A Quantum Heat Engine Model of Photosynthesis … Noise-Induced Coherence Boosts Output by 27%

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Original abstract

The researchers modeled the special-pair chlorophylls of the photosynthetic reaction center as a quantum heat engine and interpreted, in quantum-thermodynamic terms, how solar photons are converted into charge separation (electric current). The core of the model is noise-induced coherence, in which coherence is driven not by an external coherent laser but by natural noise, showing that quantum effects can operate even under nature's incoherent sunlight. Theoretical calculations showed that a model with noise-induced coherence can enhance the charge-separation output by up to about 27% compared with the same model without coherence. [Quantum Biology Society] Over the past dozen years or so, quantum-biological phenomena in photosynthesis have been a major point of debate in the field. Two-dimensional spectroscopy experiments observed signals that oscillate over time in photosynthetic pigment complexes, but because these results used coherent ultrashort laser pulses, it remained an open question whether the same phenomena would appear under nature's incoherent sunlight. In physics, meanwhile, it had been theoretically predicted that inducing quantum coherence in a photovoltaic device such as a solar cell could raise its energy-conversion efficiency. A collaboration including Konstantin E. Dorfman and co-workers, affiliated with Texas A&M University, Princeton University, the University of California, Irvine, and Baylor University in the United States, showed theoretically that these two disparate phenomena arise from the same quantum-mechanical principle. Published in the Proceedings of the National Academy of Sciences (PNAS) in 2013, the paper analyzed the photosynthetic reaction center by modeling it as a biological quantum heat engine. ■ Viewing Photosynthesis as a Quantum Heat Engine A quantum heat engine is a device that absorbs hot thermal radiation (solar photons) and converts it into useful work (such as electric current), and its ultimate efficiency is governed by the Carnot limit. The researchers proposed that the photosynthetic reaction center can be described within this same thermodynamic framework. At the special-pair chlorophylls at the heart of the reaction center, light energy is absorbed and an electron is excited, and the charge separation that occurs as this electron moves from the donor to the acceptor is precisely the source of the electric current (biological chemical energy). The researchers modeled this special pair as a five-level system containing two excited states (a1, a2). The energy gap between the two excited states, the Davydov splitting, was set to about 450–800 cm⁻¹ in bacterial reaction centers, while the special-pair coupling in the Photosystem II (Photosystem II) reaction center was set to about 160–200 cm⁻¹. ■ The Two Faces of Coherence: Oscillations and Enhanced Output The central insight of this paper is that it tied together the population oscillations observed in photosynthetic antenna experiments and the efficiency enhancement predicted in solar-cell models, showing that both originate in a common source: noise-induced coherence via Fano interference. Fano interference arises when two levels couple to the same continuum state, so that incoherent noise itself can induce coherence even without an external coherent laser. This suggests that the quantum effects of photosynthetic complexes may operate not only in artificial laboratory settings but also under nature's incoherent sunlight. ■ Coherence Made by Noise, and the 27% Enhancement The researchers analyzed how this coherence phenomenon manifests depending on the system's damping conditions. Overdamped regime: the population reaches steady state smoothly without oscillating, and here the steady-state coherence is stably maintained, so the output was calculated to be enhanced by up to about 27% relative to a model without coherence. Underdamped regime: distinct population oscillations appear on a timescale of about 130 fs, but the steady-state coherence vanishes entirely, so no output-enhancement effect appears. Intermediate regime: an output enhancement of about 18% was predicted to coexist with population oscillations. (Under open-circuit conditions, where no current flows, the coherence value also becomes 0.) In other words, the conclusion is that the oscillations observed in experiments and the output enhancement on the energy-conversion side are different manifestations of the same quantum-mechanical mechanism. ■ Significance and Limitations This study carries great significance in reinterpreting photosynthesis through a thermodynamic lens - the output of a quantum heat engine - rather than through the rate of energy transfer. It also unified population oscillations and output enhancement under the single principle of noise-induced coherence, widening the intersection of quantum optics and biology, and provided a powerful theoretical framework that could be applied to the design of artificial light-harvesting devices. At the same time, it must be clearly recognized that this paper is a theoretical and modeling study, not one based on actual biological measurement data. The figure of 27% output enhancement is a theoretical prediction derived from a simplified five-level model that assumes a particular Markov approximation and weak system-environment coupling. It does not fully reflect the complex, strongly coupled environment of real photosynthetic proteins (the non-Markovian regime), and debate persists in the field over whether the origin of the oscillatory signals in two-dimensional spectroscopy is purely electronic coherence or vibronic (vibrational-electronic) coupling. Even so, this work - which recast the primary charge separation of photosynthesis within the framework of quantum thermodynamics - is regarded as a landmark theoretical study that expanded quantum biology into a new dimension. #QuantumBiology #QuantumHeatEngine #QuantumThermodynamics #Photosynthesis #ReactionCenter #SpecialPair #NoiseInducedCoherence #FanoInterference #ChargeSeparation #QuantumCoherence https://www.pnas.org/doi/10.1073/pnas.1212666110

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