2018/09/11 by Ilya G. Ryabinkin, Tzu-Ching Yen, Ryabinkin, Ilya G. +5 · 14 citations
Computer Science · Physics and Astronomy · #Quantum Computing Algorithms and Architecture #Quantum Information and Cryptography #Quantum and electron transport phenomena
paper · pdf · doi:10.48550/arxiv.1809.03827
A unitary coupled-cluster (UCC) form for the wavefunction in the variational\nquantum eigensolver has been suggested as a systematic way to go beyond the\nmean-field approximation and include electron correlation in solving quantum\nchemistry problems on a quantum computer. Although being exact in the limit of\nincluding all possible coupled-cluster excitations, practically, the accuracy\nof this approach depends on how many and what kind of terms are included in the\nwavefunction parametrization. Another difficulty of UCC is a growth of the\nnumber of simultaneously entangled qubits even at the fixed fermionic\nexcitation rank. Not all quantum computing architectures can cope with this\ngrowth. To address both problems we introduce a qubit coupled-cluster (QCC)\nmethod that starts directly in the qubit space and uses energy response\nestimates for ranking the importance of individual entanglers for the\nvariational energy minimization. Also, we provide an exact factorization of a\nunitary rotation of more than two qubits to a product of two-qubit unitary\nrotations. Thus, the QCC method with the factorization technique can be limited\nto only two-qubit entanglement gates and allows for very efficient use of\nquantum resources in terms of the number of coupled-cluster operators. The\nmethod performance is illustrated by calculating ground-state potential energy\ncurves of H2 and LiH molecules with chemical accuracy, \≤ 1 kcal/mol.\n