Chlorophyll Power
Quantum details of photosynthesis could yield better solar cells BY MICHAEL MOYER
As nature’s own solar cells, plants
convert sunlight into energy via photosynthesis.
New details are emerging
about how the process is able to exploit
the strange behavior of quantum systems,
which could lead to entirely novel
approaches to capturing usable light
from the sun.
All photosynthetic organisms use protein-
based “antennas” in their cells to
capture incoming light, convert it to energy
and direct that energy to reaction
centers—critical trigger molecules that
release electrons and get the chemical
conversion rolling. These antennas must
strike a difficult balance: they must be
broad enough to absorb as much sunlight
as possible yet not grow so large that they
impair their own ability to shuttle the energy
on to the reaction centers.
This is where quantum mechanics
becomes useful. Quantum systems can
exist in a superposition, or mixture, of
many different states at once. What’s
more, these states can interfere with
one another—adding constructively at
some points, subtracting at others. If the
energy going into the antennas could be
broken into an elaborate superposition
and made to interfere constructively
with itself, it could be transported to the
reaction center with nearly 100 percent
efficiency.
A new study by Mohan Sarovar, a
chemist at the University of California,
Berkeley, shows that some antennas—
namely, those found on a certain type of green
photosynthetic bacteria—do just that. Moreover,
nearby antennas split incoming energy
between them, which leads not just to mixed
states but to states that are entangled over
a broad (in quantum terms) distance. Gregory
Scholes, a chemist at the University of Toronto,
shows in a soon to be published study
that a species of marine algae utilizes a similar
trick. Interestingly, the fuzzy quantum
states in these systems are relatively longlived,
even though they exist at room temperature
and in complicated biological systems.
In quantum experiments in the physics
lab, the slightest intrusion will destroy a
quantum superposition (or state).
These studies mark the first evidence
of biological organisms that exploit
strange quantum behaviors. A better understanding
of this intersection of microbiology
and quantum information, researchers
say, could lead to “bioquantum”
solar cells that are more efficient than today’s
photovoltaics.
