Gate-efficient simulation of molecular eigenstates on a quantum computer

  1. Marc Ganzhorn,
  2. Daniel J. Egger,
  3. Panagiotis Kl. Barkoutsos,
  4. Pauline Ollitrault,
  5. Gian Salis,
  6. Nikolaj Moll,
  7. Andreas Fuhrer,
  8. Peter Müller,
  9. Stefan Woerner,
  10. Ivano Tavernelli,
  11. and Stefan Filipp
A key requirement to perform simulations of large quantum systems on near-term quantum hardware is the design of quantum algorithms with short circuit depth that finish within the available coherence time. A way to stay within the limits of coherence is to reduce the number of gates by implementing a gate set that matches the requirements of the specific algorithm of interest directly in hardware. Here, we show that exchange-type gates are a promising choice for simulating molecular eigenstates on near-term quantum devices since these gates preserve the number of excitations in the system. Complementing the theoretical work by Barkoutsos et al. [PRA 98, 022322 (2018)], we report on the experimental implementation of a variational algorithm on a superconducting qubit platform to compute the eigenstate energies of molecular hydrogen. We utilize a parametrically driven tunable coupler to realize exchange-type gates that are configurable in amplitude and phase on two fixed-frequency superconducting qubits. With gate fidelities around 95% we are able to compute the eigenstates within an accuracy of 50 mHartree on average, a limit set by the coherence time of the tunable coupler.
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