Nonadiabatic Molecular Association in Thermal Gases Driven by Radio-Frequency Pulses

Abstract

The molecular association process in a thermal gas of 85 Rb is investigated where the effects of the envelope of the radio-frequency field are taken into account. For experimentally relevant parameters our analysis shows that with increasing pulse length the corresponding molecular conversion efficiency exhibits low-frequency interference fringes which are robust under thermal averaging over a wide range of temperatures. This dynamical interference phenomenon is attributed to Stückelberg phase accumulation between the low-energy continuum states and the dressed molecular state which exhibits a shift proportional to the envelope of the radio-frequency pulse intensity.External fields are widely used in order to probe, tune and control various aspects of atomic matter. For example, in the field of ultracold atomic physics dc magnetic fields constitute the main experimental means for the creation and manipulation of molecules via Feshbach resonances [1][2][3]. Techniques involving radio-frequency (RF) magnetic fields are of exceptional importance since they are highly adjustable in experiments [4,5]. Indeed, the additional magnetic RF field modulation enables the investigation of cold molecule formation [6][7][8] or heteronuclear association/dissociation processes in a microgravity environment [9], association of Efimov trimers [10][11][12][13] or manipulation of Feshbach collisions [14][15][16]. Beyond cold physics, external fields are also used in ultrafast physics where short laser pulses probe photoionization processes [17]. In such systems the pulse envelope plays a crucial role since it induces a time-dependent AC-Stark shift of the energy levels of the system whereas the light-pulse derivatives yield a dynamic interference in photoionization cross-sections [18][19][20][21][22][23][24][25], [26].In this letter, the RF-induced association process in an ultracold thermal gas is investigated and it is shown that the RF field envelope plays a crucial role. This allows us to extend the concept of dynamical interference in strong field ionization into the realm of ultracold physics and to explore its impact on the production of cold molecules. In Ref.[7] experimental evidences suggested that RF association in a thermal gas can exhibit Rabi-like oscillations in the molecular conversion efficiency (MCE) as a function of the duration of the RF field. On the other hand, the corresponding theoretical studies in Ref. [6] show that in this range of parameters any coherence in the MCE is completely smeared out by thermal averaging. Evidently, these studies still pose an intriguing question for the RF association processes in thermal gases: Under what con-ditions does the RF molecule formation display interference fringes that survive thermal averaging? Our study tackles this question: a gas of 85 Rb atoms with density n = 10 11 cm 3 is considered for which the gas temperature varies from T = 20 nK up to T = 50 nK , and for an RF field driving frequency that can associate continuum states near the dissociation threshold. In t...

Coherent Superposition of Feshbach Dimers and Efimov Trimers

“…Our sensitivity is thus limited to ∼ 100 DITRIS states (according to the SNR) and we fully benefit from the interferometric enhancement of the signal to obtain this level of sensitivity. For our experimental parameters, a trimer-excluding theory predicts the conversion of ∼ 0.018 × N 0 ≈ 540 atoms into ∼ 270 dimers after the first pulse [48]. This provides an upper bound for DITRIS states that can be created in our system in agreement with the observed results.…”

“…We associate this feature with the field where the maximal production efficiency of molecules is obtained. We observe slight variations of the field on the pulse intensity [48] but not on the pulse duration. In virtue of their close vicinity, E d and E t are unresolved in this experiment [34].…”

Quenched Magneto-association of Ultracold Feshbach Molecules

“…Ultracold molecules are today one of the physical systems most used to study a variety of physical phenomena, ranging from quantum information [1][2][3][4][5], to ultracold chemistry [6][7][8][9], to exploration of novel dipolar phases of matter [10][11][12][13][14][15], to tests of variations of fundamental constants [16][17][18]. As a result, developing efficient techniques to produce such molecules is a highly sought after goal [19][20][21][22][23][24][25]. Since most experiments using such molecules start from a gas of ultracold atoms, the central question is how to efficiently produce a dense sample of molecules while still keeping them at ultracold temperatures.…”