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Journal of Contemporary Physics (Armenian Academy of Sciences)

Reversible Generation of Coherence between Metastable Quantum States Using Laser Pulses with Quadratic Frequency Chirp

Gagik P. Djotyan, A. A. Avetisyan, A. P. Djotyan

Published Mar 1, 2026• 0 citations
Publisher

Abstract

Abstract In this paper, we show that the scheme we previously developed for creating a controlled coherent superposition of metastable states in a quantum system, under certain conditions, allows the reversible generation of arbitrary coherence between these states. This scheme utilizes laser pulses with a quadratic frequency chirp (QFC). While the first QFC laser pulse generates the required superposition of metastable states, a subsequent QFC laser pulse with the same parameters as the previous one, but with the opposite direction of frequency chirp, returns the quantum system to its original state, allowing another required coherence value to be formed by applying the next QFC pulse with the appropriate parameters. The case of a single “broadband” QFC laser pulse is considered, when the frequency spectrum of the pulse (without chirp) exceeds the frequency distance between the metastable energy levels of the system.

DOI
10.1134/s1068337226700337

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Abstract In this paper, we show that the scheme we previously developed for creating a controlled coherent superposition of metastable states in a quantum system, under certain conditions, allows the reversible generation of arbitrary coherence between these states. This scheme utilizes laser pulses with a quadratic frequency chirp (QFC). While the first QFC laser pulse generates the required superposition of metastable states, a subsequent QFC laser pulse with the same parameters as the previous one, but with the opposite direction of frequency chirp, returns the quantum system to its original state, allowing another required coherence value to be formed by applying the next QFC pulse with the appropriate parameters. The case of a single “broadband” QFC laser pulse is considered, when the frequency spectrum of the pulse (without chirp) exceeds the frequency distance between the metastable energy levels of the system.
We present a novel scheme for coherent manipulation of populations and robust creation of arbitrary coherent superposition of metastable states of a quantum system with lambda configuration of operating energy levels using laser pulses with a quadratically chirped carrier frequency. The case of a single broadband laser pulse is considered, when the frequency spectrum of the pulse exceeds the frequency distance between the metastable energy levels of the system. The results of the dressed state analysis and numerical simulation are presented, demonstrating reliable and robust creation of an arbitrary coherent superposition of metastable states by changing the parameters of laser pulses.
We present a scheme of population transfer between two metastable (ground) states of the $\ensuremath{\Lambda}$ atom without considerable excitation of the atom using single frequency-chirped laser pulses. The physics of the process is generation of the ``trapped'' superposition of the ground states by the laser pulse at sufficiently high laser peak intensity. The main conditions for realization of this regime are the following: The width of the transform-limited laser pulse envelope frequency spectrum (without chirp) must be smaller and the peak Rabi frequency of the pulse must be larger than the frequency interval between the two ground states of the $\ensuremath{\Lambda}$ atom. During the frequency chirp, the laser pulse must first come into resonance with the transition from the initially occupied ground state to the excited state and after that with the transition between the excited and second initially empty ground states. In the case when the envelope frequency spectrum width (without chirp) of the pulse exceeds the frequency interval between the two ground states, we show a possibility of controllable generation of superposition of the ground states with a controllable excitation of the $\ensuremath{\Lambda}$ atom.
Traditional schemes for coherent population transfer or generation of coherent superposition states in multilevel atoms or molecules usually utilize two or more laser beams with radiation bandwidth smaller than the frequency interval between the working levels. We show the possibility of creation of the coherent superposition of three metastable states of a four-level atom with tripodlike level structure using a single short frequency-chirped laser pulse. The bandwidth of the pulse envelope (without chirp) must be comparable to or exceed the frequency distance between the two metastable levels. No appreciable excitation of the atom takes place during the creation of the coherent superposition state, thus diminishing significantly the effect of decoherence due to the spontaneous decay of the excited state. The proposed method of creation of superposition states is robust against variations in the laser pulse parameters. Since this method does not require maintaining steady resonance with the atomic transitions (owing to the frequency chirp of the laser pulse), it is effective both in homogeneously and inhomogeneously broadened media.
We propose a scheme for coherent control of population transfer and arbitrary superpositions of quantum states by a single-chirped laser pulse in a Λ-type excited-doublet four-level system. It is shown that selective, efficient, and robust population transfer from the initial state to the target states (excited or ground) can be achieved via chirped adiabatic passage when the pulse bandwidth is smaller than about 1/10 of the energy separations between the excited-doublet levels and between the ground states. Moreover, arbitrary coherent superpositions between the excited-doublet states, or between the higher ground state and either of the excited-doublet states, can be created by appropriately tuning the chirp rate and detuning the laser field. The proposed flexible method of coherent control of population transfer and arbitrary superpositions between the initially unpopulated states requires only a single pulse, and is robust against variations in the laser parameters, which has potential applications in quantum information processing.
This paper shows that a single laser pulse with a chirped frequency can generate trapped superposition of the ground states in the /spl Lambda/-atom when the width of the transform-limited laser pulse envelope frequency spectrum (without chirp) is smaller but the peak Rabi frequency of the pulse is larger than frequency interval between the two ground states of the /spl Lambda/-atom. This study also analyzes the propagation of the frequency-chirped laser pulse in a medium consisting of /spl Lambda/-atoms analytically and by exact numerical solution of coupled Bloch-Maxwell equations. The results show that there is nearly absorptionless propagation of the single frequency-chirped laser pulse on distances more than 40 times the pulse duration, when the formulated above conditions of negligible excitation of the /spl Lambda/-atom are fulfilled. The described effect of absorptionless propagation of the single frequency-chirped pulse is referred to as electromagnetically self-induced transparency.
We propose and analyze a scheme for creation of coherent superposition of metastable states in a tripod-structured atom using frequency-chirped laser pulses with negligible excitation of the atoms. The underlying physics of the scheme is explained using the formalism of adiabatic states. We show that the proposed scheme may be equally efficient in homogeneously and Doppler-broadened media. By numerically solving the master equation for the density matrix operator, we analyze the influence of relaxation processes on the efficiency of the creation of superposition states. We show that the proposed scheme is robust against small-to-medium variations of the laser field’s parameters.