Greening the World: Quantum Computing and Artificial Photosynthesis
Book: Quantum Supremacy: How the Quantum Computer Revolution Will Change Everything Author: Michio Kaku ISBN: 978-0385548366
Chapter 7 is about something we all take for granted. Plants. Green stuff everywhere. You walk through a forest, everything is alive and growing, and you don’t think much about it. Kaku makes you stop and consider what is actually happening at the molecular level though. Scientists still don’t fully understand how photosynthesis works. After 3 billion years of it happening on Earth, we still can’t explain the first step properly.
Both humbling and interesting.
The Quantum Magic Inside Every Leaf
Photosynthesis creates about 15,000 tons of biomass per second. Staggering number. The basic process sounds simple: plants take carbon dioxide, sunlight, and water, then produce sugar and oxygen. We learned this in school.
The part nobody taught us in school is what happens right at the beginning, when a photon of light hits a leaf. The photon hits chlorophyll, which absorbs red and blue light but reflects green back. That is why plants are green. If plants absorbed all light, they would be black. A side note I never considered before.
When that photon hits the chlorophyll, it creates energy vibrations called excitons. These excitons need to travel across the leaf surface to reach a collection center, where the actual chemical conversion happens. According to the Second Law of Thermodynamics, most of this energy should be lost as waste heat along the way. It is not though. The process is nearly 100 percent efficient.
Kaku uses a great analogy. Imagine a golf tournament where every player fires a ball randomly in all directions. Then, somehow, every single ball scores a hole in one. That should not happen. With photosynthesis, it does.
Path Integrals and Room Temperature Quantum Effects
The leading explanation involves Richard Feynman’s path integrals. The exciton does not pick one path to the collection center. Instead, it “explores” all possible paths simultaneously and picks the most efficient one. Quantum behavior, the same kind of superposition we talked about in earlier chapters.
In 2007, Graham Fleming at Berkeley actually measured this. Using ultrafast lasers that fire pulses lasting a femtosecond (one millionth of one billionth of a second), he detected quantum coherence in photosynthesis. Light waves existing in multiple quantum states simultaneously, exploring multiple pathways to the reaction center at the same time.
His colleague K. Birgitta Whaley explained it like this: the excitation “picks” the most efficient route from a quantum menu of possible paths.
The second mystery is the really interesting part for engineers though. Quantum computers need to be cooled to near absolute zero to maintain coherence. Random thermal motion destroys quantum states. Yet photosynthesis does this at room temperature. Every day. On every leaf. We still don’t know how plants pull this off. If we could figure that out, it would change how we build quantum systems.
The Artificial Leaf
What if we could replicate photosynthesis artificially? This is where the chapter gets practical.
The idea is straightforward in concept but hard in execution. Step one: use sunlight to split water into hydrogen and oxygen. The hydrogen can power fuel cells, which burn cleanly and produce only water as waste. Step two: combine that hydrogen with CO2 to create fuel and useful hydrocarbons. You burn the fuel, it produces CO2, and you capture that CO2 and recycle it back. A closed loop. No net gain of greenhouse gas.
Harry Atwater, director of the Joint Center for Artificial Photosynthesis at the Department of Energy, calls it “closing the carbon fuel cycle.” An audacious idea. CO2 goes from being the villain of climate change to being a useful resource that keeps getting recycled.
The first proof of concept came back in 1972 when Fujishima and Honda showed light could split water using titanium dioxide and platinum electrodes. Only 0.1 percent efficient, but it proved the idea works. Since then, researchers at JCAP got the efficiency up to 10 percent using semiconductors and nickel catalysts instead of expensive platinum.
The harder part is combining hydrogen with CO2 to make fuel. CO2 is a very stable molecule, which is exactly why it hangs around in the atmosphere causing problems. Harvard chemist Daniel Nocera found a way using a bacterium called Ralstonia eutropha that combines hydrogen with CO2 at 11 percent efficiency. He claims this is 10 to 100 times better than nature. His take: the chemistry problem is solved. Now it is an economics problem, whether industry and government will invest enough to scale it.
Peidong Yang at Berkeley takes a different approach, growing bioengineered bacteria on semiconducting nanowires. The nanowires split water with light, and the bacteria use the hydrogen to create useful chemicals like butanol and natural gas.
Where Quantum Computers Come In
Most of this artificial photosynthesis research so far is done by trial and error. Hundreds of experiments with exotic chemicals. The process of using hydrogen to fix CO2 into fuel requires transferring many electrons and breaking many molecular bonds. Classical computers cannot simulate these processes accurately because they are fundamentally quantum mechanical.
Quantum computers could simulate these molecular processes natively. Model new catalysts for CO2 recycling, find cheaper materials to replace platinum in water splitting, and improve the entire artificial photosynthesis process. Quantum researcher Ali El Kaafarani wrote in Forbes that quantum computers may accelerate discovery of new CO2 catalysts for efficient carbon dioxide recycling.
If quantum computers can crack artificial photosynthesis, it opens up real possibilities. More efficient solar cells. Modified crops that grow in harsh environments. Even plants engineered for Mars colonization. Most importantly, a viable way to recycle CO2, which would be a major step in fighting climate change.
My Take
This chapter connects well to what we discussed in Chapter 1 about decoherence. Nature figured out quantum coherence at room temperature billions of years ago, and we still can’t replicate it in our labs. A reminder that nature has a massive head start on us.
The artificial leaf concept is more convincing to me than some of the other applications Kaku discusses. The science is real, the prototypes exist, and the efficiency numbers are improving. The bottleneck is economics and engineering scale, not fundamental physics. As someone who works on infrastructure problems, that’s the kind of problem I understand. Not “can we do it” but “can we do it cheaply enough.”
The quantum computing angle is legitimate here. Molecular simulation is where quantum computers have their clearest advantage over classical machines. If they can help improve artificial photosynthesis, the impact on clean energy could be substantial.
We should be realistic though. We are talking about two hard problems stacked on top of each other: building reliable quantum computers and perfecting artificial photosynthesis. Both are still in early stages. The potential is real, but so is the distance between “lab demo” and “deployed at scale.”