Molecules are complicated. Forget the grade-school picture of electrons orbiting a nucleus like planets around the sun. Electrons can be shared among many atomic nuclei. They interact with one another in ways described by the equations of quantum mechanics. It’s these complex interactions, which grow exponentially with the number of electrons, that largely govern chemical reactions and the properties of molecules.
Simulating these electrons with perfect precision might take a conventional computer millions of years. But algorithms running on quantum computers might be able to perform precise computations in days or even hours. This would provide clues on how to precisely design molecules with desired properties and tailor their reactions with amazing control.
Sufficiently precise quantum simulation might allow chemists to create new compounds like better high-temperature superconductors, catalysts that could take nitrogen or carbon dioxide out of the air, new drugs, more efficient solar cells, strong lightweight materials for airplanes, and so forth. It would be a way to quickly figure out how a new substance would behave without actually having to synthesize it. It might herald a new age of materials science.
Between 2014 and 2020, Ryan Babbush published dozens of papers—together with collaborators at Google and elsewhere—that outlined dramatically more efficient quantum simulation algorithms. The upshot is that some quantum simulation calculations could, in principle, be done in hours, on a sufficiently powerful quantum computer.
Take the case of nitrogenase, an enzyme that some bacteria use to remove nitrogen from the air and create ammonia, a compound of nitrogen and hydrogen. This process, known as nitrogen fixation, is essential for agriculture, which is why nitrogen-based fertilizers are a linchpin of the world’s food system. Nitrogenase is a big molecule that includes a catalytic site known as FeMoco.
Currently, an energy-intensive technique known as the Haber-Bosch process produces most fertilizers, accounting for about 2% of humanity’s total energy usage. “If we could figure out how that enzyme [nitrogenase] is doing this, then we might be able to design an industrially viable alternative for producing fertilizer, which could scale and save a huge amount of energy,” Babbush says.
He and his collaborators have found a potential way to use a quantum computer to analyze FeMoco and shed light on the mechanism by which it first breaks the bonds between nitrogen atoms that are bound together in nitrogen gas and then succeeds in combining the nitrogen with hydrogen. (Babbush acknowledges that competing approaches using clever approximations to simulate molecules on classical computers might get there first.)
Another line of research that Babbush has advanced aims to figure out how quantum computers can calculate the behavior of electrons in metals and crystals. Potential applications could include finding better superconductors or making more efficient solar cells. In these materials, the repeating pattern of the atoms creates very complicated behavior among the interdependent electrons. And Babbush is figuring out how quantum computers can be used to make sense of these interactions.
If quantum computers succeed in remaking our material world, Babbush’s work will be one reason why.
