📝 SummarXiv

Abstract

Quantum Phase Estimation (QPE) is a cornerstone algorithm for fault-tolerant quantum computation, especially for electronic structure calculations of chemical systems. To accommodate the diverse characteristics of quantum chemical systems, numerous variants of QPE have been developed, each with distinct qubit and gate cost implications. In this paper, we quantify the impact of three key parameters on the overall quantum resource costs for the QPE algorithm: the choice between trotterization and qubitization, the use of molecular orbitals versus plane-wave basis-sets, and the selection of the fermion-to-qubit encoding scheme. From this, we establish clear performance trade-offs and delineate specific parameter regimes that minimize resource costs for relevant molecular systems. When performing phase estimation on large molecules in the fault-tolerant setting, we found the first-quantized qubitization circuit using the plane-wave basis to be the most efficient, with a gate cost scaling of $\tilde{\mathcal{O}}([N^{4/3}M^{2/3}+N^{8/3}M^{1/3}]/\varepsilon)$ for a system of $N$ electrons and $M$ orbitals, which is the best known scaling to date. On the other hand, when only noisy intermediate-scale or near-term fault-tolerant systems are available, the phase estimation of small molecules can be performed with gate cost of $\mathcal{O}(M^{7}/\varepsilon^{2})$ via trotterization in the MO basis. Furthermore, we provide numerical estimations of qubit and T gate costs required to perform QPE for several real-world molecular systems under these different parameter choices.