M. Fang

Observation of topological transitions in interacting quantum circuits

  1. P. Roushan,
  2. C. Neill,
  3. Yu Chen,
  4. M. Kolodrubetz,
  5. C. Quintana,
  6. N. Leung,
  7. M. Fang,
  8. R. Barends,
  9. B. Campbell,
  10. Z. Chen,
  11. B. Chiaro,
  12. A. Dunsworth,
  13. E. Jeffrey,
  14. J. Kelly,
  15. A. Megrant,
  16. J. Mutus,
  17. P. O'Malley,
  18. D. Sank,
  19. A. Vainsencher,
  20. J. Wenner,
  21. T. White,
  22. A. Polkovnikov,
  23. A. N. Cleland,
  24. and J.M. Martinis
The discovery of topological phases in condensed matter systems has changed the modern conception of phases of matter. The global nature of topological ordering makes these phases robust
and hence promising for applications. However, the non-locality of this ordering makes direct experimental studies an outstanding challenge, even in the simplest model topological systems, and interactions among the constituent particles adds to this challenge. Here we demonstrate a novel dynamical method to explore topological phases in both interacting and non-interacting systems, by employing the exquisite control afforded by state-of-the-art superconducting quantum circuits. We utilize this method to experimentally explore the well-known Haldane model of topological phase transitions by directly measuring the topological invariants of the system. We construct the topological phase diagram of this model and visualize the microscopic evolution of states across the phase transition, tasks whose experimental realizations have remained elusive. Furthermore, we developed a new qubit architecture that allows simultaneous control over every term in a two-qubit Hamiltonian, with which we extend our studies to an interacting Hamiltonian and discover the emergence of an interaction-induced topological phase. Our implementation, involving the measurement of both global and local textures of quantum systems, is close to the original idea of quantum simulation as envisioned by R. Feynman, where a controllable quantum system is used to investigate otherwise inaccessible quantum phenomena. This approach demonstrates the potential of superconducting qubits for quantum simulation and establishes a powerful platform for the study of topological phases in quantum systems.
arxiv pdf

Qubit architecture with high coherence and fast tunable coupling

  1. Yu Chen,
  2. C. Neill,
  3. P. Roushan,
  4. N. Leung,
  5. M. Fang,
  6. R. Barends,
  7. J. Kelly,
  8. B. Campbell,
  9. Z. Chen,
  10. B. Chiaro,
  11. A. Dunsworth,
  12. E. Jeffrey,
  13. A. Megrant,
  14. J. Y. Mutus,
  15. P. J. J. O'Malley,
  16. C. M. Quintana,
  17. D. Sank,
  18. A. Vainsencher,
  19. J. Wenner,
  20. T. C. White,
  21. Michael R. Geller,
  22. A. N. Cleland,
  23. and John M. Martinis
We introduce a superconducting qubit architecture that combines high-coherence qubits and tunable qubit-qubit coupling. With the ability to set the coupling to zero, we demonstrate
that this architecture is protected from the frequency crowding problems that arise from fixed coupling. More importantly, the coupling can be tuned dynamically with nanosecond resolution, making this architecture a versatile platform with applications ranging from quantum logic gates to quantum simulation. We illustrate the advantages of dynamic coupling by implementing a novel adiabatic controlled-Z gate, at a speed approaching that of single-qubit gates. Integrating coherence and scalable control, our „gmon“ architecture is a promising path towards large-scale quantum computation and simulation.
arxiv pdf