Quantum chemistry calculations, long envisioned as a transformative application for quantum computers in fields like drug development and agriculture, may face significant limitations, according to recent analysis. While quantum computing has seen rapid advancement, the identification of applications that truly justify substantial investment remains an ongoing challenge.
One leading candidate for early adoption is the solution of quantum chemistry problems, specifically the calculation of molecular energy levels crucial for biomedical and industrial applications. Such tasks inherently involve managing the simultaneous quantum behavior of numerous particles, primarily electrons within a molecule, making them appear suited for quantum computing architectures built on quantum principles.
However, a new study by Xavier Waintal and his colleagues at CEA Grenoble in France suggests that two prominent quantum algorithms designed for these calculations might offer only limited utility, even at their best.
“My personal thinking is that it’s probably doomed, not proven doomed, but probably doomed,” Waintal stated regarding the prospect of using quantum computers for molecular energy calculations.
Analyzing the Challenges: Error-Prone and Fault-Tolerant Systems
The researchers divided their comprehensive mathematical analysis into two distinct segments. The first part addresses the capabilities of current, error-prone quantum computers, often referred to as noisy quantum computers. The second part focuses on the potential of future, theoretically “fault-tolerant” quantum computers, which would be impervious to errors.
Error-Prone Quantum Computers and the VQE Algorithm
For noisy quantum computers, the variational quantum eigensolver (VQE) algorithm is a method for calculating molecular energy levels. The accuracy of the results derived from VQE is directly influenced by the degree of noise present in the quantum system.
The team’s findings indicate that for VQE to achieve accuracy comparable to algorithms running on conventional classical computers, the quantum computer’s noise levels would need to be suppressed to such an extreme degree that it would essentially require fault-tolerance. It is important to note that a practical, fully fault-tolerant quantum computer has not yet been developed.
Future Fault-Tolerant Machines and the Orthogonality Catastrophe
Looking ahead, several companies are working towards developing fault-tolerant quantum computers within the next five years. These advanced machines are expected to compute molecular energies using a different algorithm known as quantum phase estimation (QPE).
While QPE largely circumvents the issue of errors, the study highlights a significant obstacle known ominously as the “orthogonality catastrophe.” This phenomenon means that as the complexity or size of molecules increases, the probability of QPE successfully calculating their lowest energy level diminishes exponentially.
Consequently, Thibaud Louvet, a team member from the French quantum computing firm Quobly, explained that even with highly advanced quantum computers, the practical advantages of using QPE for calculating molecular energies would be restricted to a narrow set of scenarios. He suggested that the ability to run QPE should be viewed more as a benchmark for the maturity of quantum hardware rather than a tool that will become standard for practicing chemists.
Expert Commentary and Broader Implications
George Booth, a researcher at King’s College London not involved in the study, commented on the findings. “It is easy to over-hype the prospects of quantum computers in this domain, with many thinking that the advent of quantum computers will instantly render any classical approach to quantum chemistry obsolete,” he noted.
“This study is clear to point out significant challenges for accurate molecular simulation, which will remain even in the ‘fault-tolerant era,’ and cast doubt on whether quantum chemistry is really such a quick win for quantum computers,” Booth added.
Despite these specific limitations for energy calculations, Booth suggested that quantum computers might still find utility in other areas of chemistry, such as simulating the dynamic behavior of chemical systems when subjected to external stimuli, like laser light.
Journal reference: Physical Review B DOI: 10.1103/hpt6-9tnk