Qubit coupled cluster singles and doubles variational quantum eigensolver ansatz for electronic structure calculations

Variational quantum eigensolver (VQE) for electronic structure calculations is believed to be one major potential application of near term quantum computing. Among all proposed VQE algorithms, the unitary coupled cluster singles and doubles excitations (UCCSD) VQE ansatz has achieved high accuracy and received a lot of research interest. However, the UCCSD VQE based on fermionic excitations needs extra terms for the parity when using Jordan-Wigner transformation. Here we introduce a new VQE ansatz based on the particle preserving exchange gate to achieve qubit excitations. The proposed VQE ansatz has gate complexity up-bounded to $O(n^4)$ where $n$ is the number of qubits of the Hamiltonian. Numerical results of simple molecular systems such as BeH$_2$, H$_2$O, N$_2$, H$_4$ and H$_6$ using the proposed VQE ansatz gives very accurate results within errors about $10^{-3}$ Hartree.