Untangling the Quantum Entanglement Behind Photosynthesis: Berkeley scientists shine new light on green plant secrets

[Henk Elegeert]

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5/13/2010 7:32:02 PM
*Untangling the Quantum Entanglement Behind Photosynthesis: Berkeley
scientists shine new light on green plant
secrets<http://nanotechwire.com/news.asp?nid=9875>
*

The future of clean green solar power may well hinge on scientists being
able to unravel the mysteries of photosynthesis, the process by which green
plants convert sunlight into electrochemical energy. To this end,
researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley
National Laboratory (Berkeley Lab) and the University of California (UC),
Berkeley have recorded the first observation and characterization of a
critical physical phenomenon behind photosynthesis known as quantum
entanglement.

Previous experiments led by Graham Fleming, a physical chemist holding joint
appointments with Berkeley Lab and UC Berkeley, pointed to quantum
mechanical effects as the key to the ability of green plants, through
photosynthesis, to almost instantaneously transfer solar energy from
molecules in light harvesting complexes to molecules in electrochemical
reaction centers. Now a new collaborative team that includes Fleming have
identified entanglement as a natural feature of these quantum effects. When
two quantum-sized particles, for example a pair of electrons, are
“entangled,” any change to one will be instantly reflected in the other, no
matter how far apart they might be. Though physically separated, the two
particles act as a single entity.

“This is the first study to show that entanglement, perhaps the most
distinctive property of quantum mechanical systems, is present across an
entire light harvesting complex,” says Mohan Sarovar, a post-doctoral
researcher under UC Berkeley chemistry professor Birgitta Whaley at the
Berkeley Center for Quantum Information and Computation. “While there have
been prior investigations of entanglement in toy systems that were motivated
by biology, this is the first instance in which entanglement has been
examined and quantified in a real biological system.”

The results of this study hold implications not only for the development of
artificial photosynthesis systems as a renewable non-polluting source of
electrical energy, but also for the future development of quantum-based
technologies in areas such as computing – a quantum computer could perform
certain operations thousands of times faster than any conventional computer.

“The lessons we’re learning about the quantum aspects of light harvesting in
natural systems can be applied to the design of artificial photosynthetic
systems that are even better,” Sarovar says. “The organic structures in
light harvesting complexes and their synthetic mimics could also serve as
useful components of quantum computers or other quantum-enhanced devices,
such as wires for the transfer of information.”

*The schematic on the left shows the absorption of light by a light
harvesting complex and the transport of the resulting excitation energy to
the reaction center through the FMO protein. On the right is a monomer of
the FMO protein, showing its orientation relative to the antenna and the
reaction center. The numbers label FMO’s seven pigment molecules. (Image
from Mohan Sarovar)*

What may prove to be this study’s most significant revelation is that
contrary to the popular scientific notion that entanglement is a fragile and
exotic property, difficult to engineer and maintain, the Berkeley
researchers have demonstrated that entanglement can exist and persist in the
chaotic chemical complexity of a biological system.

“We present strong evidence for quantum entanglement in noisy
non-equilibrium systems at high temperatures by determining the timescales
and temperatures for which entanglement is observable in a protein structure
that is central to photosynthesis in certain bacteria,” Sarovar says.

Sarovar is a co-author with Fleming and Whaley of a paper describing this
research that appears on-line in the journal Nature Physics titled “Quantum
entanglement in photosynthetic light-harvesting complexes.” Also
co-authoring this paper was Akihito Ishizaki in Fleming’s research group.

Green plants and certain bacteria are able to transfer the energy harvested
from sunlight through a network of light harvesting pigment-protein
complexes and into reaction centers with nearly 100-percent efficiency.
Speed is the key – the transfer of the solar energy takes place so fast that
little energy is wasted as heat. In 2007, Fleming and his research group
reported the first direct evidence that this essentially instantaneous
energy transfer was made possible by a remarkably long-lived, wavelike
electronic quantum coherence.

Using electronic spectroscopy measurements made on a femtosecond (millionths
of a billionth of a second) time-scale, Fleming and his group discovered the
existence of “quantum beating” signals, coherent electronic oscillations in
both donor and acceptor molecules. These oscillations are generated by the
excitation energy from captured solar photons, like the waves formed when
stones are tossed into a pond. The wavelike quality of the oscillations
enables them to simultaneously sample all the potential energy transfer
pathways in the photosynthetic system and choose the most efficient.
Subsequent studies by Fleming and his group identified a closely packed
pigment-protein complex in the light harvesting portion of the
photosynthetic system as the source of coherent oscillations.

“Our results suggested that correlated protein environments surrounding
pigment molecules (such as chlorophyll) preserve quantum coherence in
photosynthetic complexes, allowing the excitation energy to move coherently
in space, which in turn enables highly efficient energy harvesting and
trapping in photosynthesis,” Fleming says.

In this new study, a reliable model of light harvesting dynamics developed
by Ishizaki and Fleming was combined with the quantum information research
of Whaley and Sarovar to show that quantum entanglement emerges as the
quantum coherence in photosynthesis systems evolves. The focus of their
study was the Fenna-Matthews-Olson (FMO) photosynthetic light-harvesting
protein, a molecular complex found in green sulfur bacteria that is
considered a model system for studying photosynthetic energy transfer
because it consists of only seven pigment molecules whose chemistry has been
well characterized.

“We found numerical evidence for the existence of entanglement in the FMO
complex that persisted over picosecond timescales, essentially until the
excitation energy was trapped by the reaction center,” Sarovar says.

“This is remarkable in a biological or disordered system at physiological
temperatures, and illustrates that non-equilibrium multipartite entanglement
can exist for relatively long times, even in highly decoherent
environments.”

The research team also found that entanglement persisted across distances of
about 30 angstroms (one angstrom is the diameter of a hydrogen atom), but
this length-scale was viewed as a product of the relatively small size of
the FMO complex, rather than a limitation of the effect itself.

“We expect that long-lived, non-equilibrium entanglement will also be
present in larger light harvesting antenna complexes, such as LH1 and LH2,
and that in such larger light harvesting complexes it may also be possible
to create and support multiple excitations in order to access a richer
variety of entangled states,” says Sarovar.

The research team was surprised to see that significant entanglement
persisted between molecules in the light harvesting complex that were not
strongly coupled (connected) through their electronic and vibrational
states. They were also surprised to see how little impact temperature had on
the degree of entanglement.

“In the field of quantum information, temperature is usually considered very
deleterious to quantum properties such as entanglement,” Sarovar says. “But
in systems such as light harvesting complexes, we see that entanglement can
be relatively immune to the effects of increased temperature.”

This research was supported in part by U.S. Department of Energy’s Office of
Science, and in part by a grant from the Defense Advanced Research Projects
Agency (DARPA).

Berkeley Lab is a U.S. Department of Energy national laboratory located in
Berkeley, California. It conducts unclassified scientific research and is
managed by the University of California. Visit our website at
http://www.lbl.gov.

"

Het lijkt nu nog slechts een kwestie van tijd ...

Henk Elegeert

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