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Physics of Particles and Nuclei Letters

Coherent signal amplification in rhodopsin media

M. V. Altaĭsky, В. Н. Горбачев, Fabio Pichierri

Published Mar 1, 2007• 3 citations
Publisher

Abstract

A quantum optics model that explains the sensitivity of the dark-adapted rhodopsin to a single photon is presented. We describe the absorption of photons by rhodopsin molecules using the three-level A-type model for its retinal chromofore, with 3 states being the all-trans, 11-cis and the photoexcited S 1 state. The abinitio calculations have been performed to estimate the parameters of the model.

DOI
10.1134/s1547477107020124

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A quantum optics model that explains the sensitivity of the dark-adapted rhodopsin to a single photon is presented. We describe the absorption of photons by rhodopsin molecules using the three-level A-type model for its retinal chromofore, with 3 states being the all-trans, 11-cis and the photoexcited S 1 state. The abinitio calculations have been performed to estimate the parameters of the model.
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The two striking components of retina, i.e., the light sensitive neural layer in the eye, by which it responds to light are (the three types of) color sensitive Cones and color insensitive Rods (which outnumber the cones 20:1). The interaction between electromagnetic radiation and these photoreceptors (causing transitions between cis- and trans- states of rhodopsin molecules in the latter) offers a prime example of physical processes at the nano-bio interface. After a brief review of the basic facts about vision, we propose a quantum mechanical model (paralleling the Jaynes-Cummings model (JCM) of interaction of light with matter) of early vision describing the interaction of light with the two states of rhodopsin mentioned above. Here we model the early essential steps in vision incorporating, separately, the two well-known features of retinal transduction (converting light to neural signals): small numbers of cones respond to bright light (large number of photons) and large numbers of rods respond to faint light (small number of photons) with an amplification scheme. An outline of the method of solution of these respective models based on quantum density matrix is also indicated. This includes a brief overview of the theory, based on JCM, of signal amplification required for the perception of faint light. We envision this methodology, which brings a novel quantum approach to modeling neural activity, to be a useful paradigm in developing a better understanding of key visual processes than is possible with currently available models that completely ignore quantum effects at the relevant neural level.
We numerically study the implementation of a NOT gate by laser pulses in a model molecular system presenting two electronic surfaces coupled by nonadiabatic interactions. The two states of the bit are the fundamental states of the cis-trans isomers of the molecule. The gate is classical in the sense that it involves a one-qubit flip so that the encoding of the outputs is based on population analysis which does not take the phases into account. This gate can also be viewed as a double photoswitch process with the property that the same electric field controls the two isomerizations. As an example, we consider one-dimensional cuts in a model of the retinal in rhodopsin already proposed in the literature. The laser pulses are computed by the multitarget optimal control theory with chirped pulses as trial fields. Very high fidelities are obtained. We also examine the stability of the control when the system is coupled to a bath of oscillators modeled by an ohmic spectral density. The bath correlation time scale being smaller than the pulse duration, the dynamics is carried out in the Markovian approximation
Abstract Excitation of molecules by incident incoherent electromagnetic radiation, such as sunlight, is described in detail and contrasted with the effect of coherent (e.g. laser) light. The nature of the quantum coherences induced by the former, relevant to transport processes in nature and in technology, is emphasized. Both equilibrium and steady state scenarios are discussed, three examples: simple models, calcium excitation in polarized light, and the isomerization of retinal in rhodopsin are used to expose the underlying qualitative nature of the established coherences.