Trap-loss Collisions Research Articles

Observation of microwave shielding of ultracold molecules

Harnessing the potential wide-ranging quantum science applications of molecules will require control of their interactions. Here, we used microwave radiation to directly engineer and tune the interaction potentials between ultracold calcium monofluoride (CaF) molecules. By merging two optical tweezers, each containing a single molecule, we probed collisions in three dimensions. The correct combination of microwave frequency and power created an effective repulsive shield, which suppressed the inelastic loss rate by a factor of six, in agreement with theoretical calculations. The demonstrated microwave shielding shows a general route to the creation of long-lived, dense samples of ultracold polar molecules and evaporative cooling.

Shielding ultracold molecules

Ultracold Molecules Ultracold molecules hold promise for a wide range of exciting applications. However, such applications are currently hampered by the limited number of ultracold molecular ensembles that can be created and by their short lifetimes. Anderegg et al. used a microwave dressing field to tune the collisional properties of calcium monofluoride molecules trapped in optical tweezers. This approach allowed a sixfold suppression of inelastic trap-loss collisions. This scheme paves the way for the creation of a variety of long-lived ultracold molecular ensembles. Science , abg9502, this issue p. [779][1] [1]: /lookup/doi/10.1126/science.abg9502

Semi-classical calculations of ultracold and cold collisions with frequency-chirped light

We conduct semi-classical Monte-Carlo simulations of ultracold collisions utilizing frequency-chirped laser light on a nanosecond timescale. Recent experiments demonstrated partial control of light-assisted collisional mechanisms with relatively slow chirp rates (10 GHz/μs). Collisions induced with positive chirped light enhance the inelastic collisional loss rate of atoms from a magneto-optical trap due to rapid adiabatic passage, whereas trap loss collisions can be coherently blocked when negative chirped light is used. Early quantum and classical simulations show that for negative chirps, the laser’s frequency continually interacts with the atom pair during the collision. We investigate how this process depends on the chirp rate and show that by moderately speeding up the chirp (>50 GHz/μs), we can significantly enhance coherent processes. We extend our semi-classical model to examine using pulse shaping as a means to coherently control collisions and show that features in the pulse shape should be on the order of or less than 1 ns. We also show that coherent control of collisions using this technique can be extended to temperatures exceeding 1 K.

Quantum dynamical calculations of ultracold collisions induced by nonlinearly chirped light

We describe quantum dynamical calculations of ultracold ${}^{85}$Rb trap-loss collisions induced by pulses of light whose frequency is chirped on the nanosecond time scale. The chirped light excites the ground-state collisional wave function to the long-range attractive potential and escape from the trap is modeled by an absorbing boundary at short range. Both positive and negative chirps are considered and various chirp shapes and detunings are examined. For positive chirps, the loss rates are rather independent of the chirp shape. Negative chirps, on the other hand, show a dependence on chirp shape for detunings where collisional flux can be coherently returned to the ground state. These trends are consistent with the results of a recent experiment.

Coherent control of ultracoldRb85trap-loss collisions with nonlinearly frequency-chirped light

We present results on coherent control of ultracold trap-loss collisions using 40 ns pulses of nonlinearly frequency-chirped light. The chirps, either positive or negative, sweep ~1 GHz in 100 ns and are centered at various detunings below the D2 line of 85Rb. At each center detuning, we compare the collisional rate constant beta for chirps that are linear in time, concave-down and concave-up. For positive chirps, we find that beta generally depends very little on the shape of the chirp. For negative chirps, however, we find that beta can be enhanced by up to 50(20)% for the case of the concave-down shape. This occurs at detunings where the evolution of the wavepacket is expected to be coherent. An enhancement at these detunings is also seen in quantum mechanical simulations of the collisional process.

Coherent control of ultracold collisions with chirped light: Direction matters

We demonstrate the ability to coherently control ultracold atomic Rb collisions using frequency-chirped light on the nanosecond time scale. For certain center frequencies of the chirp, the rate of inelastic trap-loss collisions induced by negatively chirped light is dramatically suppressed compared to the case of a positive chirp. We attribute this to a fundamental asymmetry in the system: an excited wave packet moves inward on the attractive molecular potential. For a positive chirp, the resonance condition moves outward in time, while for a negative chirp, it moves inward, in the same direction as the excited wave packet; this allows multiple interactions between the wave packet and the light, enabling the wave packet to be returned coherently to the ground state. Classical and quantum calculations support this interpretation.

Probing ultracold collisional dynamics with frequency-chirped pulses

We report on the dynamics of ultracold collisions induced by near-resonant frequency-chirped light. A series of identical chirped pulses, separated by a variable delay, is applied to an ultracold sample of $^{85}\mathrm{Rb}$, and the rate of inelastic trap-loss collisions is measured. For small detunings of the chirped light below the atomic resonance, we observe that the rate of collisions induced by a given pulse can be increased by the presence of an earlier pulse. We attribute this to the enhancement of short-range collisional flux by the long-range excitation of atom pairs to an attractive molecular potential. For larger detunings and short delays, we find that a leading pulse can suppress the rate of collisions caused by a following pulse. This is due to a depletion of short-range atom pairs by the earlier pulse. Comparison of our data to classical Monte Carlo simulations of the collisions yields reasonable agreement.

Trap loss collisions of 6Li and 7Li with 23Na in a combined magneto-optical trap

We investigated heteronuclear inelastic cold collisions in a mixed-species Li–Na magneto-optical trap, where the Li trap loss from collisions with Na was measured. Experimental results for the rate coefficient βNaLi for the isotopes 6Li as well as 7Li as a function of the Li-MOT laser intensity are presented. Furthermore, we discuss some unique characteristics of cold collisions of Na and Li which arise from (1) the very small spin–orbit splitting of Li, (2) the very small C6 coefficients of the Na(3 2S) + Li(2 2P) asymptotes and (3) the very small and inverted hyperfine structure of Li.

Heating rates in collisionally opaque alkali-metal atom traps: Role of secondary collisions

Grazing collisions with background gas are the major cause of trap loss and trap heating in atom traps. To first order, these effects do not depend on the trap density. In collisionally opaque trapped atom clouds, however, scattered atoms with an energy E larger than the effective trap depth ${\mathcal{E}}_{\mathrm{eff}},$ which are destined to escape from the atom cloud, will have a finite probability for a secondary collision. This results in a contribution to the heating rate that depends on the column density $〈\mathrm{nl}〉$ of the trapped atoms, i.e., the product of density and characteristic size of the trap. For alkali-metal atom traps, secondary collisions are quite important due to the strong long-range interaction with like atoms. We derive a simple analytical expression for the secondary heating rate, showing a dependency proportional to $〈\mathrm{nl}〉 {\mathcal{E}}_{\mathrm{eff}}^{1/2}.$ When extrapolating to a vanishing column density, only primary collisions with the background gas will contribute to the heating rate. This contribution is rather small, due to the weak long-range interaction of the usual background gas species in an ultrahigh-vacuum system---He, Ne, or Ar---with the trapped alkali-metal atoms. We conclude that the transition between trap-loss collisions and heating collisions is determined by a cutoff energy $200 \ensuremath{\mu}\mathrm{K}<~{\mathcal{E}}_{\mathrm{eff}}<~400 \ensuremath{\mu}\mathrm{K},$ much smaller than the actual trap depth $\mathcal{E}$ in most magnetic traps. Atoms with an energy ${\mathcal{E}}_{\mathrm{eff}}<E<\mathcal{E}$ escape into the Oort cloud: a mechanism of effective traploss in the microkelvin range of trap temperatures. We present results of secondary heating rates for the alkali-metal atoms Li through Cs as a function of the effective trap depth, the column density of the trap, and the species in the background gas. The predictions of our model are in good agreement with the experimental data of Myatt for heating rates in high-density ${}^{87}\mathrm{Rb}$-atom magnetic traps at JILA, including the effect of the rf shield and the composition of the background gas. It is shown that collisions with atoms from the Oort cloud also contribute to the heating rate. For ${}^{85}\mathrm{Rb}$ the calculated heating rate is below the experimentally observed value at JILA, supporting the idea that inelastic collisions in the trap are the major source of heating.

Cold inelastic collisions between lithium and cesium in a two-species magneto-optical trap

We investigate collisional properties of lithium and cesium which are simultaneously confined in a combined magneto-optical trap. Trap-loss collisions between the two species are comprehensively studied. Different inelastic collision channels are identified, and inter-species rate coefficients as well as cross-sections are determined. It is found that loss rates are independent of the optical excitation of Li, as a consequence of the repulsive Li*-Cs interaction. Li and Cs loss by inelastic inter-species collisions can completely be attributed to processes involving optically excited cesium (fine-structure changing collisions and radiative escape). By lowering the trap depth for Li, an additional loss channel of Li is observed which results from ground-state Li-Cs collisions changing the hyperfine state of cesium.

Trap-loss collisions of85Rband87Rb:Dependence on trap parameters

We report on measurements of trap-loss collisions between ultracold Rb atoms that are confined in a magneto-optical trap. Both isotopes, ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb},$ have been studied over a wide range of trap laser intensities and detunings. The trap-loss collisional rate constant exhibits variations over three orders of magnitude. At low intensities and/or large detunings, the trap loss is dominated by ground-state hyperfine-changing collisions, while at high intensities and small detunings, collisions involving excited atoms are more important. We see significant differences between the isotopes in both regimes. At large detunings, the ability of the trap to recapture products of a hyperfine-changing collision is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room-temperature background gas have also been measured. A surprisingly large increase in this rate is seen when the confining power of the trap is reduced.

Observation of flux enhancement in collisions between ultracold atoms.

We have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of {approximately}3 have been observed. This indicates that the atomic pair-distribution function can be significantly distorted in a typical laser trap environment. {copyright} {ital 1996 The American Physical Society.}

Trap-depth measurements using ultracold collisions.

We measure the depth of an optical atom trap using repulsive trap-loss collisions. These collisions involve excitation by a laser tuned to a frequency above the atomic resonance. Simple conservation of energy considerations give direct quantitative measurements of the trap depth. Further analysis allows measurement of the escape probability as a function of energy. We apply the methods to Rb traps, and use the results to form conclusions about ground-state hyperfine-changing collisions.

Suppression of trap loss collisions at low temperature.

We report on measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below \ensuremath{\sim}70 \ensuremath{\mu}K. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.

Comparison of trap-loss collision spectra for 85Rb and 87Rb.

We present measurements of excited-state trap-loss collisions of optically trapped $^{85}\mathrm{Rb}$ and $^{87}\mathrm{Rb}$ atoms as a function of the frequency of the light used to excite the colliding atom pairs. For detunings outside the excited-state hyperfine structure, the trap-loss rates are found to be the same for the two isotopes. For detunings inside the excited-state hyperfine structure, $^{87}\mathrm{Rb}$ collisions occur at a substantially lower rate than those of $^{85}\mathrm{Rb}$. The spectra make it clear that the long-range dynamics of the collisions are strongly modified by the excited-state hyperfine structure, and that the dynamics are different for the two isotopes.

Isotopic difference in trap loss collisions of laser cooled rubidium atoms

We report measurements of collisional rate constants for cold Rb atoms held in a magneto-optical trap. $^{85}\mathrm{Rb}$ and $^{87}\mathrm{Rb}$ display significantly different behavior. At low trap laser intensities, atoms which undergo a hyperfine-changing collision gain sufficient velocity to escape the trap. Since the hyperfine splittings are quite different for the isotopes, the loss rates display a large isotopic dependence in this regime. At higher trap laser intensities, where trap loss is due to inelastic collisions involving excited atoms, a dramatic and unexpected isotopic dependence is seen. Under identical conditions, the loss rate for $^{85}\mathrm{Rb}$ is 3.3 times that of $^{87}\mathrm{Rb}$.

Excited-state collisions of trappedRb85atoms

We descrbe a new method for measuring excited-state collisions between optically trapped atoms. With this method, trap-loss collision rates are deduced from the loading behavior of clouds of trapped atoms in the regime where radiation trapping limits the atom density. Our measurements indicate that $^{85}\mathrm{Rb}$ trap-loss collisions occur at significantly smaller rates than expected both from previous work on Cs and from recent models. In addition, the dependence of the trap-loss collisions on the frequency of the light used to excite the atom pairs is also different from that of Cs, suggesting that assumptions about the dynamics in these models need modification.