Ask a Caltech Expert: Garnet Chan on Quantum Computers and Chemistry

How Will Quantum Computers Affect Chemistry?

We asked Garnet Chan, Bren Professor of Chemistry at Caltech, to answer this question.

I am in the field of quantum chemistry, the application of quantum mechanics to chemistry, which has existed in some form since the discovery of quantum mechanics itself almost 100 years ago. Modern-day quantum chemists like me develop computational methods to model atoms and molecules in the settings in which they appear in nature and in the course of chemical reactions. Like the bulk of my discipline, most of my work uses classical (i.e., standard) computers to approximately solve the equations of quantum mechanics and thereby simulate chemistry.

I also work and collaborate with other scientists in the area of quantum computing. My recent focus is mainly on trying to clearly establish the boundary between things that you can do using classical computers and things you can't imagine doing with them but potentially could one day tackle on a quantum computer. It's very difficult because it is hard to fully predict what quantum computers can do without having the devices available. But we can at least better define the maximum capabilities of classical computers so that we know what it is we are looking to beat when using quantum computers for these problems.

Although the literature abounds with proposals for problems quantum computers might solve, when we actually develop mature quantum computers, I think there will be many applications that we aren't able to think of now. Actually, it is my opinion that many applications that are being proposed will not be the ones that we will end up really valuing these devices for.

It is often stated that quantum computers will have a big impact on quantum chemistry, but I see several problems with some of the current discussion. The first is that most of chemistry is not, in fact, very quantum mechanical: the atoms in a big biological molecule like a protein behave almost like a set of balls connected by springs. This is why biologists do not talk much about quantum computing! Even in the more quantum aspects of chemistry, such as in the interaction of molecules with light, the fact that quantum chemistry has existed as a discipline for so many years really says that much of the associated quantum mechanics can be modeled quite well using the existing power of classical computing, and it's not clear that quantum computers have a big advantage in this case. So, the problems that might need quantum computers only form a small part of the problems of the field.

The second issue is that people sometimes overstate the importance of quantum chemistry. For example, people have argued that quantum computers could model how natural processes use enzymes to collect nitrogen from the air and convert it into a form that plants use as fertilizer. Because these natural processes do not need the extreme temperatures and pressures required for industrial fertilizer production, the idea is that, by studying them, we might find a way to make fertilizer in everyday conditions. But chemists and chemical engineers understand that biological processes are not necessarily optimized for industry. For example, the natural processes involve bacteria distributed in the soil all throughout the world, each microbe doing a little bit of chemistry, but if you want to manufacture fertilizer, you want something very concentrated: all the feedstock comes into a factory and a gigantic amount of fertilizer comes out. Even if a quantum computer could better solve the quantum chemistry problem of these particular enzymes, we would be very far away from replacing the industrial Haber process.

It is not that I think quantum computing will not impact quantum chemistry—as with any new computational device or computational paradigm, there is no doubt we will learn how to harness it and will do new things with it. Some subset of our current computational problems will hopefully become easier to solve. But it is not clear to me that this will end up being the most important application of quantum technology to chemistry.

What is most amazing to me is that we are actually beginning to be able to build devices that we can control so precisely that we can carry out the delicate operations of quantum computing. The ability to control the quantum state of atoms, molecules, materials, and light is the more fundamental technology, and in some ways, building quantum computers is really an application of it. Ultimately, I think it is this new level of control that will have the biggest impact on chemistry. In fact, some of my colleagues, such as chemist Scott Cushing and physicist Nick Hutzler, are already using these techniques to carry out new types of molecular chemistry experiments.

— Garnet Chan, Bren Professor of Chemistry

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