1 / 13

Limits of quantum speedup in photosynthetic light harvesting

Limits of quantum speedup in photosynthetic light harvesting. Stephan Hoyer , Mohan Sarovar and K. Birgitta Whaley APS March Meeting 2010. Berkeley Quantum Information and Computation Center. Quantum random walks. Use adjacency matrix for a graph as the Hamiltonian

varden
Download Presentation

Limits of quantum speedup in photosynthetic light harvesting

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Limits of quantum speedup in photosynthetic light harvesting Stephan Hoyer, Mohan Sarovar and K. Birgitta Whaley APS March Meeting 2010 Berkeley Quantum Information and Computation Center

  2. Quantum random walks • Use adjacency matrix for a graph as the Hamiltonian • Powerful tool for quantum algorithms, including search • Characteristic “quantum speedup” over classical walks Quantum walk Classical walk Ballistic Reviews: arXiv:quant-ph/0303081, arXiv:quant-ph/0403120

  3. Is photosynthesis doing quantum computing? • Experiments at Berkeley found long lasting quantum coherence in light-harvesting complexes • Claim: dynamics like quantum search • Most direct analogy – quantum random walk • Quantum speedup in photosynthesis? Engel et al., Nature 446, 782 (2007)

  4. 1 2 6 7 5 3 4 Modeling light harvesting complexes • Single particle, tight-binding approach • Energy landscapes • Disordered – random environment • Ordered – energy funnels • Decoherence • Room temperature in a protein cage • Simplest noise model is pure dephasing Energy

  5. Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria • FMO serves as a “quantum wire” between light harvesting antenna and reaction center • Extremely low light conditions (~1 photon/minute) • Highly optimized by evolution

  6. FMO as 1D quantum walk • Energy transport in FMO is through a single monomer Source 6 1 5 7 2 4 3 Trapping to reaction center Energies in cm-1 FMO Hamiltonian for C. Tepidum from Adolphs and Renger, Biophys. J. 91, 2778 (2006)

  7. Quantum speedup in FMO • Three timescales: • Quantum speedup until ~70 fs • Coherence until ~500 fs • Transport completed after ~5 ps • Results also hold with more realistic noise model Ballistic Subdiffusive Most transport is subdiffusive For better noise model, see Ishizaki and Fleming, J. Chem. Phys. 130, 234111 (2009)

  8. Coherent transport with energetic disorder • Non-constant site energies En cause transition from ballistic to localized transport Localized Random energies lead to Anderson localization Linearly increasing energies lead to Stark localization (Bloch oscillations) Localized • Quantum speedup limited to times beforelocalization • ~70 fs in FMO corresponds to localization (2 sites)

  9. Decoherence in light harvesting complexes • Pure dephasing helps transport in disordered systems • Low levels of dephasing allow escape from localization • High levels of dephasing suppress transport with the quantum Zeno effect • But – dephasing assisted transport is (at best) diffusive • Quantum speedup still limited to times before localization Ballistic Diffusive Localized Rebentrostet al., arXiv:0807.0929 Plenio and Huelga, arXiv:0807.4902

  10. Conclusions • Quantum speedup in FMO is too short lived (only 70 fs) for quantum search • Distinguishing features of light harvesting complexes(disorder, funnel, dephasing) make them unsuitable for scalable quantum walks or even fast diffusive transport • More realistically, coherence might help efficiency or robustness, but its functional role remains unclear Reference: New Journal of Physics (to appear), arXiv:0910.1847

  11. Systematic investigation of diffusive transport • With non-zero dephasing, transport eventually becomes diffusivefor an infinite chains • Distinguishing features of LHCs (energetic disorder and dephasing) are also not favorable for diffusive transport Quantum walk (not diffusive)

  12. Realistic noise on FMO • Results hold under more realistic noise models incorporating phonon dynamics in bath Ballistic Diffusive Localized Saturated Model from Ishizaki and Fleming, J. Chem. Phys. 130, 234111 (2009)

  13. Implications of Stark localization • Quantum particles on a lattice do not roll downhill • Energy funnels do not work • Different from continuous space wave packet • Discrete space and any variation in site energies leads to localization Discrete space Continuous space

More Related