Jack Raymond

Essentially exact numerical modelling of flux qubit chains subject to charge and flux noise

  1. Matthew R.C. Fitzpatrick,
  2. Jack Raymond,
  3. and Malcolm P. Kennett
We present an essentially exact numerical method for modelling flux qubit chains subject to charge and flux noise. We define an essentially exact method as one that introduces errors
that are completely controlled such that they can be made arbitrarily small by tuning the simulation parameters. The method adopts the quasi-adiabatic path integral formalism to express the system’s reduced density matrix as a time-discretized path integral, comprising a series of influence functionals that encode the non-Markovian dynamics of the system. We present a detailed derivation of the path integral expression for the system’s reduced density matrix and describe in detail the tensor network algorithm used to evaluate the path integral expression. We have implemented our method in an open-sourced Python library called „spinbosonchain“. When appropriate, we draw connections between concepts covered in this manuscript and the library’s code.
arxiv pdf

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