Observation of Power Superbroadening of Spectral Line Profiles on IBM Quantum

TL;DR

This paper reports the experimental observation of power narrowing in spectral lines using Lorentzian-shaped pulses on IBM Quantum, contrasting with traditional power broadening, and introduces shaped pulses that significantly enhance power broadening.

Contribution

The study demonstrates power narrowing with Lorentzian pulses and introduces new pulse shapes that greatly increase power broadening in quantum systems.

Findings

Power narrowing observed with Lorentzian pulses, reducing spectral width by up to a factor of 10.

Shaped pulses can produce power broadening more than three times that of rectangular pulses.

Quadratic and even-exponent pulse families significantly enhance power broadening effects.

Abstract

Power broadening refers to the widening of the spectral line profile in a two-state quantum transition as the strength of the driving field increases. This phenomenon commonly arises in continuous-wave driving when the radiation field's intensity exceeds the transition's saturation intensity and it has been extensively studied in spectroscopy. For pulsed-field excitation, the spectral response of the quantum system may differ significantly: while a rectangular-shaped pulse leads to a linear power broadening, pulses with smooth shapes show significantly reduced power broadening, for instance, logarithmic for the Gaussian shape and none for the hyperbolic-secant shape. Recently [Phys. Rev. Lett. 132, 020802 (2024)], in a dramatic paradigm shift, we have demonstrated experimentally that for Lorentzian-shaped pulses, the opposite effect - power narrowing - takes place: the width of the spectral profile decreases when the driving pulse amplitude increases, with a narrowing factor of as much as 10 observed. While in high-resolution spectroscopy the push is for eliminating or even inverting the power broadening, there are applications where it is used to an advantage for it facilitates off-resonance excitation. Here, we present a number of shaped pulses that exhibit power broadening much greater than that of the rectangular pulse of the same pulse area. They are grouped in two families of pulse shapes. In particular, in regard to the width of the second Rabi oscillation maximum, the quadratic pulse family shows an increase by a factor of 3.3 whereas the even-exponent pulse family exhibits an increase by a factor of more than 3.5.