Quantum computers could be the future of computational chemistry if they can calculate properties of molecules that conventional digital computers can’t handle. Today’s quantum systems are a long way from reaching that goal. But a quantum computer at IBM just passed a milestone: It performed the first calculations involving molecules containing more than just hydrogen and helium.
Digital computers crunch numbers to describe properties such as a molecule’s ground-state energy by using the Schrödinger equation to calculate mathematical parameters called wave functions. However, digital computers can solve such problems exactly only for elementary molecules because of the great complexity of the many interactions of the multiple subatomic particles found in larger compounds.
With digital computers, “exact solutions rapidly become unfeasible, even for the fastest computers working over the entire lifetime of the universe,” says theoretical chemist Donald Truhlar of the University of Minnesota, who was not involved in the new study. “Quantum computers do not require exponentially increasing time to solve larger and larger systems, so they do not suffer the same limitations.”
Instead of the digital 1s and 0s in digital computers, quantum computers calculate wave functions with qubits—typically sensitive magnetic detectors called superconducting quantum interference devices. Because qubits are quantum systems themselves, they can represent the quantum chemistry of molecules directly, which digital bits cannot do. But technical factors currently limit the number of qubits quantum computers can use and their ability to correct qubit errors. As a result, quantum computers so far have been limited to calculating properties of very simple molecules: dihydrogen and helium hydride, using 2-qubit processors.
Abhinav Kandala, Antonio Mezzacapo, and coworkers at IBM Thomas J. Watson Research Center have now used six of the qubits on a 7-qubit quantum computer processor to calculate the ground-state energies of lithium hydride and beryllium hydride (Nature 2017, DOI: 10.1038/nature23879). They achieved this advance by using a processor with more qubits than in previous quantum chemistry studies and by optimizing a previously developed algorithm to reduce the number of qubits and quantum operations needed to simulate larger molecules.
The work puts IBM temporarily at the head of the pack in chemistry applications of quantum computing, comments quantum computing expert Alán Aspuru-Guzik of Harvard University. However, he says, the achievement could soon be leapfrogged by the company’s major quantum computing competitors, Google and Microsoft. Potential chemistry applications of quantum computing include discovering small-molecule drugs and determining the properties of new materials, Aspuru-Guzik says.
IBM promotes its quantum chemistry program, called IBM Q, by giving chemists free access to an online quantum computer that carries out ground-state energy calculations on molecules such as H2 and LiH.