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arXiv:1508.06732 · Journal of the Optical Society of America B

Efficiency limitation for realizing an atom–molecule adiabatic transfer based on a chainwise system

Jingjing Zhai, Lu Zhang, Keye Zhang, Jing Qian, Weiping Zhang

Published Sep 23, 2015• 4 citations
arXivHTMLPDF

Abstract

In a recent work, we have developed a robust chainwise atom–molecule adiabatic passage scheme to produce ultracold ground–state molecules via photo-associating free atoms [Phys. Rev. A81, 013632 (2010)10.1103/PhysRevA.81.013632PLRAAN1050-2947]. With the help of intermediate auxiliary levels, the pump laser intensity requested in the atomic photo-association process can be greatly reduced. In the present work, we extend the scheme to a more generalized (2n+1)-level system and investigate its efficiency limitations. As the intermediate levels and auxiliary lasers increase, the atom–molecule adiabatic passage would be gradually closed, leading to a poor transfer efficiency. For the purpose of enhancing the efficiency, we present various optimization approaches to the laser parameters, involving order number n, relative strength ratio, and absolute strength. We show that there can remain a limit on the population transfer efficiency given by a three-level Λ system. In addition, we illustrate the importance of selecting an appropriate number of intermediate levels for maintaining a highly efficient transfer under mild experimental conditions.

DOI
10.1364/josab.32.002164

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In a recent work, we have developed a robust chainwise atom–molecule adiabatic passage scheme to produce ultracold ground–state molecules via photo-associating free atoms [Phys. Rev. A81, 013632 (2010)10.1103/PhysRevA.81.013632PLRAAN1050-2947]. With the help of intermediate auxiliary levels, the pump laser intensity requested in the atomic photo-association process can be greatly reduced. In the present work, we extend the scheme to a more generalized (2n+1)-level system and investigate its efficiency limitations. As the intermediate levels and auxiliary lasers increase, the atom–molecule adiabatic passage would be gradually closed, leading to a poor transfer efficiency. For the purpose of enhancing the efficiency, we present various optimization approaches to the laser parameters, involving order number n, relative strength ratio, and absolute strength. We show that there can remain a limit on the population transfer efficiency given by a three-level Λ system. In addition, we illustrate the importance of selecting an appropriate number of intermediate levels for maintaining a highly efficient transfer under mild experimental conditions.
We generalize the idea of chainwise stimulated Raman adiabatic passage (STIRAP) [Kuznetsova et al., Phys. Rev. A 78, 021402(R) (2008)] to a photoassociation-based chainwise atom-molecule system, with the goal of directly converting two-species atomic Bose-Einstein condensates (BEC) into a ground polar molecular BEC. We pay particular attention to the intermediate Raman laser fields, a control knob inaccessible to the usual three-level model. We find that an appropriate exploration of both the intermediate laser fields and the stability property of the atom-molecule STIRAP can greatly reduce the power demand on the photoassociation laser, a key concern for STIRAPs starting from free atoms due to the small Franck-Condon factor in the free-bound transition.
Phys. Rev. A 78, 021402(R) (2008)] to a photoassociation-based chainwise atom-moleculesystem, with the goal of directly convertingtwo-species atomic Bose-Einstein condensates(BEC) intoa ground polar molecular BEC. We pay particular attention to the intermediate Raman laser fields,a control knob inaccessible to the usual three-level model. We find that an appropriate explorationof both the intermediate laser fields and the stability property of the atom-molecule STIRAP cangreatly reduce the power demand on the photoassociation laser, a key concern for STIRAPs startingfrom free atoms due to the small Franck-Condon factor in the free-bound transition.
We suggest and analyze a technique for efficient and robust creation of dense ultracold molecular ensembles in their ground rovibrational state. In our approach a molecule is brought to the ground state through a series of intermediate vibrational states via a multistate chainwise stimulated Raman adiabatic passage technique. We study the influence of the intermediate states decay on the transfer process and suggest an approach that minimizes the population of these states, resulting in a maximal transfer efficiency. As an example, we analyze the formation of 87Rb2^{87}\mathrm{Rb}_{2}87Rb2​ starting from an initial Feshbach molecular state and taking into account major decay mechanisms due to inelastic atom-molecule and molecule-molecule collisions. Numerical analysis suggests a transfer efficiency >90>90%>90, even in the presence of strong collisional relaxation as are present in a high density atomic gas.
A quantum gas of ultracold molecules, with long-range and anisotropic interactions, will enable a series of fundamental studies in physics and chemistry. In particular, samples of ground-state molecules at ultralow temperatures and high number densities will facilitate the explorations of a large number of many-body physical phenomena and applications in quantum information processing. However, due to the lack of efficiently cooling techniques such as laser cooling for atomic gases, high number densities for ultracold molecular samples are not readily attainable. Associating ultracold atoms to weakly bound dimer molecules via Feshbach resonance and subsequently transferring them to a wanted molecular ro-vibronic ground state by a stimulated Raman adiabatic passages (STIRAP) have proved to be an effective way in producing ideal ultracold molecular samples. As a typical illustration, in a recent study (2010 Nat. Phys. 6 265) Danzl et al. experimentally realized the preparation of Cs2 molecule into its ro-vibronic ground state via two different multi-level STIRAPs:one is based on a single conversion route and the others are based on a cascade-connected route (labeled by 4p-STIRAP and s-STIRAP, respectively). In this work, we present a theoretical study for these two STIRAP schemes, focusing on the differences in physical principle and realistic performance between them. On the one hand, according to the theoretical approach of quasi-dark eigenstates, we conclude that a highly efficient population transfer is achievable in both schemes. On the other hand, by systematically studying the influences of the relevant parameters, including the spontaneous decays and the detunings from the intermediate states, and the temporal sequence and the amplitude of the laser pulses, we disclose their respective advantages and weaknesses in the realistic implementation. We theoretically predict that for both schemes their maximal conversion efficiencies each can attain 0.97 as long as the spontaneous decays from the intermediate excited states are sufficiently suppressed. Yet considering the fact that the already implemented efficiency is only around 0.6 for both schemes, there is still room for optimization, e.g. using stable Rydberg energy levels in future experiment. Furthermore, the success of these two schemes can provide a new route to the controllable entanglement preparation, opening more applications in the fields of quantum logic gate and so on.
We investigate the relationship between stability, adiabaticity and transfer efficiency in a \Lambda-type atom-molecule coupling system via a nonlinear stimulated Raman adiabatic passage. We find that only when the pump and control lasers overlap in time domain, the coherent population trapping (CPT) state could become unstable. If the overlapping time of the two lasers is short so that unstable growth of the deviation from the CPT state is negligible, then good adiabaticity of the CPT state could be maintained even in the unstable region. In this case, a high atom-molecule transfer efficiency could be obtained by chirping applied laser pulses to elegantly compensate the frequency shift induced by intra-atomic collision. Our results could be useful for efficiently photoassociating ground-state molecules from a cold atomic gas with strong atom-atom collisional interaction.
We investigate the relationship between stability, adiabaticity and transfer efficiency in a Λ-type atom-molecule coupling system via a nonlinear stimulated Raman adiabatic passage. We find that only when the pump and control lasers overlap in time domain, the coherent population trapping (CPT) state could become unstable. If the overlapping time of the two lasers is short so that unstable growth of the deviation from the CPT state is negligible, then good adiabaticity of the CPT state could be maintained even in the unstable region. In this case, a high atom-molecule transfer efficiency could be obtained by chirping applied laser pulses to elegantly compensate the frequency shift induced by intra-atomic collision. Our results could be useful for efficiently photoassociating ground-state molecules from a cold atomic gas with strong atom-atom collisional interaction.