Freiburg 2024 – scientific programme

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Q: Fachverband Quantenoptik und Photonik

Q 53: Quantum Control

Q 53.1: Talk

Thursday, March 14, 2024, 14:30–14:45, HS 3118

Quantum Control on a Quantum Computer: Theory and experiment — •Nikolay Vitanov — Sofia University, Bulgaria

Recent results from quantum control experiments on some quantum processors offered by IBM Quantum will be reported. The experiments have been performed using the back-end Qiskit Pulse package, which offers full control over the experimental parameters: pulse amplitude, frequency, phase and shape. The results include the demonstration of composite pulses - trains of pulses with well-defined relative phases used as control parameters - for complete (X gates) and partial (Hadamard and general rotation gates) population inversion, which cancel the experimental errors to an arbitrary order. Another example is the newly proposed quantum control technique of polychromatic pulse trains - sequences of pulses of different appropriately chosen frequencies used, instead of the phases, as control parameters.

Conventional wisdom suggests that the excitation line profile should broaden when the Rabi frequency increases - this is the textbook effect of power broadening. Earlier work demonstrated that power broadening may not occur in pulsed excitation and revealed the near absence of power broadening in excitation by Gaussian pulses. Quite remarkably, we have observed the counterintuitive phenomenon of power narrowing with driving pulses of Lorentzian shape - the squeezing of the excitation line profile when the Rabi frequency increases. While this stunning effect had been predicted earlier it has never been observed in an experiment.

Keywords: composite pulses; pulse shape effects; power narrowing