State preparation of a fluxonium qubit with feedback from a custom FPGA-based platform

  1. Richard Gebauer,
  2. Nick Karcher,
  3. Daria Gusenkova,
  4. Martin Spiecker,
  5. Lukas Grünhaupt,
  6. Ivan Takmakov,
  7. Patrick Winkel,
  8. Luca Planat,
  9. Nicolas Roch,
  10. Wolfgang Wernsdorfer,
  11. Alexey V. Ustinov,
  12. Marc Weber,
  13. Martin Weides,
  14. Ioan M. Pop,
  15. and Oliver Sander
We developed a versatile integrated control and readout instrument for experiments with superconducting quantum bits (qubits), based on a field-programmable gate array (FPGA) platform. Using this platform, we perform measurement-based, closed-loop feedback operations with 428ns platform latency. The feedback capability is instrumental in realizing active reset initialization of the qubit into the ground state in a time much shorter than its energy relaxation time T1. We show experimental results demonstrating reset of a fluxonium qubit with 99.4% fidelity, using a readout-and-drive pulse sequence approximately 1.5μs long. Compared to passive ground state initialization through thermalization, with the time constant given by T1= 80μs, the use of the FPGA-based platform allows us to improve both the fidelity and the time of the qubit initialization by an order of magnitude.
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