No abstract

The Coupled Cluster (CC) method is used to compute the electronic correlation energy in atoms and molecules and often leads to highly accurate results. However, its usual implementation in the projected form becomes nonvariational when the excitations are truncated and therefore fails to describe quantum states characterized by strong electronic correlations. Thanks to its exponential form, CC can be naturally adapted to quantum algorithms. In particular, the quantum unitary CC (q-UCC) is a popular wavefunction Ansatz for the Variational Quantum Eigensolver (VQE). The variational nature of this approach can lead to significant advantages compared to its classical equivalent (in the projected form), in particular for the description of strong electronic correlation. However, due to the large number of gate operations required in q-UCC, approximations need to be introduced in order to make this approach implementable in a stateof-the-art quantum computer. In this work, we propose several variants of the standard q-UCCSD Ansatz in which only a subset of excitations is included. In particular, we investigate the singlet and pair q-UCCD approaches combined with orbital optimization. We show that these approaches can capture the dissociation/distortion profiles of challenging systems such as H 4 , H 2 O and N 2 molecules, as well as the one-dimensional periodic Fermi-Hubbard chain. The results, which are in good agreement with the exact solutions, promote the future use of q-UCC methods for the solution of challenging electronic structure problems in quantum chemistry.

No abstract

No abstract

No abstract

The development of tailored materials for specific applications is an active field of research in chemistry, material science and drug discovery. The number of possible molecules obtainable from a set...

A central building block of many quantum algorithms is the diagonalization of Pauli operators. Although it is always possible to construct a quantum circuit that simultaneously diagonalizes a given set of commuting Pauli operators, only resource-efficient circuits are reliably executable on near-term quantum computers. Generic diagonalization circuits can lead to an unaffordable Swapgate overhead on quantum devices with limited hardware connectivity. A common alternative is excluding two-qubit gates, however, this comes at the cost of restricting the class of diagonalizable sets of Pauli operators to tensor product bases (TPBs). In this letter, we introduce a theoretical framework for constructing hardware-tailored (HT) diagonalization circuits. We apply our framework to group the Pauli operators occurring in the decomposition of a given Hamiltonian into jointly-HT-diagonalizable sets. We investigate several classes of popular Hamiltonians and observe that our approach requires a smaller number of measurements than conventional TPB approaches. Finally, we experimentally demonstrate the practical applicability of our technique, which showcases the great potential of our circuits for near-term quantum computing.

No abstract