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      Molecular Nanomagnets as Qubits with Embedded Quantum-Error Correction

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          Abstract

          We show that molecular nanomagnets have a potential advantage in the crucial rush toward quantum computers. Indeed, the sizable number of accessible low-energy states of these systems can be exploited to define qubits with embedded quantum error correction. We derive the scheme to achieve this crucial objective and the corresponding sequence of microwave/radiofrequency pulses needed for the error correction procedure. The effectiveness of our approach is shown already with a minimal S = 3/2 unit corresponding to an existing molecule, and the scaling to larger spin systems is quantitatively analyzed.

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          Quantum supremacy using a programmable superconducting processor

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            Quantum Computing in the NISQ era and beyond

            Noisy Intermediate-Scale Quantum (NISQ) technology will be available in the near future. Quantum computers with 50-100 qubits may be able to perform tasks which surpass the capabilities of today's classical digital computers, but noise in quantum gates will limit the size of quantum circuits that can be executed reliably. NISQ devices will be useful tools for exploring many-body quantum physics, and may have other useful applications, but the 100-qubit quantum computer will not change the world right away - we should regard it as a significant step toward the more powerful quantum technologies of the future. Quantum technologists should continue to strive for more accurate quantum gates and, eventually, fully fault-tolerant quantum computing.
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              Hardware-efficient variational quantum eigensolver for small molecules and quantum magnets

              Quantum computers can be used to address electronic-structure problems and problems in materials science and condensed matter physics that can be formulated as interacting fermionic problems, problems which stretch the limits of existing high-performance computers. Finding exact solutions to such problems numerically has a computational cost that scales exponentially with the size of the system, and Monte Carlo methods are unsuitable owing to the fermionic sign problem. These limitations of classical computational methods have made solving even few-atom electronic-structure problems interesting for implementation using medium-sized quantum computers. Yet experimental implementations have so far been restricted to molecules involving only hydrogen and helium. Here we demonstrate the experimental optimization of Hamiltonian problems with up to six qubits and more than one hundred Pauli terms, determining the ground-state energy for molecules of increasing size, up to BeH2. We achieve this result by using a variational quantum eigenvalue solver (eigensolver) with efficiently prepared trial states that are tailored specifically to the interactions that are available in our quantum processor, combined with a compact encoding of fermionic Hamiltonians and a robust stochastic optimization routine. We demonstrate the flexibility of our approach by applying it to a problem of quantum magnetism, an antiferromagnetic Heisenberg model in an external magnetic field. In all cases, we find agreement between our experiments and numerical simulations using a model of the device with noise. Our results help to elucidate the requirements for scaling the method to larger systems and for bridging the gap between key problems in high-performance computing and their implementation on quantum hardware.
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                Author and article information

                Journal
                J Phys Chem Lett
                J Phys Chem Lett
                jz
                jpclcd
                The Journal of Physical Chemistry Letters
                American Chemical Society
                1948-7185
                16 September 2020
                15 October 2020
                : 11
                : 20
                : 8610-8615
                Affiliations
                []Dipartimento di Scienze Matematiche, Fisiche e Informatiche, Università di Parma , I-43124 Parma, Italy
                []UdR Parma, INSTM , I-43124 Parma, Italy
                []INFN, Sezione di Milano Bicocca, Gruppo Collegato di Parma , Parma, Italy
                Author notes
                Article
                10.1021/acs.jpclett.0c02213
                8011924
                32936660
                c99d4ba2-f1d8-47ca-befa-4c0aabf3b832

                Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained ( https://creativecommons.org/licenses/by/4.0/).

                History
                : 21 July 2020
                : 16 September 2020
                Categories
                Letter
                Custom metadata
                jz0c02213
                jz0c02213

                Physical chemistry
                Physical chemistry

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