arXiv subjects

Ivo S. Mihov

Publications and source records attributed to Ivo S. Mihov.

5 recordsLinked to original sources

arXiv2026

DRAG-Compatible Leakage Suppression in Landau--Zener Control via Isoprobability Twins

Analytically solvable models -- particularly the Landau-Majorana-Stückelberg-Zener (LMSZ) and Allen-Eberly-Hioe (AEH) models -- underpin many quantum-gate implementations and population-transfer protocols. However, their canonical pulse shapes are incompatible with modern leakage-suppression techniques and some systems. Most notably, the constant Rabi envelope of the LMSZ pulse prevents many leakage-suppression approaches, which require smoothness. We address both limitations by developing the concept of isoprobability twin models: distinct pairs of Rabi frequency $Ω(t)$ and detuning $Δ(t)$ that yield identical post-pulse transition probabilities based on the Delos-Thorson transformation. In this work, we formalise the method by experimentally demonstrating the equivalence of multiple LMSZ and AEH twin models on IBM's ibm_kyiv processor. Finally, we show a staggering leakage reduction by more than 3 orders of magnitude using a custom DRAG implementation of a cosine LMSZ isoprobability model.

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arXiv2025

Observation of Power Superbroadening of Spectral Line Profiles on IBM Quantum

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.

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arXiv2024

Qubit dynamics driven by smooth pulses of finite duration

We present a study of the dynamics of a qubit driven by a pulsed field of finite duration. The pulse shape starts and ends linearly in time. The most typical example of such a shape is the sine function between two of its nodes, but several other pulse shapes are also studied. All of them present smooth alternatives to the commonly used rectangular pulse shape, resulting in much weaker power broadening, much faster vanishing wings in the excitation line profile and hence much reduced sidebands. In the same time, such shapes with a well-defined finite duration do not suffer from the spurious effects arising when truncating a pulse of infinite duration, e.g. Gaussian. We derive two approximate analytic solutions which describe the ensuing quantum dynamics. Both approximations assume that the field changes linearly at the beginning and the end of the driving pulse, and adiabatically in between. The first approximation matches the linear and adiabatic parts at an appropriate instant of time and is expressed in terms of Weber's parabolic cylinder functions. The second, much simpler, approximation uses the asymptotics of the Weber function in order to replace it by simpler functions, and some additional transformations. Both approximations prove highly accurate when compared to experimental data obtained with two of the IBM Quantum processors. Both the greatly reduced power broadening and the greatly suppressed sidebands are observed for all pulse shapes, in a nearly complete agreement between theory and experiment.

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arXiv2024

Defying Conventional Wisdom in Spectroscopy: Power Narrowing on IBM Quantum

Power broadening $-$ the broadening of the spectral line profile of a two-state quantum transition as the amplitude of the driving field increases $-$ is a well-known and thoroughly examined phenomenon in spectroscopy. It typically occurs in continuous-wave driving when the intensity of the radiation field increases beyond the saturation intensity of the transition. In pulsed-field excitation, linear power broadening occurs for a pulse of rectangular temporal shape. Pulses with smooth shapes are known to exhibit much less power broadening, e.g. logarithmic for a Gaussian pulse shape. It has been predicted, but never experimentally verified, that pulse shapes which vanish in time as $\sim |t|^{-λ}$ should exhibit the opposite effect $-$ power narrowing $-$ in which the post-pulse transition line width decreases as the amplitude of the driving pulse increases. In this work, power narrowing is demonstrated experimentally for a class of powers-of-Lorentzian pulse shapes on the IBM Quantum processor ibmq_manila. Reduction of the line width by a factor of over 10 is observed when increasing the pulse area from $π$ to $7π$, in a complete reversal of the power broadening paradigm. Moreover, thorough theoretical and experimental study is conducted on the truncation of the pulse wings which introduces a (small) power-broadened term which prevents power narrowing from reaching extreme values $-$ a hitherto unknown cut-off broadening effect for which an explicit analytical formula is derived. In the absence of other power broadening mechanisms, Lorentzian pulses truncated at sufficiently small values can achieve as narrow line profiles as desired.

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arXiv2023

Pulse shape effects in qubit dynamics demonstrated on an IBM quantum computer

We present a study of the coherent interaction of a qubit with a pulse-shaped external field of a constant carrier frequency. We explore, theoretically and experimentally, the transition line profile -- the dependence of the transition probability on the detuning -- for five different pulse shapes: rectangular, Gaussian, hyperbolic-secant, squared hyperbolic-secant and exponential. The theoretical description for all cases but sech$^2$ is based on the analytical solutions to the Schrödinger equation or accurate approximations available in the literature. For the sech$^2$ pulse we derive an analytical expression for the transition probability using the Rosen-Zener conjecture, which proves very accurate. The same conjecture turns out to provide a very accurate approximation for the Gaussian and exponential pulses too. The experimental results are obtained with one of IBMQ's quantum processors. An excellent agreement between theory and experiment is observed, demonstrating some pulse-shape-dependent fine features of the transition probability profile. The mean absolute error -- a measure of the accuracy of the fit -- features an improvement by a factor of 4 to 8 for the analytic models compared to the commonly used Lorentzian fits. Moreover, the uncertainty of the qubit's resonance frequency is reduced by a factor of 4 for the analytic models compared to the Lorentzian fits. These results demonstrate both the accuracy of the analytic modelling of quantum dynamics and the excellent coherent properties of IBMQ's qubit.

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