The Quantum Superhighway
In the process of photosynthesis, the capture and transfer of a photon's energy happen with an efficiency that classical physics struggles to explain. When light strikes a pigment molecule, its energy creates an excitation called an exciton. This packet of energy must travel from the antenna complex to a reaction center where its energy can be converted into chemical form. For years, scientists modeled this as a "random walk," where the exciton inefficiently hops between molecules until it finds its destination. This model, however, does not account for the nearly 100% quantum efficiency observed in many photosynthetic systems.
The alternative involves quantum mechanics. An important structure for studying this phenomenon is the Fenna-Matthews-Olson (FMO) complex, a pigment-protein complex found in green sulfur bacteria like Chlorobaculum tepidum. These bacteria are adapted to survive in extremely low-light environments, making energy efficiency important. Instead of hopping, the exciton enters a state of quantum coherence, a wavelike superposition where it effectively exists across multiple pigment molecules at once. This allows the exciton to "sample" all possible paths to the reaction center simultaneously, instantly identifying the most efficient route. This quantum superhighway model helps explain how energy transfer can be so rapid and lossless.
Observing the Quantum Beat
The coordinates for this location point to the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, where chemist Graham R. Fleming and his research group provided some of the first direct evidence for these long-lived quantum effects in biological systems. In a landmark 2007 experiment, they used a technique called two-dimensional electronic spectroscopy. This method uses a sequence of ultrafast laser pulses, on the order of femtoseconds (a quadrillionth of a second), to follow the flow of energy through the FMO complex.
The results were unexpected. The team observed "quantum beats"—oscillating signals indicating that the excitons maintained their wavelike coherence for surprisingly long durations. Initially observed at a cryogenic temperature of 77 K, the coherence was shown to persist for over 660 femtoseconds. Subsequent studies demonstrated that this coherence lasts for hundreds of femtoseconds even at physiological temperatures. This duration, while short by human standards, is long enough for the energy to traverse the protein complex and find the reaction center. The protein structure of the FMO complex appears to shield the delicate quantum state from environmental noise that would normally destroy it almost instantly. While the exact role and necessity of these effects are still debated, with some studies showing coherence decays in as little as 60 femtoseconds, the observations started a new field of quantum biology.