Physical and flexible qubit coupled cluster for quantum chemistry simulation on a noisy intermediate scale quantum computer

Abstract

In quantum chemistry simulation employing variational quantum eigensolver(VQE) architecture under the noisy and intermediate scale quantum computer(NISQ), there are many studies describe the ground state of an electronic structure of a molecular system, consuming less precious quantum resources quantified as the number of qubits and the time for building the final quantum state. In these aspects, one of the viable way is qubit coupled cluster(QCC) ansatz which has entanglers with non-zero gradient of energy. In this paper, we suggest two main modifications on the QCC method. One is limited QCC(L-QCC) which puts a limit on the number of entanglers. With this approach, we show a strategy to choose the effective entanglers. It introduces a way of trading-off between the circuit depth and accuracy. The other one is physical QCC(P-QCC). This prevents the QCC simulation from non physical behavior, that is the change of the number of electrons. Also, with the experiment of VQE with QCC ansatz under noisy environment, we find the requirement for noise specs to simulate a given molecule with small errors.