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      Quantum equation of motion for computing molecular excitation energies on a noisy quantum processor

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          Abstract

          The computation of molecular excitation energies is essential for predicting photo-induced reactions of chemical and technological interest. While the classical computing resources needed for this task scale poorly, quantum algorithms emerge as promising alternatives. In particular, the extension of the variational quantum eigensolver algorithm to the computation of the excitation energies is an attractive choice. However, there is currently a lack of such algorithms for correlated molecular systems that is amenable to near-term, noisy hardware. Here, we introduce an efficient excited states quantum algorithm, that employs a quantum version of the well-established classical equation of motion approach, which allows the calculation of the excitation energies of a given system using an approximated description of its ground state wave function. We numerically test the algorithm for several small molecules and experimentally demonstrate the robustness of the algorithm by computing the excitation energies of a LiH molecule at varying noise levels.

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          Error mitigation extends the computational reach of a noisy quantum processor

          Quantum computation, a paradigm of computing that is completely different from classical methods, benefits from theoretically proved speed-ups for certain problems and can be used to study the properties of quantum systems1. Yet, because of the inherently fragile nature of the physical computing elements (qubits), achieving quantum advantages over classical computation requires extremely low error rates for qubit operations, as well as substantial physical qubits, to realize fault tolerance via quantum error correction2,3. However, recent theoretical work4,5 has shown that the accuracy of computation (based on expectation values of quantum observables) can be enhanced through an extrapolation of results from a collection of experiments of varying noise. Here we demonstrate this error mitigation protocol on a superconducting quantum processor, enhancing its computational capability, with no additional hardware modifications. We apply the protocol to mitigate errors in canonical single- and two-qubit experiments and then extend its application to the variational optimization6-8 of Hamiltonians for quantum chemistry and magnetism9. We effectively demonstrate that the suppression of incoherent errors helps to achieve an otherwise inaccessible level of accuracy in the variational solutions using our noisy processor. These results demonstrate that error mitigation techniques will enable substantial improvements in the capabilities of near-term quantum computing hardware.
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            Does chlorine peroxide exhibit a strong ultraviolet absorption near 250 nm?

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              The Equations of Motion Method An Approach to the Dynamical Properties of Atoms and Molecules

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                Author and article information

                Journal
                28 October 2019
                Article
                1910.12890
                86584d8f-e7f0-43ba-96e9-e68b47a4644c

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                Custom metadata
                quant-ph cond-mat.str-el physics.chem-ph

                Condensed matter, Quantum physics & Field theory, Physical chemistry

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