Defying Conventional Wisdom in Spectroscopy: Power Narrowing on IBM Quantum

Area of Science:

• Quantum Spectroscopy
• Quantum Information Science

Background:

• Power broadening is a known spectroscopic effect where spectral line profiles widen with increasing driving field amplitude.
• While typically observed in continuous-wave driving, pulsed excitation shows varying power broadening depending on pulse shape (e.g., logarithmic for Gaussian).
• Theoretical predictions suggested 'power narrowing' for specific pulse shapes vanishing as ~|t|^{-λ}, but experimental verification was lacking.

Purpose of the Study:

• To experimentally demonstrate and investigate the phenomenon of power narrowing.
• To explore the impact of pulse shape on spectral line broadening in quantum transitions.
• To analyze the role of pulse wing truncation in achieving power narrowing.

Main Methods:

• Utilized powers-of-Lorentzian pulse shapes for excitation in a two-state quantum system.
• Performed experiments on the IBM Quantum processor ibmq_manila.
• Systematically varied the pulse area and studied the resulting spectral line width.

Main Results:

• Successfully demonstrated power narrowing, observing a reduction in spectral line width by over a factor of 10.
• Observed a reversal of the conventional power broadening effect with increasing pulse area (π to 7π).
• Quantified the effect of pulse wing truncation, identifying a power-broadened term that limits extreme narrowing.

Conclusions:

• Experimental evidence confirms power narrowing for specific pulse shapes, challenging the universal nature of power broadening.
• Truncated Lorentzian pulses offer a pathway to achieve arbitrarily narrow spectral line profiles, limited only by experimental imperfections.
• This finding has implications for high-fidelity quantum control and spectroscopy in quantum systems.