Andrew D. King

Observation of topological phenomena in a programmable lattice of 1,800 qubits

  1. Andrew D. King,
  2. Juan Carrasquilla,
  3. Isil Ozfidan,
  4. Jack Raymond,
  5. Evgeny Andriyash,
  6. Andrew Berkley,
  7. Mauricio Reis,
  8. Trevor M. Lanting,
  9. Richard Harris,
  10. Gabriel Poulin-Lamarre,
  11. Anatoly Yu. Smirnov,
  12. Christopher Rich,
  13. Fabio Altomare,
  14. Paul Bunyk,
  15. Jed Whittaker,
  16. Loren Swenson,
  17. Emile Hoskinson,
  18. Yuki Sato,
  19. Mark Volkmann,
  20. Eric Ladizinsky,
  21. Mark Johnson,
  22. Jeremy Hilton,
  23. and Mohammad H. Amin
The celebrated work of Berezinskii, Kosterlitz and Thouless in the 1970s revealed exotic phases of matter governed by topological properties of low-dimensional materials such as thin
films of superfluids and superconductors. Key to this phenomenon is the appearance and interaction of vortices and antivortices in an angular degree of freedom—typified by the classical XY model—due to thermal fluctuations. In the 2D Ising model this angular degree of freedom is absent in the classical case, but with the addition of a transverse field it can emerge from the interplay between frustration and quantum fluctuations. Consequently a Kosterlitz-Thouless (KT) phase transition has been predicted in the quantum system by theory and simulation. Here we demonstrate a large-scale quantum simulation of this phenomenon in a network of 1,800 in situ programmable superconducting flux qubits arranged in a fully-frustrated square-octagonal lattice. Essential to the critical behavior, we observe the emergence of a complex order parameter with continuous rotational symmetry, and the onset of quasi-long-range order as the system approaches a critical temperature. We use a simple but previously undemonstrated approach to statistical estimation with an annealing-based quantum processor, performing Monte Carlo sampling in a chain of reverse quantum annealing protocols. Observations are consistent with classical simulations across a range of Hamiltonian parameters. We anticipate that our approach of using a quantum processor as a programmable magnetic lattice will find widespread use in the simulation and development of exotic materials.
arxiv pdf

Performance of a quantum annealer on range-limited constraint satisfaction problems

  1. Andrew D. King
The performance of a D-Wave Vesuvius quantum annealer was recently compared to a suite of classical algorithms on a class of constraint satisfaction instances based on frustrated loops.
However, the construction of these instances leads the maximum coupling strength to increase with problem size. As a result, larger instances are subject to amplified analog control error, and are effectively annealed at higher temperatures in both hardware and software. We generate similar constraint satisfaction instances with limited range of coupling strength and perform a similar comparison to classical algorithms. On these instances the D-Wave Vesuvius processor, run with a fixed 20μs anneal time, shows a scaling advantage over the software solvers for the hardest regime studied. This scaling advantage opens the possibility of quantum speedup on these problems. Our results support the hypothesis that performance of D-Wave Vesuvius processors is heavily influenced by analog control error, which can be reduced and mitigated as the technology matures.
arxiv pdf