L. Richard

Niobium Air Bridges as a Low-Loss Component for Superconducting Quantum Hardware

  1. N. Bruckmoser,
  2. L. Koch,
  3. I. Tsitsilin,
  4. M.Grammer,
  5. D. Bunch,
  6. L. Richard,
  7. J. Schirk,
  8. F. Wallner,
  9. J. Feigl,
  10. C. M. F. Schneider,
  11. S. Geprägs,
  12. V.P. Bader,
  13. M. Althammer,
  14. L. Södergren,
  15. and S. Filipp
Scaling up superconducting quantum processors requires a high routing density for readout and control lines, relying on low-loss interconnects to maintain design flexibility and device
performance. We propose and demonstrate a universal subtractive fabrication process for air bridges based on an aluminum hard mask and niobium as the superconducting film. Using this technology, we fabricate superconducting CPW resonators incorporating multiple niobium air bridges in and across their center conductors. Through rigorous cleaning methods, we achieve mean internal quality factors in the single-photon limit exceeding Qint=8.2×106. Notably, the loss per air bridge remains below the detection threshold of the resonators. Due to the larger superconducting energy gap of niobium compared to conventional aluminum air bridges, our approach enables stable performance at elevated temperatures and magnetic fields, which we experimentally confirm in temperatures up to 3.9 K and in a magnetic field of up to 1.60 T. Additionally, we utilize air bridges to realize low-loss vacuum-gap capacitors and demonstrate their successful integration into transmon qubits by achieving median qubit lifetimes of T1=51.6μs.
arxiv pdf

Parametric multi-element coupling architecture for coherent and dissipative control of superconducting qubits

  1. G. B. P. Huber,
  2. F. A. Roy,
  3. L. Koch,
  4. I. Tsitsilin,
  5. J. Schirk,
  6. N. J. Glaser,
  7. N. Bruckmoser,
  8. C. Schweizer,
  9. J. Romeiro,
  10. G. Krylov,
  11. M. Singh,
  12. F. X. Haslbeck,
  13. M. Knudsen,
  14. A. Marx,
  15. F. Pfeiffer,
  16. C. Schneider,
  17. F. Wallner,
  18. D. Bunch,
  19. L. Richard,
  20. L. Södergren,
  21. K. Liegener,
  22. M. Werninghaus,
  23. and S. Filipp
As systems for quantum computing keep growing in size and number of qubits, challenges in scaling the control capabilities are becoming increasingly relevant. Efficient schemes to simultaneously
mediate coherent interactions between multiple quantum systems and to reduce decoherence errors can minimize the control overhead in next-generation quantum processors. Here, we present a superconducting qubit architecture based on tunable parametric interactions to perform two-qubit gates, reset, leakage recovery and to read out the qubits. In this architecture, parametrically driven multi-element couplers selectively couple qubits to resonators and neighbouring qubits, according to the frequency of the drive. We consider a system with two qubits and one readout resonator interacting via a single coupling circuit and experimentally demonstrate a controlled-Z gate with a fidelity of 98.30±0.23%, a reset operation that unconditionally prepares the qubit ground state with a fidelity of 99.80±0.02% and a leakage recovery operation with a 98.5±0.3% success probability. Furthermore, we implement a parametric readout with a single-shot assignment fidelity of 88.0±0.4%. These operations are all realized using a single tunable coupler, demonstrating the experimental feasibility of the proposed architecture and its potential for reducing the system complexity in scalable quantum processors.
arxiv pdf