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Laser Physics

Sub-Doppler laser cooling via spontaneous generated coherence

Vase Moeini, Seyedeh Hamideh Kazemi, Mohammad Mahmoudi

Published Dec 20, 2019• 3 citations
Publisher

Abstract

Abstract We propose a system of four-level atoms which can generate a sub-Doppler cooling in the presence of quantum interference effects. The atom is modeled as a double -type system where dipole-allowed transitions are correlated through the spontaneously generated coherence (SGC). We will show that the including of such interference effects created by spontaneous emission gives rise to a cooling mechanism in which the lowest achievable temperatures approach the single-photon recoil limit. Indeed, this mechanism benefits from the features generated by the phenomenon of SGC-induced electromagnetically induced transparency; in the regime of weak coupling fields, atoms can be subject to a strong radiation pressure force, allowing them to be cooled to near, and even below, the recoil temperature.

DOI
10.1088/1555-6611/ab5f12

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Abstract We propose a system of four-level atoms which can generate a sub-Doppler cooling in the presence of quantum interference effects. The atom is modeled as a double -type system where dipole-allowed transitions are correlated through the spontaneously generated coherence (SGC). We will show that the including of such interference effects created by spontaneous emission gives rise to a cooling mechanism in which the lowest achievable temperatures approach the single-photon recoil limit. Indeed, this mechanism benefits from the features generated by the phenomenon of SGC-induced electromagnetically induced transparency; in the regime of weak coupling fields, atoms can be subject to a strong radiation pressure force, allowing them to be cooled to near, and even below, the recoil temperature.
We propose a sub-Doppler laser-cooling mechanism that takes advantage of the unique spectral features and extreme dispersion generated by the phenomenon of electromagnetically induced transparency (EIT). EIT is a destructive quantum interference phenomenon experienced by atoms with multiple internal quantum states when illuminated by laser fields with appropriate frequencies. By detuning the lasers slightly from the ``dark resonance,'' we observe that, within the transparency window, atoms can be subject to a strong viscous force, while being only slightly heated by the diffusion caused by spontaneous photon scattering. In contrast to other laser-cooling schemes, such as polarization gradient cooling or EIT-sideband cooling, no external magnetic field or strong external confining potential is required. Using a semiclassical approximation, we derive analytically quantitative expressions for the steady-state temperature, which is confirmed by full quantum mechanical numerical simulations. We find that the lowest achievable temperatures approach the single-photon recoil energy. In addition to dissipative forces, the atoms are subject to a stationary conservative potential, leading to the possibility of spatial confinement. We find that under typical experimental parameters, this effect is weak and stable trapping is not possible.
We propose a laser cooling mechanism based on velocity-selective double-dark resonance that leads to a temperature significantly lower than the one-photon recoil limit. This mechanism benefits from sharp and high-contrast spectra which are induced by the two electromagnetically induced transparency windows due to the interacting dark-state resonances. It is theoretically demonstrated that four-level atoms illuminated by two counter-propagating probe beams and two additional beams directed perpendicularly to the other two exhibit new cooling effects; for red detuned probe lasers, atoms can be subject to a strong viscous force with an extremely small diffusion, characteristic of heating caused by the stochastic nature of spontaneous emission processes. By quantum mechanical simulations, we then find that the lowest temperature in EIT windows approaches 1 nK for the case of mercury. However, the different broadening mechanisms can destroy the cooling, so that the lowest temperature can increase to the recoil temperature.
We discuss various aspects of the incoherent spontaneous emission in atomic few-level systems arising from the coupling of the atom to the surrounding vacuum. First, we consider systems where the decoherence due to spontaneous emission acts as a limiting factor. Here, we combine collective effects in larger samples of atoms with control mechanisms known from single-atom schemes, or modify the system dynamics by externally inducing multiphoton quantum interference effects. In the second part, we discuss ground-state laser cooling of trapped atoms and ions. Here, the momentum transfer in the spontaneous emission events is required to cool the particles, but needs to be controlled in order to achieve a low cooling limit. In our scheme, we make use of double electromagnetically induced transparency in order to design the absorption spectrum of the trapped particle. In the final part, we show that the incoherent part of the resonance fluorescence spectrum of a two-level system may serve as an interesting candidate for high-precision spectroscopy. For this, we discuss relativistic and radiative corrections to the resonance fluorescence spectra of laser-driven few-level systems.
The coherence of a four-level Λ-type system with spontaneously generated coherence(SGC) is studied.With a weak probe and a strong coupling fields,whether the atomic system is on two-photon resonance or not,there is light without population inversion(LWI) on the probe field.On two-photon resonance,LWI is strengthened by SGC.On non-two-photon resonance,LWI can be controlled simply by adjusting the parameter of SGC and the two-upper-level separation.Meanwhile,the analytical solutions to the SGC parameter where LWI occurs are given in both cases.
In this work, the effects of quantum interference and spontaneously generated coherence (SGC) are theoretically analyzed in a four-level system of a 87Rb^{87}\mathrm{Rb}87Rb atom. For the effects of SGC, we find that a new kind of electromagnetically induced transparency channel can be induced due to destructive interference, and the nonlinear Kerr absorption can be coherently narrowed or eliminated under different strengths of the coupling and switching fields.