QubitSpectroscopyAnalysis (QAE-1158)

Explanation of experiment and analysis

The goal of the "qubit spectroscopy" a.k.a "two-tone" experiment is to verify that the qubit circuit is working, i.e. that we can excite a qubit in the ground state to some non-trivial superposition of |0⟩ and |1⟩. The etymology of the name “two-tone” stems from that we are sending two pulses (historically tones). First, we send a stimulus pulse at some variable frequency, and then we send a readout pulse at the characteristic frequency of the resonator (which we know from resonator spectroscopy).

We can determine that the qubit circuit is working if, when the modulation frequency of the stimulus pulse is swept, we see a large amplitude in the measurement signal at some point, because this means that the readout resonator has shifted due to the qubit's excitation.

In the analysis we fit a Lorentz distribution to the data. If the median of this distribution corresponds to our rough estimate (known from fabrication or previous experiments) then we identify this as the |0⟩ → |1⟩ frequency of the qubit.

Explanation of changes

• Added fitting function defined for Lorentzian P.D.F. in quantify_core/analysis/fitting_models.py.
• Added a corresponding lmfit.model.Model class for the Lorentzian defined in quantify_core/analysis/fitting_models.py
• Added QubitSpectroscopyAnalysis analysis class defined in quantify_core/analysis/spectroscopy_analysis.py

Motivation of changes

This analysis class is useful when in characterizing e.g. transmon qubits, but did not exist yet in the codebase.

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Merge checklist

See also merge request guidelines

• Merge request has been reviewed (in-depth by a knowledgeable contributor), and is approved by a project maintainer.
• New code is covered by unit tests (or N/A).
• New code is documented and docstrings use numpydoc format (or N/A).
• New functionality: considered making private instead of extending public API (or N/A).
• Public API changed: added @deprecated (or N/A).
• Tested on hardware (or N/A).
• CHANGELOG.md and AUTHORS.md have been updated (or N/A).
• Windows tests in CI pipeline pass (manually triggered by maintainers before merging).
  • Maintainers do not hit Auto-merge, we need to actively check as manual tests do not block pipeline

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