qiskit/releasenotes/notes/0.45/single-pulse-rx-cal-347aadc...

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Two new transpiler passes are added to generate single-pulse RX gate calibrations on the fly.
These single-pulse RX calibrations will reduce the gate time in half, as described in
P.Gokhale et al, Optimized Quantum Compilation for Near-Term Algorithms with OpenPulse
(2020), `arXiv:2004.11205 <https://arxiv.org/abs/2004.11205>`__.
To reduce the amount of RX calibration data that needs to be generated,
:class:`~qiskit.transpiler.passes.optimization.normalize_rx_angle.NormalizeRXAngle`
performs three optimizations: wrapping RX gate rotation angles to [0, pi],
replacing RX(pi/2) and RX(pi) with SX and X gates, and quantizing the rotation angles.
This pass is required to be run before
:class:`~qiskit.transpiler.passes.calibration.rx_builder.RXCalibrationBuilder`,
which generates RX calibrations on the fly.
The optimizations performed by ``NormalizeRXAngle`` reduce the amount of calibration data and
enable us to take advantage of the more accurate, hardware-calibrated
pulses. The calibrations generated by ``RXCalibrationBuilder`` are bootstrapped from
the SX gate calibration, which should be already present in the target.
The amplitude is linearly scaled to achieve the desired arbitrary rotation angle.
Such single-pulse calibrations reduces the RX gate time in half, compared to the
conventional sequence that consists of two SX pulses.
There could be an improvement in fidelity due to this reduction in gate time.