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.
Mohan Sarovar (seated) and (from left) Akihito Ishizaki, Birgitta Whaley and Graham Fleming carried out the first observation and characterization of quantum entanglement in a real biological system. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
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.”
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).