Andrea Lizzit

Syndrome-Resolved Decoding of Qubit Readout Errors

  1. Han Zheng,
  2. Vladimir Pešić,
  3. Nikolay Gusarov,
  4. Andrea Lizzit,
  5. Taketo Imaizumi,
  6. Chulwoo Ahn,
  7. Marco Nicolas Mueller,
  8. Raymond Mencia,
  9. Edoardo Charbon,
  10. and Vladimir E. Manucharyan
Readout errors in superconducting qubits include incorrect state assignment, bit flips within the computational subspace, and leakage to non-computational states. These errors require
different mitigation strategies, yet conventional measurement fidelity and repeatability metrics do not resolve their individual contributions. Previous repeated-readout studies have shown that temporal correlations reveal additional information about readout errors. Here we introduce the Readout Error Decoder (RED), which converts repeated thresholded binary records into a channel-resolved error budget. Controlled error injections validate the channel-specific response of the decoder. We demonstrate RED on both fluxonium and transmon qubits and across distinct leakage-lifetime regimes. RED identifies the error channels limiting readout performance without requiring dedicated discrimination of non-computational states, providing a basis for targeted readout optimization across different qubit modalities.
arxiv pdf

Raising the Cavity Frequency in cQED

  1. Raymond A. Mencia,
  2. Taketo Imaizumi,
  3. Igor A. Golovchanskiy,
  4. Andrea Lizzit,
  5. and Vladimir E. Manucharyan
The basic element of circuit quantum electrodynamics (cQED) is a cavity resonator strongly coupled to a superconducting qubit. Since the inception of the field, the choice of the cavity
frequency was, with a few exceptions, been limited to a narrow range around 7 GHz due to a variety of fundamental and practical considerations. Here we report the first cQED implementation, where the qubit remains a regular transmon at about 5 GHz frequency, but the cavity’s fundamental mode raises to 21 GHz. We demonstrate that (i) the dispersive shift remains in the conventional MHz range despite the large qubit-cavity detuning, (ii) the quantum efficiency of the qubit readout reaches 8%, (iii) the qubit’s energy relaxation quality factor exceeds 107, (iv) the qubit coherence time reproducibly exceeds 100 μs and can reach above 300 μs with a single echoing π-pulse correction. The readout error is currently limited by an accidental resonant excitation of a non-computational state, the elimination of which requires minor adjustments to the device parameters. Nevertheless, we were able to initialize the qubit in a repeated measurement by post-selection with 2×10−3 error and achieve 4×10−3 state assignment error. These results encourage in-depth explorations of potentially transformative advantages of high-frequency cavities without compromising existing qubit functionality.
arxiv pdf