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arXiv:1509.07439 · Physical Review A

Suppression of two-body collisional losses in an ultracold gas via the Fano effect

Jianwen Jie, Ya‐Wen Zhang, Peng Zhang

Published Feb 9, 2016• 2 citations
arXivHTMLPDF

Abstract

The Fano effect [U. Fano, Phys. Rev. 124, 1866 (1961)] shows that an inelastic scattering process can be suppressed when the output channel (OC) is coupled to an isolated bound state. The Fano effect was originally derived via a first-order perturbation treatment for coupling between the incident channel (IC) and the OC. In this paper, we first generalize the Fano effect to systems with arbitrarily strong IC-OC couplings. We analytically prove that, in a system with one IC and one OC, when the interatomic interaction potentials are real functions of the interatomic distance, the exact $s$-wave inelastic scattering amplitude can always be suppressed to zero by coupling between the IC or the OC (or both of them) and an extra isolated bound state. We further investigate the application of this generalized Fano effect for the suppression of two-body collisional losses of ultracold atoms and show that when the low-energy inelastic collision between two ultracold atoms is suppressed by this effect, the real part of the elastic scattering length between the atoms is still capable of being much larger than the range of interatomic interaction. In addition, when open scattering channels are coupled to two bound states, with the help of the Fano effect, independent control of the elastic and inelastic scattering amplitudes of two ultracold atoms can be achieved. Possible experimental realizations of our scheme are also discussed.

DOI
10.1103/physreva.93.022705

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The Fano effect [U. Fano, Phys. Rev. 124, 1866 (1961)] shows that an inelastic scattering process can be suppressed when the output channel (OC) is coupled to an isolated bound state. The Fano effect was originally derived via a first-order perturbation treatment for coupling between the incident channel (IC) and the OC. In this paper, we first generalize the Fano effect to systems with arbitrarily strong IC-OC couplings. We analytically prove that, in a system with one IC and one OC, when the interatomic interaction potentials are real functions of the interatomic distance, the exact sss-wave inelastic scattering amplitude can always be suppressed to zero by coupling between the IC or the OC (or both of them) and an extra isolated bound state. We further investigate the application of this generalized Fano effect for the suppression of two-body collisional losses of ultracold atoms and show that when the low-energy inelastic collision between two ultracold atoms is suppressed by this effect, the real part of the elastic scattering length between the atoms is still capable of being much larger than the range of interatomic interaction. In addition, when open scattering channels are coupled to two bound states, with the help of the Fano effect, independent control of the elastic and inelastic scattering amplitudes of two ultracold atoms can be achieved. Possible experimental realizations of our scheme are also discussed.
Inelastic collisions of ultracold atoms or molecules are analyzed using very general arguments. In free space, the deactivation rate can be enhanced or suppressed together with the scattering length of the corresponding elastic collision via a Feshbach resonance, and by interference of deactivation of the closed and open channels. In reduced dimensional geometries, the deactivation rate decreases with decreasing collision energy and does not increase with resonant elastic scattering length. This has broad implications; e.g., stabilization of molecules in a strongly confining two-dimensional optical lattice, since collisional decay of excited states due to inelastic collisions is suppressed. The relation of our results to the Lieb-Liniger model for bosonic atoms is addressed.
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.
In this work we investigate the potential of controlling cold (O(K)−mK) and ultracold (mK-μK) atom-molecule collisions by tuning scattering states across Feshbach resonances using magnetic fields. We are interested in particular in the prospect of suppressing the often undesirable inelastic collisions. The He-O_2 system provides the vehicle for our study. We calculate bound and quasi-bound states of several isotopic combinations, including their Zeeman structure, to reveal the underlaying pattern for easier characterization of quasi-bound states in terms of rigorous and approximately good quantum numbers. These calculations also help us locate the fields at which zero-energy resonances will occur. Scattering calculations are then performed for collisions of 3^He and 4^He with {16}^O_2 at fixed (1 μK) energy but varying magnetic field. The field is varied to sweep the scattering state across resonance. At low and ultralow energies we enter the Wigner threshold regime where the S-partial wave dominates the wavefunction. The cross sections, and the real and imaginary parts of the scattering length, vary dramatically across resonance. Their profiles are used to analyze the resonances. In a highlight of our results we show that dramatic suppression of inelastic cross sections occur for 4^He-{16}^O_2 . The resonances are relatively wide (of order 100 Gauss), with suppression of inelastic scattering over a similarly wide range of fields and for temperatures ranging from 10 mK down to 1 μK. We conclude that under certain conditions it is possible to almost completely eliminate inelastic collisions. This is potentially very important for cooling techniques, such as evaporative and sympathetic cooling, that require efficient elastic cross sections. Suppression of inelastic collisions can not only increase thermalization efficiency but it can also result in longer trap-lifetimes by reducing transitions to untrapable states.
In the absence of inelastic scattering, Feshbach resonances produce poles in scattering lengths and very large peaks in elastic cross-sections. However, inelastic scattering removes the poles. Whenever the resonant state is coupled comparably to the elastic and inelastic channels, the scattering length exhibits only a small oscillation and peaks in cross-sections are significantly suppressed. A resonant scattering length is defined to characterize the amplitude of the oscillation, and is shown to be small for many collisions of ultracold molecules. The results suggest that cross-sections for some ultracold collision processes will be much less sensitive to details of the potential than has been expected.
We derive a model to explain the observed suppression of optically induced loss in a weakly interacting Fermi gas as the sss-wave scattering length is increased [C. A. Royse et al., Phys. Rev. Lett. 133, 083404 (2024)]. We incorporate spin-dependent loss into a quasiclassical collective spin vector model to show that loss suppression occurs via a transition to a magnetized dynamical state, where two-body sss-wave scattering is inhibited via the Pauli principle. By comparing measurements in mixtures and coherently prepared samples, we show that the data are quantitatively explained by the model, which is applicable to the optical control of energy-space lattices for new quantum simulators.