Molecules are complicated. Forget the elementary school image of electrons orbiting a nucleus like planets around the sun. Electrons can be shared by many atomic nuclei. They interact with each other in a way described by the equations of quantum mechanics. It is these complex interactions, which grow exponentially with the number of electrons, that significantly determine chemical reactions and the properties of molecules.
It could take a conventional computer millions of years to simulate these electrons with perfect precision. But algorithms running on quantum computers could be capable of performing precise calculations in days or even hours. This would provide clues on how to precisely design molecules with the desired properties and tailor their reactions with amazing control.
A sufficiently accurate quantum simulation could enable chemists to create new compounds such as better high-temperature superconductors, catalysts that could remove nitrogen or carbon dioxide from the air, new drugs, more efficient solar cells, strong lightweight materials for aircraft, etc. It would be a way to quickly find out how a new substance would behave without actually synthesizing it. It could herald a new age in materials science.
Between 2014 and 2020, Ryan Babbush, along with coworkers at Google and elsewhere, published dozens of papers outlining dramatically more efficient quantum simulation algorithms. The result is that some quantum simulation calculations could in principle be carried out 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 to make ammonia, a compound made up of nitrogen and hydrogen. This process, known as nitrogen fixation, is essential for agriculture, which is why nitrogen fertilizers are a linchpin of the global food system. Nitrogenase is a large molecule that contains a catalytic site known as FeMoco.
Currently, an energy-intensive process known as the Haber-Bosch process produces most of the fertilizers, which make up about 2% of all human energy consumption. “If we could find out how this enzyme works [nitrogenase] If we do this, we can potentially develop an industrially viable alternative to making fertilizers that could scale and save a lot of energy, ”says Babbush.
He and his co-workers have found a possible way to analyze FeMoco with a quantum computer and elucidate the mechanism by which it first breaks the bonds between nitrogen atoms that are linked in nitrogen gas and then connects the nitrogen with hydrogen. (Babbush admits that competing approaches that use clever approximations to simulate molecules on classic computers may get there first.)
Another line of research that Babbush has pursued aims to find out how quantum computers can calculate the behavior of electrons in metals and crystals. Potential applications could be to find better superconductors or to make more efficient solar cells. In these materials, the repeating pattern of the atoms creates a very complex behavior between the interdependent electrons. And Babbush is figuring out how quantum computers can be used to understand these interactions.
If quantum computers succeed in recreating our material world, Babbush’s work will be a reason.