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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Optical Shielding of Ultracold 39K-Cs Collision

Ting Xie, Andrea Orbán, Olivier Dulieu, Nadia Bouloufa-Maafa

Published Jun 1, 2019• 0 citations
Publisher

Abstract

Research focusing on the formation of ultracold atomic and molecular quantum gases is a continuously expanding field due to its envisioned applications such as quantum-controlled chemistry or quantum simulation. The aim of our theoretical work is to find ways to suppress inelastic or reactive processes between colliding particles in ultracold quantum gases. Using a laser field detuned to the blue of a relevant transition, we propose to couple the initial colliding particle state to a repulsive excited one, thus preventing the particles to come close to each other. This "blueshielding" technique is applied for ultracold quantum gases, close to quantum degeneracy. We investigate the optical shielding in ultracold39K-Cs collision by using close-coupling channel method. As the (3)3Σ+and (2)3Π+molecular states are repulsive in long range[1], one verified effective optical shielding can be realized via driving the transition from the ground α3Σ+state to these excited states in the presence of linear-polarized light. We demonstrate that a strong field with a small blue detuning to the atomic transition is to the benefit of shielding since the Condon point is farther away from the inner region of ground triplet state. The spontaneous emission effect is taken into account by considering excited optical potentials with the modified formula of shielding rate [2]. We expect the shielding efficiency could be improved by using circular-polarized light [3].

DOI
10.1109/cleoe-eqec.2019.8873276

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Research focusing on the formation of ultracold atomic and molecular quantum gases is a continuously expanding field due to its envisioned applications such as quantum-controlled chemistry or quantum simulation. The aim of our theoretical work is to find ways to suppress inelastic or reactive processes between colliding particles in ultracold quantum gases. Using a laser field detuned to the blue of a relevant transition, we propose to couple the initial colliding particle state to a repulsive excited one, thus preventing the particles to come close to each other. This "blueshielding" technique is applied for ultracold quantum gases, close to quantum degeneracy. We investigate the optical shielding in ultracold39K-Cs collision by using close-coupling channel method. As the (3)3Σ+and (2)3Π+molecular states are repulsive in long range[1], one verified effective optical shielding can be realized via driving the transition from the ground α3Σ+state to these excited states in the presence of linear-polarized light. We demonstrate that a strong field with a small blue detuning to the atomic transition is to the benefit of shielding since the Condon point is farther away from the inner region of ground triplet state. The spontaneous emission effect is taken into account by considering excited optical potentials with the modified formula of shielding rate [2]. We expect the shielding efficiency could be improved by using circular-polarized light [3].
Abstract Ultracold temperatures in dilute quantum gases opened the way to an exquisite control of matter at the quantum level. Here we focus on the control of ultracold atomic collisions using a laser to engineer their interactions at large interatomic distances. We show that the entrance channel of two colliding ultracold atoms can be coupled to a repulsive collisional channel by the laser light so that the overall interaction between the two atoms becomes repulsive: this prevents them to come close together and to undergo inelastic processes, thus protecting the atomic gases from unwanted losses. We illustrate such an optical shielding (OS) mechanism with 39 K and 133 Cs atoms colliding at ultracold temperature (<1 μK). The process is described in the framework of the dressed-state picture and we then solve the resulting stationary coupled Schrödinger equations. The role of spontaneous emission and photoinduced inelastic scattering is also investigated as possible limitations of the shielding efficiency. We predict an almost complete suppression of inelastic collisions over a broad range of Rabi frequencies and detunings from the 39 K D2 line of the OS laser, both within the [0, 200 MHz] interval. We found that the polarization of the shielding laser has a minor influence on this efficiency. This proposal could easily be formulated for other bialkali-metal pairs as their long-range interaction are all very similar to each other.
We propose a method to suppress the chemical reactions between ultracold bosonic ground-state ^{23}Na^{87}Rb molecules based on optical shielding. By applying a laser with a frequency blue-detuned from the transition between the lowest rovibrational level of the electronic ground state X^{1}Σ^{+}(v_{X}=0,j_{X}=0), and the long-lived excited level b^{3}Π_{0}(v_{b}=0,j_{b}=1), the long-range dipole-dipole interaction between the colliding molecules can be engineered, leading to a dramatic suppression of reactive and photoinduced inelastic collisions, for both linear and circular laser polarizations. We demonstrate that the spontaneous emission from b^{3}Π_{0}(v_{b}=0,j_{b}=1) does not deteriorate the shielding process. This opens the possibility for a strong increase of the lifetime of cold molecule traps and for an efficient evaporative cooling. We also anticipate that the proposed mechanism is valid for alkali-metal diatomics with sufficiently large dipole-dipole interactions.
We theoretically investigate the collisions between ultracold polar molecules in the presence of two lasers ensuring a Raman resonant transition on individual molecules to suppress photon scattering, taking the example of bosonic Na 23 K 39 molecules. By varying laser detunings and intensities, we enable a repulsive long-range interaction potential between molecules. After solving a set of coupled Schrödinger equations with the Hamiltonian written in the basis of laser-dressed states of the molecule pair at infinite distance, we identify quasiresonant conditions under which elastic collisions are favored over inelastic and reactive ones, by a factor of about 2, thus demonstrating a promising pathway for efficient two-photon optical shielding of ultracold molecular collisions. The results are analyzed in terms of scattering length of the colliding laser-dressed molecules, which exhibit prominent resonances assigned to the interaction of the entrance channel with other specific channels, consistent with the existence of a quasibound level of the long-range molecular pair induced by the lasers.
Introduction Photoassociation uses optical fields to produce bound molecules from free atoms. Optical fields can also prevent atoms from closely approaching, thereby shielding them from shortrange inelastic or reactive interactions and suppressing the rates of these processes. Recently several groups have demonstrated shielding and suppression by shining an optical field on a cold atom sample. Figure 6.1(a) shows how a simple semiclassical picture can be used to interpret the shielding effect as the rerouting of a ground-state entrance channel scattering flux to an excited repulsive curve at an internuclear distance localized around a Condon point. An optical field, blue detuned with respect to the asymptotic atomic transition, resonantly couples the ground and excited states. In the cold and ultracold regime particles approach on the ground state with very little kinetic energy. Excitation to the repulsive state effectively halts their approach in the immediate vicinity of the Condon point, and the scattering flux then exits either on the repulsive excited state or on the ground state. Figure 6.1(b) shows how this picture can be represented as a Landau–Zener (LZ) avoided crossing of field-dressed potentials. As the blue-detuned suppressor laser intensity increases, the avoided crossing gap around the Condon point widens, and the semiclassical particle moves through the optical coupling region adiabatically. The flux effectively enters and exits on the ground state, and the collision becomes elastic.
Two recent developments in ultracold atomic collision physics are discussed. The first is the “optical shielding” of ultracold atomic collisions by optical radiation that is tuned to the blue of an atomic resonance. This radiation couples a pair of colliding atoms to repulsive excited states and prevents their close approach. This effect may substantially suppress inelastic collision rates, and is an interesting example of the control of a cold collision process by a weak perturbing field. The second topic is ultracold atom photoassociation spectroscopy. In this work, high resolution free‐bound absorption spectra are obtained which provide detailed information on the long‐range states of the two interacting atoms. The physics of these states has an appealing simplicity, and in addition is of importance for a complete understanding of the effects of atomic interactions in cold atom experiments. The photoassociation spectra also provide a unique probe of cold collisions.