Coupler microwave-activated controlled phase gate on fluxonium qubits

  1. Ilya A. Simakov,
  2. Grigoriy S. Mazhorin,
  3. Ilya N. Moskalenko,
  4. Nikolay N. Abramov,
  5. Alexander A. Grigorev,
  6. Dmitry O. Moskalev,
  7. Anastasiya A. Pishchimova,
  8. Nikita S. Smirnov,
  9. Evgeniy V. Zikiy,
  10. Ilya A. Rodionov,
  11. and Ilya S. Besedin
Tunable couplers have recently become one of the most powerful tools for implementing two-qubit gates between superconducting qubits. A tunable coupler typically includes a nonlinear
element, such as a SQUID, which is used to tune the resonance frequency of an LC circuit connecting two qubits. Here we propose a complimentary approach where instead of tuning the resonance frequency of the tunable coupler by applying a quasistatic control signal, we excite by microwave the degree of freedom associated with the coupler itself. Due to strong effective longitudinal coupling between the coupler and the qubits, the frequency of this transition strongly depends on the computational state, leading to different phase accumulations in different states. Using this method, we experimentally demonstrate a CZ gate of 44 ns duration on a fluxonium-based quantum processor, obtaining a fidelity of 97.6±0.4% characterized by cross-entropy benchmarking.

Topological photon pairs in a superconducting quantum metamaterial

  1. Ilya S. Besedin,
  2. Maxim A. Gorlach,
  3. Nikolay N. Abramov,
  4. Ivan Tsitsilin,
  5. Ilya N. Moskalenko,
  6. Alina A. Dobronosova,
  7. Dmitry O. Moskalev,
  8. Alexey R. Matanin,
  9. Nikita S. Smirnov,
  10. Ilya A. Rodionov,
  11. Alexander N. Poddubny,
  12. and Alexey V. Ustinov
Recent discoveries in topological physics hold a promise for disorder-robust quantum systems and technologies. Topological states provide the crucial ingredient of such systems featuring
increased robustness to disorder and imperfections. Here, we use an array of superconducting qubits to engineer a one-dimensional topologically nontrivial quantum metamaterial. By performing microwave spectroscopy of the fabricated array, we experimentally observe the spectrum of elementary excitations. We find not only the single-photon topological states but also the bands of exotic bound photon pairs arising due to the inherent anharmonicity of qubits. Furthermore, we detect the signatures of the two-photon bound edge-localized state which hints towards interaction-induced localization in our system. Our work demonstrates an experimental implementation of the topological model with attractive photon-photon interaction in a quantum metamaterial.