Limits of quantum speedup in photosynthetic light harvesting

Abstract

It has been suggested that excitation transport in photosynthetic light harvesting complexes features speedups analogous to those found in quantum algorithms. Here we compare the dynamics in these light harvesting systems to the dynamics of quantum walks, in order to elucidate the limits of such quantum speedups. For the Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria, we show that while there is indeed speedup at short times, this is short lived (70 fs) despite longer lived (ps) quantum coherence. Remarkably, this time scale is independent of the details of the decoherence model. More generally, we show that the distinguishing features of light-harvesting complexes not only limit the extent of quantum speedup but also reduce rates of diffusive transport. These results suggest that quantum coherent effects in biological systems are optimized for efficiency or robustness rather than the more elusive goal of quantum speedup.

How Markovian is exciton dynamics in purple bacteria?

“…This is the form typically used when modelling the interaction of the environment with excitonic systems. [9][10][11][12][13][14][15][16][17][18][19][20][21] It is clear however that there are significant contributions from other Lindblad terms and that a pure dephasing Lindblad is not nearly the optimal choice. Diagonalising h we find that the optimal Lindblad is dominated by two processes, …”

“…[9][10][11][12][13][14][15][16][17][18][19][20][21] Ideas such as noise-assisted transport, 9,10,[12][13][14]16 quantum locking, and momentum rejuvenation 14 have been used to suggest how interactions with the environment can help speed up energy transport in networks of chromophores. a) n.linden@bristol.ac.uk b) fred.manby@bristol.ac.uk However, it is still not certain whether these phenomena play a role in the real biological context, where the assumptions inherent in the master-equation approach may not apply.…”

Geometrical effects on energy transfer in disordered open quantum systems

“…ETE measures the likelihood of successful trapping, weighted by trapping rate: it quantifies the excitation availability whenever the reaction center is ready to operate within a period much shorter than the exciton life-time. 26 Note that this definition is very different than the first passage time measure, 21,28 which quantifies the time scale of the first arrival of an exciton to the trapping sites. The latter definition is not necessarily correlated with the efficiency of quantum transport.…”

Microscopic quantum coherence in a photosynthetic-light-harvesting antenna

“…The Hamiltonian for electronic excitations is time dependent because it contains random fluctuations due to the presence of classical noise. This assumption corresponds to stochastic approaches such as the temperature-independent Lindblad equation or the Haken-Strobl model [61], which are extensively employed for examining quantum effects in photosynthetic EET [11,18,20,21,38,39,42,43,62,63]. It should be noticed that the classical noise in this model forces the electronic excitation to remain in a pure quantum state, and therefore an ensemble dephasing effect can be discussed without any influence of decoherence.…”