Abstract
One of the key applications for quantum computers will be the simulation of other quantum systems that arise in chemistry, materials science, etc., in order to accelerate the process of discovery. It is important to ask the following question: Can this simulation be achieved using near-future quantum processors, of modest size and under imperfect control, or must it await the more distant era of large-scale fault-tolerant quantum computing? Here, we propose a variational method involving closely integrated classical and quantum coprocessors. We presume that all operations in the quantum coprocessor are prone to error. The impact of such errors is minimized by boosting them artificially and then extrapolating to the zero-error case. In comparison to a more conventional optimized Trotterization technique, we find that our protocol is efficient and appears to be fundamentally more robust against error accumulation.
- Received 14 December 2016
DOI:https://doi.org/10.1103/PhysRevX.7.021050
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Quantum computers hold the potential to solve complex problems in chemistry, physics, and materials science that cannot be handled by conventional digital computers. While practical large-scale quantum computing is still a ways off, simple devices could become available in the next few years. Tantalizingly, a mere 50 physical qubits (the quantum equivalent of a digital bit) suffice to enter uncharted territory, where the processor is so complex that modern supercomputers cannot predict its behavior. However, quantum algorithms require error-correcting hardware, which may need millions of qubits. The challenge, then, is to find useful tasks within this gap, involving hundreds or thousands of qubits rather than millions. We propose a novel approach to simulating quantum systems that requires relatively few qubits and yet can tolerate error accumulation.
We follow a hybrid approach by closely coupling a quantum computer with a traditional one. Our proposed algorithm simply tolerates errors rather than needing hardware-level error correction. To do this, it deliberately increases the error rate in order to understand its effect. This allows us to estimate how the system would behave in the absence of errors. Through numerical analysis, we find that the algorithm can perform useful, conventionally unfeasible tasks using a modest number of qubits.
By coupling a classical computer to a quantum processor, our algorithm can substantially reduce the number of qubits needed, which in turn makes it easier to have several quantum processors working in parallel. We believe that this is a promising approach to realizing the first generation of quantum computers.