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ChemPhysChem

Effective Atom–Molecule Conversions Using Radio Frequency Fields

Yijue Ding, Jesús Pérez‐Ríos, Chris H. Greene

Published Aug 11, 2016• 4 citations
Publisher

Abstract

Abstract The present study is inspired by the Wieman group experiment [Phys. Rev. Lett. 2005, 95, 190404], in which they use a slow modulated magnetic field to effectively transfer rubidium atoms into cold molecules near a Feshbach resonance. We develop a time‐dependent collision theory based on two channel model potentials to study the atom–molecule population transfer induced by a single‐color radio frequency field in an ultracold 87Rb gas. Wave‐packet dynamical simulations allow an investigation of both bound–bound transitions and free–bound transitions. The effects of temperature, detuning and the RF amplitude on the population transfer are discussed in detail. Some of our simulations suggest that oscillatory atom–molecule conversion could originate from the long coherence time of the wave packet. This coherence time is unusually long in ultracold gases because the collision energy is typically quite well‐defined.

DOI
10.1002/cphc.201600646

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Abstract The present study is inspired by the Wieman group experiment [Phys. Rev. Lett. 2005, 95, 190404], in which they use a slow modulated magnetic field to effectively transfer rubidium atoms into cold molecules near a Feshbach resonance. We develop a time‐dependent collision theory based on two channel model potentials to study the atom–molecule population transfer induced by a single‐color radio frequency field in an ultracold 87Rb gas. Wave‐packet dynamical simulations allow an investigation of both bound–bound transitions and free–bound transitions. The effects of temperature, detuning and the RF amplitude on the population transfer are discussed in detail. Some of our simulations suggest that oscillatory atom–molecule conversion could originate from the long coherence time of the wave packet. This coherence time is unusually long in ultracold gases because the collision energy is typically quite well‐defined.
We study 87Rb^{87}\mathrm{Rb}87Rb cold collisions in a static magnetic field and a single-color radio-frequency (RF) field by employing the multichannel quantum defect theory in combination with the Floquet method to solve the two-body time-dependent Schr\"odinger equation. Our results show that RF fields can modify the two-body scattering length by a large scale through Feshbach resonances in both low- and high-static magnetic-field regimes. Such RF-induced Feshbach resonances can be applied to quenching experiments or control of interactions in spinor condensates. Here, we also show that, analogously to photoassociation, RF fields can also associate cold atoms into molecules at a useful rate.
Abstract The conversion of ultracold atoms to molecules via a magnetic Feshbach resonance with a sinusoidal modulation of the field is studied. Different practical realizations of this method in Bose atomic gases are analyzed. Our model incorporates many-body effects through an effective reduction of the complete microscopic dynamics. Moreover, we simulate the experimental conditions corresponding to the preparation of the system as a thermal gas and as a condensate. Some of the experimental findings are clarified. The origin of the observed dependence of the production efficiency on the frequency, amplitude, and application time of the magnetic modulation is elucidated. Our results uncover also the role of the atomic density in the dynamics, specifically, in the observed saturation of the atom–molecule conversion process.
This thesis has investigated the dynamics of ultracold atomic collisions in the presence of both static magnetic and oscillating radio-frequency (rf) fields. The bound state structure and scattering length of 39K+133Cs was examined in the presence of only a static magnetic field, where it was found that no Feshbach resonance of suitable width for magnetoassociation existed at a magnetic field where caesium can be cooled to degeneracy. We then showed that zero-energy Feshbach resonances may be engineered using an rf field in places where they did not previously exist. An rf field with frequency 79.9 MHz was chosen to induce a resonance near 21 G. The widths of such rf-induced Feshbach resonances increase quadratically with rf field strength. The resonances presented are lossless with circularly polarized rf, and the molecules created are long-lived even with plane-polarized RF. Collisional losses in rf-dressed magnetic traps were also investigated. An rf-induced loss mechanism that does not exist in the absence of rf radiation was identified. This mechanism is not suppressed by a centrifugal barrier in the outgoing channel, and can be much faster than spin relaxation, which is centrifugally suppressed. We explore the dependence of the rf-induced loss rate on singlet and triplet scattering lengths, hyperfine splittings and the strength of the rf field. The results were interpreted in terms of an adiabatic model of the collision dynamics, and calculate the corresponding nonadiabatic couplings. The loss rate can vary by 10 orders of magnitude as a function of singlet and triplet scattering lengths. 87Rb is a special case, where several factors combine to reduce rf-induced losses; as a result, they are slow compared to spin-relaxation losses. For most other alkali-metal pairs, rf-induced losses are expected to be much faster and may dominate. For heteronuclear mixtures an rf-modified spin-exchange mechanism was identified that results in loss rates orders of magnitude greater than the rf-induced loss rates in homonuclear cases. Fast loss is expected in mixtures where the Lande g-factors differ.
A rigorous quantum theory of atomic collisions in the presence of radio frequency (rf) magnetic fields is developed and applied to elucidate the effects of combined dc and rf magnetic fields on ultracold collisions of Rb atoms. We show that rf fields can be used to induce Feshbach resonances, which can be tuned by varying the amplitude and frequency of the rf field. The rf-induced Feshbach resonances occur also in collisions of atoms in low-field-seeking states at moderate rf field strengths easily available in atom chip experiments, which opens up the world of tunable interactions to magnetically trappable atomic quantum gases.
We have calculated the time-dependent dynamics of two ultracold Na atoms in an atom trap where a time-dependent magnetic field B(t)B(t)B(t) moves a Feshbach resonance state across the energy threshold for a binary collision. Our coupled-channel scattering calculations, which reproduce the observed properties of such resonances in sodium atom collisions, can be reduced to an effective two-channel configuration-interaction model for one bound state and one continuum. The model is adapted to describe the time-dependent dynamics induced by B(t)B(t)B(t) for two atoms trapped either in a strongly confining single well of an optical lattice or in an optical potential in the presence of a Bose-Einstein condensate. We show that a simple Landau-Zener curve crossing model gives quantitative agreement with exact calculations of field-induced transition rates. If B(t)B(t)B(t) sweeps the resonance across threshold from above, two atoms in the ground state of the trap potential can be efficiently converted to translationally cold dimer molecules. If the resonance is swept from below, the atoms can be removed from the ground state and placed in hot vibrational levels of the trap. Our calculations reproduce the rapid atom loss rates observed in a Na Bose-Einstein condensate due to sweeping a Feshbach resonance state through the binary collision threshold.
The heteronuclear atomic collision in the presence of a radio frequency (RF) electric field and a dc magnetic field is investigated theoretically by using the asymptotic bound state model. A RF field can be used to modify Feshbach resonance by choosing suitable intensity and frequency. The control mechanism is based on the interaction of the RF electric field with the instantaneous dipole moment of the collision atomic pair. We find an interesting phenomenon: the character exchange of the Feshbach resonances and resonance interference modulated by the RF field takes place in the ultracold collision of 6 Li and 40 K. This resonance interference may be used to modify the interactions in ultracold quantum gases.