Optical Trapping of Atoms: Characterization and Optimization

1. Optical Trapping of Atoms: Characterization and Optimization Charlie Fieseler University of Kentucky UW REU 2011 Subhadeep Gupta

2. So… why? • Studying superfluid/degenerate gas properties • Two species, Lithium and Ytterbium, can use one as a probe • Condensed matter simulations • Molecule formation, specifically polar • Quantum computing in a lattice • Fundamental physics, of course • Fine structure constant measurement through Yb BEC atom interferometry • Electric dipole moment of electron

3. Getting Cool Atoms: Laser cooling • Zeeman Slower • absorbs on-resonance light in the atom’s frame of reference, with a magnetic field to counteract Doppler • MOT (also using Zeeman effect) • A 3D trap that catches the cooler atoms • Oppositely polarized light is preferentially absorbed • After compression, ends up: • >10^6 atoms at ~20 μK

4. Getting Cold Atoms • Using an optical dipole trap (ODT), cool evaporatively (two step) • >2*10^4 Yb atoms below ~170nK (critical temperature), and can go below 30nK • >10^4 Li atoms below ~300nK, and can go below 100nK • With both species, can cool sympathetically (different trap depths)

5. Optical Dipole Trap

6. ODT (cont.) • The atoms are high-field seeking, i.e. optical tweezers • None of the other measurements make any sense unless you know the waist and position of the focus • Cameras are usually used, but they can be quite expensive and (firsthand) very unreliable • Do something simple: razorblade

7. A different method of beam profiling • A more conventional method scans perpendicular to the beam • This is not very sensitive to small waists • Hard to know where the minimum is • For the proof of concept, the beam is single-mode with a Gaussian shape • A scan along the axis of propagation can measure small waists with <5% error

8. The shape of the beam • This method can also measure deviations from a Gaussian shape • A Gaussian intensity function gives the power by integrating:

9. Setup: single-mode fiber

10. Setup: “razor” blade

11. 3D Power as a function of razor position P z x

12. It really does fit well!

13. Progression of plots

14. Modeling the trap geometry • What do you want the waists to be? • In reaching degeneracy, trapping frequencies (i.e. of an harmonic oscillator) are key:

15. Optimizing (or at least a first guess) • Symmetrical makes the most sense: same power, circular, same size • But then gravity… poof, nonlinear • Harder to model, but there are some theoretical benefits • Weaken dependence of frequency on trap depth: if gravity were tunable, could get down to .075 from .5

16. Effects of Gravity • To the right: Trap at 10W and .25W • The trap becomes dominated by gravity at low power: two effects • Lower exponent • Smaller curvature and therefore coefficients

17. Trap depth vs. frequency

18. The aforementioned first guess • The trap disappears in one dimension before the others • The power in that beam should be held at a minimum, while the other beam continues the evaporation • Gravity is not a large enough effect to break the symmetry earlier

19. Next steps • Actually build this setup! • Will be used for a Ytterbium BEC interferometry experiment • The curves shown do not really show a benefit, but other tweaks need to be tested.

20. References • http://lanl.arxiv.org/PS_cache/arxiv/pdf/1105/1105.5751v1.pdf • NWAPS 2010 (Walla Walla, WA) Invited Talk by Deep Gupta • http://grad.physics.sunysb.edu/~fdimler/index1.html