Recent analyses suggest quantum chemical calculations, which could enhance drug development and agricultural innovation, may not be the game-changer for quantum computers that many hoped.

As advancements in quantum computer technology progress rapidly, the most compelling applications for continued investment remain uncertain. One widely considered option is solving complex quantum chemistry problems, including energy level calculations for molecules critical to biomedicine and industry. This requires managing the behavior of numerous quantum particles (electrons in a molecule) simultaneously, aligning well with quantum computing’s strengths.

However, Xavier Weintal and his team at CEA Grenoble in France have demonstrated that the leading quantum algorithms for this purpose may be of limited utility.

“In my view, it’s likely doomed; it’s not definitively doomed, but it’s probably facing insurmountable challenges,” remarks Weintal on the feasibility of using quantum computers for molecular energy calculations.

The team categorized their analysis into two segments: one focused on current noisy quantum computers, and another on future fault-tolerant quantum systems.

Using error-prone quantum computers, energy levels can be computed via variational quantum eigensolver (VQE) algorithms, yet the outcome’s accuracy is heavily influenced by noise levels.

According to their findings, for VQE to match the accuracy of chemical algorithms running on classical systems, noise levels in quantum computers would need significant reduction, essentially qualifying them as fault-tolerant. Notably, no practical fault-tolerant quantum computer yet exists.

Several firms are racing to develop fault-tolerant quantum systems within the next five years. These advanced devices aim to utilize quantum phase estimation (QPE) for calculating molecular energy levels. While the error issue may be largely addressed here, the study uncovers a daunting challenge dubbed the “orthogonality catastrophe.”

Simply stated, as molecular size increases, the likelihood of QPE accurately determining the lowest energy level diminishes exponentially. Consequently, Thibault Louve, from French quantum computing enterprise Quobly, states that even with superior quantum computers, instances where QPE is practically viable are extremely limited. He argues that the ability to execute this algorithm should be viewed as a benchmark for quantum computer maturity rather than a primary tool for chemists.

“There’s a tendency to overstate quantum computers’ potential in this area; many assume the arrival of quantum capabilities will render classical methods for quantum chemistry obsolete,” asserts George Booth, a professor at King’s College London, who wasn’t involved in this research. “This study calls attention to considerable challenges in achieving accurate molecular simulations that will persist even in the fault-tolerant era, raising doubts about the immediate success of quantum chemistry within quantum computing.”

Nevertheless, quantum computers hold promise for various chemistry applications. For instance, they can simulate the alterations in a chemical system when subjected to disruptions, such as exposure to laser beams.