Efficient Low-Depth UCC Algorithm for Strongly Correlated Systems

The unitary coupled cluster (UCC) ansatz is one of the most promising chemistry ansatzes for quantum computation of electronic structure calculations. However, for large systems and for many orbitals, the number of UCC factors quickly leads to very deep quantum circuits. To address this, the circuit depth can be greatly reduced at the cost of more measurements by using Taylor expansions for the UCC factors that have small amplitudes, while treating the large amplitude factors exactly. This approach is further optimized by employing the hidden SU(2) group symmetry of the UCC factors of arbitrary rank. Initial work on this method demonstrated the initial viability of obtaining results with reasonable accuracy for electronic structure calculations. In this talk, we implement this method for strongly correlated systems and demonstrate the ability of this method to produce good results in systems with strong correlations. We find that often, this Taylor expansion UCC method outperforms conventional methods in these regimes at significantly reduced circuit depth. To demonstrate this utility, we consider linear hydrogen chains, rectangular H4, and BeH2.