Tailoring Term Truncations for Electronic Structure Calculations Using a Linear Combination of Unitaries
1Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom
2Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, United Kingdom
3AWS Center for Quantum Computing, Pasadena, CA 91125, USA
Published: | 2022-02-02, volume 6, page 637 |
Eprint: | arXiv:2007.11624v3 |
Doi: | https://doi.org/10.22331/q-2022-02-02-637 |
Citation: | Quantum 6, 637 (2022). |
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Abstract
A highly anticipated use of quantum computers is the simulation of complex quantum systems including molecules and other many-body systems. One promising method involves directly applying a linear combination of unitaries (LCU) to approximate a Taylor series by truncating after some order. Here we present an adaptation of that method, optimized for Hamiltonians with terms of widely varying magnitude, as is commonly the case in electronic structure calculations. We show that it is more efficient to apply LCU using a truncation that retains larger magnitude terms as determined by an iterative procedure. We obtain bounds on the simulation error for this generalized truncated Taylor method, and for a range of molecular simulations, we report these bounds as well as exact numerical results. We find that our adaptive method can typically improve the simulation accuracy by an order of magnitude, for a given circuit depth.
![](https://quantum-journal.org/wp-content/uploads/2022/01/truncated_taylor.png)
Featured image: Pruning some terms from the circuit construction of a Taylor expansion while retaining others can lead to a significant reduction of the simulation error.
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