The role of symmetries in adiabatic quantum algorithms

  • Authors:
  • Gernot Schaller;Ralf Schützhold

  • Affiliations:
  • Institut für Theoretische Physik, Technische Universität Berlin, Berlin, Germany;Fakultät für Physik am Campus Duisburg, Universität Duisburg-Essen, Duisburg, Germany

  • Venue:
  • Quantum Information & Computation
  • Year:
  • 2010

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Abstract

Exploiting the similarity between adiabatic quantum algorithms and quantum phasetransitions, we argue that second-order transitions - typically associated with broken orrestored symmetries - should be advantageous in comparison to first-order transitions.Guided by simple examples we construct an alternative adiabatic algorithm for the NPcompleteproblem Exact Cover 3. We show numerically that its average performance (forthe considered cases up to O{20} qubits) is better than that of the conventional scheme.The run-time of adiabatic algorithms is not just determined by the minimum value of thefundamental energy gap (between the ground state and the exited states), but also byits curvature at the critical point. The proposed symmetry-restoring adiabatic quantumalgorithm only contains contributions linear and quadratic in the Pauli matrices and canbe generalized to other problem Hamiltonians which are decomposed of terms involvingone and two qubits. We show how the factoring problem can be cast into such a quadraticform. These findings suggest that adiabatic quantum algorithms can solve a large classof NP problems much faster than the Grover search routine (which corresponds to afirst-order transition and yields a quadratic enhancement only).