K. V. Shulga

Time molecules with periodically driven interacting qubits

  1. K. V. Shulga,
  2. I. Vakulchyk,
  3. Y. Nakamura,
  4. S. Flach,
  5. and M. V. Fistul
We provide numerical evidence for a temporal quantum-mechanical interference phenomenon: time molecules (TM). A variety of such stroboscopic states are observed in the dynamics of two
interacting qubits subject to a periodic sequence of π-pulses with the period T. The TMs appear periodically in time and have a large duration, δtTM≫T. All TMs demonstrate an almost zero value of the total polarization and a strong enhancement of the entanglement entropy S up to the maximum value S=ln2 of a corresponding Bell state. The TMs are generated by the commensurability of the Floquet eigenvalues and the presence of maximally entangled Floquet eigenstates. The TMs remain stable with detuned system parameters and with an increased number of qubits. The TMs can be observed in microwave experiments with an array of superconducting qubits.
arxiv pdf

Fabrication and measurements of hybrid Nb/Al Josephson junctions and flux qubits with pi-shifters

  1. A. V. Shcherbakova,
  2. K. G. Fedorov,
  3. K. V. Shulga,
  4. V. V. Ryazanov,
  5. V. V. Bolginov,
  6. V. A Oboznov,
  7. S. V. Egorov,
  8. V. O. Shkolnikov,
  9. M. J. Wolf,
  10. D. Beckmann,
  11. and A. V. Ustinov
We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing
pi-phase shifters fabricated using well-established Nb-based technology of superconductor-ferromagnet-superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in-situ Ar etching using e-beam gun. Shadow-evaporated Al/AlOx/Al Josephson junctions with Nb bias pads show the expected current-voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb pi-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without pi-junction is attributed to the pi phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.
arxiv pdf