LowDepth Unitary Coupled Cluster Theory for Quantum Computation
Abstract
The unitary coupled cluster (UCC) approximation is one of the more promising wavefunction ansätze for electronic structure calculations on quantum computers via the variational quantum eigensolver algorithm. However, for large systems with many orbitals, the required number of UCC factors still leads to very deep quantum circuits, which can be challenging to implement. Based on the observation that most UCC amplitudes are small for weakly correlated molecules, we devise an algorithm that employs a Taylor expansion in the small amplitudes, trading off circuit depth for extra measurements. Strong correlations can be taken into account by performing the expansion about a small set of UCC factors, which are treated exactly. Near equilibrium, the Taylor series expansion often works well without the need to include any exact factors; as the molecule is stretched and correlations increase, we find only a small number of factors need to be treated exactly.
 Publication:

arXiv eprints
 Pub Date:
 September 2021
 arXiv:
 arXiv:2109.14134
 Bibcode:
 2021arXiv210914134C
 Keywords:

 Quantum Physics;
 Physics  Chemical Physics
 EPrint:
 17 pages, 2 figures