of freedom. He analyzed the predictive power of this model regarding the physics of light harvesting complexes and pointed out that the spectral density of environmental fluctuations, in particular, the tailoring of the effective spectral density by the protein matrix, is essential to achieve large coherence times. While his model calculations indicate that quantum coherence can in fact survive even at room temperature, provided high frequency fluctuations are filtered out, Ross McKenzie argued that the positive (or perhaps even negative) effect of coherence on the efficiency of light harvesting remains to be understood. The discussion following the talk brought up the important question of how much can be learned from “universal” low-dimensional models and where details matter. Based on the model introduced in the preceding talk, Michael Thorwart presented results on the quantum entanglement of two pairs of chromphores mediated by the coupling to the environment. He pointed out that the natural protein-solvent environment induces rather slow polarization fluctuations to the exciton and that it is essential to treat the quantum dynamics of the exciton in terms of a non-Markovian theory ruling out more conventional approaches to quantum dissipation, such as quantum master equations. His calculations, based on a numerical path integral approach, predict sizable quantum entanglement between chromophores on the experimentally relevant time scales.
This first session of the workshop ended with an open discussion. John Briggs (University of Freiburg) raised the question of how quantum coherence can be distinguished from classical phase coherence. This led to a lively exchange of views regarding the range of quantum phenomena accessible through classical concepts. Some of the ideas brought up during the discussion were taken up again in lectures during the following days. Thursday ended with an after dinner poster session, where in particular young scientists participating in the workshop presented their results as well as work in progress on quantum phenomena in complex systems. Active discussions among the different disciplines of physics, chemistry and engineering developed which not only addressed the existence of coherent quantum phenomena in biological systems, but also led to a potential transfer of concepts to artificial hybrid nanosystems, with the aim to exploit quantum coherence as technological resource.
The second day of the workshop started with a talk by Andreas Buchleitner from the University of Freiburg in which he looked at robust features of large quantum systems. His recent research on entanglement dynamics in large open systems shows that nonlinear resonances can shield quantum entanglement against certain types of noise and that under specific conditions quantum coherence can survive better the larger the system. Specifically in the context of the light harvesting complex, he stressed that in disordered systems high transport efficiencies can be achieved when of finite size, since the transport is then dominated by large fluctuations, and that efficient transport is then unambiguously conditioned on large values of certain types of entanglement. After this new insight from basic theoretical studies of multipartite entanglement, Eicke Weber, director of the Fraunhofer Institute for Solar Energy Research (ISE) in Freiburg, emphasized the urgent need to turn the exploitation of solar energy into a terawatt industry. Solar energy is the only resource than can satisfy an increasing worldwide energy demand without raising the level of carbon dioxide. The abundance of materials needed for the production of solar cells and recycling will become crucial factors for an industry of that scale so that alternatives to current technologies need to be carefully explored. Uli Würfel from the same institute discussed the state of the art of organic solar cells and described current activities at ISE and the Freiburg Materials Research Center (FMF) to improve these low cost solar cells. He emphasized the fundamental role played by disorder which favors incoherent hopping transport of charge carriers, while increased coherence could enhance the efficiency of organic solar cells. Paul Alivisatos, interim director of the Lawrence Berkeley National Laboratory, then presented promising new photovoltaic material based on nanotechnology. The nano solar cell can be integrated with other building materials and offers the promise of cheap production costs. Taking up the workshop theme, Paul Alivisatos discussed the role of quantum effects in nano solar cells based on two-dimensional arrays of quantum dots. In these devices excitons are delocalized over many dots which enhances ballistic transport. Other fascinating prospects are offered by nanorod systems than can be used as solar cells or produce photo-electrochemical hydrogen. The morning session ended with a talk by Villy Sundström (Lund University, Sweden) describing the time-scales and efficiencies of the sequence of processes leading from photo absorption in a light harvesting complex to the conversion into chemical energy. Villy Sundström pointed out that the large variety of photosynthetic organisms with vastly different composition and architecture display surprisingly similar quantum efficiencies and overall timescales. He compared these features with those of current artificial structures.
After a rainy afternoon filled with discussions in smaller groups the late afternoon session started with a talk by Joseph Wachtveitl from the University of Frankfurt on quantum efficiency in photosynthetic systems. Following an overview of a variety of photobiological systems he expanded on the intriguing mechanism of non-photochemical quenching which protects a plant in the case of strong sun irradiation. If light excitation abounds specific beta-carotenoid molecules convert dangerous radical byproducts to heat. The talk initiated a general discussion on the fascination and importance of self-repair in biological systems as well as on the role of quantum coherence for down-regulation of photosynthetic systems. Jürgen Köhler (University of Bayreuth) talked about the amazing ability of purple bacteria to exploit very weak sunlight in the sea to gain energy. Discussing the three-dimensional molecular structure of various pigment proteins, he outlined several general principles of efficient light harvesting. For example, the ubiquitous circular (instead of linear) arrangement of chromophores allows the plant to exploit all possible polarizations of the light. Jürgen Köhler emphasized that an impressive amount of microscopic detail can be learned from multi-dimensional single molecule spectroscopy combined with suitable theoretical modeling. Changing from rather complex to presumably simple systems, Peter Hamm from the University of Zürich talked about the infrared-induced cis-trans isomerization of HONO in a rare gas matrix. Arguably this environment-assisted proton transfer can be viewed as the simplest possible molecular reaction in the condensed phase. In his elaborate joint experimental/theoretical study, Peter Hamm demonstrated that in this system the quantum yield of almost 100 % is achieved through a multistep nonadiabatic vibrational energy transfer process.
After dinner the participants reassembled for an open discussion led by Uzi Landman from the Georgia Institute of Technology. Inciting a lively debate by amusing and accurate remarks of his own, the chairman guided the participants through two insightful hours during which key questions that had come up in the previous sessions where touched upon again and expanded. It was argued that light harvesting, supposedly the greatest technological challenge of our times, still faces a lot of ignorance and that the ultimate artificial realization will not just copy nature (we do not build airplanes with feathers). In order to supply several terawatts of power, solar cells need to be truly recyclable like the biological systems containing almost exclusively light elements. Self-assembly and self-repair are other factors needing more attention. Only a concerted, huge effort of all disciplines taking all aspects from basic quantum physics to fabrication costs into account will help to circumvent a carbon dioxide catastrophe.
Saturday morning started with a talk by Hans Briegel (University of Innsbruck, Austria) in which he conjectured about the role of quantum information in biological systems. He argued that quantum entanglement could build up in these systems even at room temperature since they are far from thermal equilibrium. By way of example Hans Briegel presented a model for avian magneto-sensors based on nontrivial quantum phenomena. Gerhard Stock from the University of Freiburg then described various strategies to model the dynamics of biomolecular systems. His quantum-mechanical model studies of the primary event in vision showed how vibronically coherent dynamics, arising, e.g., from the coupling of vibrational and electronic degrees of freedom, may lead to a fast reaction speed and high efficiency of the photoreaction in the protein rhodopsin. Moreover, he demonstrated that quantum mechanics can lead to a significant speed-up of the transport of vibrational energy in peptides. Shaul Mukamel from the University of California, Irvine, gave the final scientific talk of the workshop discussing the future of coherent multidimensional spectroscopy. He presented several stimulating new ideas ranging from attosecond X-ray spectroscopy, which is currently being developed in several laboratories, to nonlinear spectroscopy with entangled pairs of photons. In particular, Shaul Mukamel discussed how three-exciton correlations can be extracted by means of two-dimensional spectroscopy, opening a pathway for the detection of genuine quantum-mechanical coherence effects in biomolecular and artificial complexes. Hanspeter Helm (University of Freiburg) then wrapped up the meeting emphasizing the important new insight gained for the Freiburg group of researchers planning to launch a united research effort on the quantum physics of photosensitive processes. The aspects and intentions underlying this endeavour are outlined in the following statement.
The workshop, gathering some of the world leading experts on energy harvesting in biological and artificial systems as well as a selection of leading scientists from the Freiburg area working in related research fields, had a strong impact on the Freiburg research community. It boosted the formation of a new alliance of researchers comprising of faculty members from the Departments of Physics, Chemistry, Microsystems Engineering, and the Materials Research Center of the University of Freiburg as well as scientists from local Fraunhofer Institutes for applied research. It was felt that the expertise, infrastructure and equipment pooled in this consortium, ranging from teams addressing fundamental research over units preparing and characterizing novel materials to divisions for device development, offered unique possibilities to address successfully the urgent problem of efficient energy harvesting and conversion.
Within this century, the generation of energy will need to change entirely from fuel-based to solar-based principles. While relatively efficient solar cells are already available, current technologies will not suffice to satisfy a world demand of several terawatts of power. The two most pressing challenges to large-scale deployment of solar photovoltaics as the world moves toward a carbon neutral future are cost per kilowatt hour and total resource abundance. In addition, a large-scale solar industry needs to offer innocuous and recyclable solutions. Photobiological systems are almost exclusively comprised of light elements that are truly abundant, nontoxic, and can easily be discarded. The efficiency of these systems brought about by evolution employs principles of quantum mechanics similar to those that underlie the concept of quantum computing. However, while the promise of quantum information theory - the efficient solution of hard computational tasks - has led to intense worldwide research efforts, comparatively little attention has so far been devoted to the nontrivial quantum effects underlying the efficient conversion and transport of energy in photosynthetic organisms. There is a great necessity to comprehend in detail the microscopic principles underlying efficient light harvesting and to ascertain the potentially crucial role of fundamental quantum properties, such as many particle coherence and entanglement, in order to conceive the inexpensive and high-yield solar energy systems badly needed in the near future. Building on the unique research environment available in Freiburg, the newly formed research community aims at advancing our understanding, ameliorating our control, and, ultimately, improving the exploitation of photosensitive processes in artificial structures.
"I found the workshop stimulating, educational, and challenging. I was very impressed that FRIAS was able to both attract the international leaders in the field and that FRIAS itself has a large group of leading scientists who are committed to the multi-disciplinary approach needed to address the fundamental scientific and technological questions discussed."
"This workshop dealt with issues pertaining to some of the most challenging scientific and technological problems that scientists, and society in general, are facing at this time - namely, the identification, characterization, and elucidation of the principal issues (scientific, economic, and environmental) which must be addressed and resolved in order to make progress toward the understanding, design, and eventual large-scale implementation, of devices which can capture and convert solar energy, in an ecologically clean way, into a storable, retrievable, and usable source of energy to power our world. The presentations and lively discussions, in and out of the meeting room, explored a broad range of issues that included coherence and entanglement in bio-photosynthetic systems, the development of experimental and theoretical tools for exploration of excitation pathways in complex photoactive biological and man-made systems, and investigations of nanostructural elements to be employed in the construction of nano solar cells. The workshop made a significant step in bringing together scientists from a variety of fields, providing a forum for establishing an interdisciplinary dialogue, which will undoubtedly be essential for making future progress in this multifaceted field of endeavor, where chemists, biologists, physicists, materials scientists, and engineers will have to join forces in order to make contributions of lasting value. There is an African proverb that says: “If you want to go fast – go alone, but if you want to go far – go together”. Often, the optimal way for making scientific progress on large-scale problems is to combine and integrate individual and collaborative scientific contributions. FRIAS could facilitate progress in this important field, through workshops like the one just completed, and through fostering and coordination of individual and joint future research initiatives."