Elizaveta A. Krivko

Native CCZ Gate with Fluxonium Qubits and a Microwave-Driven Coupler

  1. Grigoriy S. Mazhorin,
  2. Tatyana A. Chudakova,
  3. Alena S. Kazmina,
  4. Nikolai G. Berezkin,
  5. Arina V. Zotova,
  6. Artyom M. Polyanskiy,
  7. Nikolay N. Abramov,
  8. Mikhail A. Tarkhov,
  9. Alexander M. Mumlyakov,
  10. Igor V. Trofimov,
  11. Elizaveta A. Krivko,
  12. Nikita Yu. Rudenko,
  13. Maxim V. Chichkov,
  14. Vladimir I. Chichkov,
  15. and Ilya A. Simakov
Native multi-qubit gates could reduce the overhead associated with decompositions into single- and two-qubit operations, but whether they can simultaneously provide high fidelity, simple
control and robustness against parasitic interactions in scalable architectures remains unclear. Here we experimentally realize a 65-ns native controlled-controlled-phase operation, locally equivalent to the Toffoli gate, with a fidelity of 99.39(5)% in a three-qubit processor unit based on fluxonium qubits coupled via a microwave-driven transmon coupler. The implemented operation would require CZ fidelities of approximately 99.94% if realized through a conventional decomposition. The gate is implemented with a single control pulse, that relies on a simple calibration procedure yielding coherence-limited performance. This processor unit naturally extends to scalable two-dimensional layouts with low parasitic interactions. Altogether, these results establish native multi-qubit gates as a viable hardware-efficient primitive for scalable superconducting quantum processors.
arxiv pdf

Wiring surface loss of a superconducting transmon qubit

  1. Nikita S. Smirnov,
  2. Elizaveta A. Krivko,
  3. Anastasiya A. Solovieva,
  4. Anton I. Ivanov,
  5. and Ilya A. Rodionov
Quantum processors using superconducting qubits suffer from dielectric loss leading to noise and dissipation. Qubits are usually designed as large capacitor pads connected to a non-linear
Josephson junction (or SQUID) by a superconducting thin metal wiring. Here, we report on finite-element simulation and experimental results confirming that more than 50% of surface loss in transmon qubits can originated from Josephson junctions wiring and can limit qubit relaxation time. Extracting dielectric loss tangents capacitor pads and wiring based on their participation ratios, we show dominant surface loss of wiring can occur for real qubits designs. Then, we simulate a qubit coupled to a bath of individual TLS defects and show that only a small fraction (~18%) of coupled defects is located within the wiring interfaces, however, their coupling strength is much higher due to stronger electromagnetic field. Finally, we fabricate six tunable floating transmon qubits and experimentally demonstrate up to 20% improvement in qubit quality factor by wiring design optimization.
arxiv pdf