We have studied the formation of ultracold RbSr molecules with laser pulses. After discussing the advantages of the Mott insulator phase for the control with pulses, we present two classes of strategies. The first class involves two electronic states. Two extensions of stimulated Raman adiabatic passage (STIRAP) for multilevel transitions are used: alternating STIRAP (A-STIRAP) and straddle STIRAP (S-STIRAP). Both transfer dynamics are modeled and compared. The second class of strategies involves only the electronic ground state and uses infrared (IR) and terahertz (THz) pulses. The molecular bond is first created by the application of a THz chirped pulse or $\ensuremath{\pi}$-pulse. Subsequently, the molecules are transferred to their rovibrational ground level using IR pulses. For this last step, different optimized pulse sequences that were obtained through optimal control techniques have been studied. The relative merits of these strategies in terms of efficiency and robustness are discussed with respect to the experimental feasibility based on present laser technologies.
We have studied the formation of ultracold RbSr molecules with laser pulses. After discussing the advantages of the Mott insulator phase for the control with pulses, we present two classes of strategies. The first class involves two electronic states. Two extensions of stimulated Raman adiabatic passage (STIRAP) for multilevel transitions are used: alternating STIRAP (A-STIRAP) and straddle STIRAP (S-STIRAP). Both transfer dynamics are modeled and compared. The second class of strategies involves only the electronic ground state and uses infrared (IR) and terahertz (THz) pulses. The molecular bond is first created by the application of a THz chirped pulse or \ensuremathπ-pulse. Subsequently, the molecules are transferred to their rovibrational ground level using IR pulses. For this last step, different optimized pulse sequences that were obtained through optimal control techniques have been studied. The relative merits of these strategies in terms of efficiency and robustness are discussed with respect to the experimental feasibility based on present laser technologies.
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.
Feshbach resonances play a major role in translationally cold-molecule preparation. In this context, their laser control is of crucial importance. This work is devoted to the depiction of some basic mechanisms of such a control using intense, short laser pulses and referring to nonlinear multiphoton processes. Our goal is to adiabatically transport a Feshbach resonance onto a zero-width resonance, the characteristics of which have already been discussed in the literature. Three processes are then addressed: (i) during the rise of the pulse and its plateau, the preparation of a so-called laser bound molecule (LBM) still stable, but structurally different from the standard chemically bound molecule; (ii) during the pulse switching off, an adiabatic transport of this LBM on a very few excited vibrational levels, and (iii) concomitantly, a filtration strategy to photodissociate all these levels except one, giving thus rise to but a single field-free excited vibrational state. With or without an eventual stimulated Raman adiabatic passage technique to bring all the population to the ground rovibrational state, this opens an alternate for a full optical control of ultracold-molecule formation. The illustrative example, offering the potentiality to be transposed to other diatomics, is H2+.
Abstract We investigate theoretically the formation of an ultracold CsYb molecule in the absolute ground state by full optical control. The laser-assisted self-induced Feshbach resonance takes place when the trap state in the optical lattice is coupled with a rovibrational state of the ground electronic state. The Feshbach molecule is formed in the resonant rovibrational state via an adiabatic population transfer by ramping the frequency of a chirped pulse. Two schemes are designed to prepare the absolute ground-state molecule starting from the Feshbach molecule: a pump–dump scheme controlled by short pulses and a stimulated-Raman-adiabatic-passage (STIRAP) scheme steered by long pulses. The probabilities of converting the Feshbach molecule to the absolute ground state molecule by using the pump–dump and the STIRAP schemes are 16% and 99%, respectively.
We develop a theoretical description of photoassociative stimulated Raman adiabatic passage (STIRAP) near a Feshbach resonance in a thermal atomic gas. We show that it is possible to use low-intensity laser pulses to directly excite pairs of atoms in the continuum near a Feshbach resonance and to transfer most of the atomic cloud to the lowest rovibrational level of the molecular ground state. For a broad resonance, commonly found in several mixtures of alkali atoms, our model predicts a transfer efficiency up to 97% for a given atom pair, and up to 70% when averaged over an atomic ensemble. The laser intensities and pulse durations needed for such an optimal transfer, 10 2 –10 3 W cm −2 and several microseconds, are easily achievable experimentally. A single pair of STIRAP pulses converts an estimated fraction f ∼10 −6 –10 −4 of atoms in an atomic ensemble, leading to the production of 10–1000 molecules in a large sample of 10 7 atoms. A total of ∼10 4 –10 6 pulse pairs are thus required to transfer most atoms into molecules. Such an efficiency compares with or surpasses currently available techniques for creating stable diatomic molecules, and the versatility of this approach simplifies its potential use for many molecular species.
Terahertz radiation has unique potential for applications to material diagnostics including semiconductors, chemical compounds, biomolecules and biotissues, imaging for medical and security applications, remote atmospheric sensing and monitoring, astronomy, etc. The search for efficient, high-power, inexpensive, portable and room temperature suitable methods of generation of THz radiation is one of the main topics in modern optoelectronics and photonics. Specifically, coherent solid-state compact efficient THz sources are desirable. Recently we suggested a new approach to the problem of generation of short coherent THz pulses in molecular gases taking advantage of the dramatic enhancement of a nonlinear response of a medium via quantum coherence. Stimulated Raman adiabatic passage (STIRAP) technique was proposed for preparation of maximal molecular coherence by two optical pump fields, which is further used to efficiently scatter off an IR probe beam and generate a Stokes THz field in a double Λ scheme. The central idea of the crystal-based technique is to generate THz radiation by resonantly inducing coherence at the corresponding transition by coherent optical fields. Electronic levels of the THz transition are coupled by a pair of optical fields to a common ground state forming a V scheme of energy levels. Coherence induced by the optical fields at the THz transition gives rise to polarization provided that the corresponding dipole moment is not zero, which will radiate out a THz pulse. We show that coherently driven molecular (such as methanol, H2F2, CH3Cl, etc.) and solid-state (ruby, alexandrite) media can potentially produce strong, controllable, short pulses of THz radiation. The pulses have energies ranging from hundreds pJ to J and time durations from several fs to ns at room temperature.
Ultracold paramagnetic and polar diatomic molecules are among the promising systems for quantum simulation of lattice-spin models. Unfortunately, their experimental observation is still challenging. Based on our recent ab initio calculations, we analyze the feasibility of all-optical schemes for the formation of ultracold 87Rb84Sr bosonic molecules. A first possibility is photoassociation followed by spontaneous emission. The photoassociation rate coefficients toward electronic states converging to the 87Rb(5s2S1/2)+84Sr(5s5p3P0,1,2) asymptotes are particularly small for vibrational levels close to the asymptote. The creation of molecules would be more interesting by using deeply bound levels which preferentially relax to the ${v}^{\ensuremath{'}\ensuremath{'}}=0$ level of the ground state. On the other hand, the photoassociation rate coefficients toward electronic states correlated to the Rb(5p2P1/2,3/2)+Sr(5s21S0) are significant for levels close to the asymptote. The spontaneous emission thus creates weakly bound molecules in a single vibrational level. A second option relies on stimulated Raman adiabatic passage implemented in a tight optical trap. It efficiently creates weakly bound ground-state molecules in a well-defined level, thus providing a promising alternative to magnetic Feshbach resonances for further population transfer toward the absolute ground state of the RbSr molecule.