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

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

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

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)

Optical Dipole Trap

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

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

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

Setup: single-mode fiber

Setup: “razor” blade

3D Power as a function of razor position zx P

It really does fit well!

Progression of plots

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:

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

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

Trap depth vs. frequency

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

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.

References 05/ v1.pdfhttp://lanl.arxiv.org/PS_cache/arxiv/pdf/11 05/ v1.pdf NWAPS 2010 (Walla Walla, WA) Invited Talk by Deep GuptaNWAPS 2010 (Walla Walla, WA) Invited Talk by Deep Gupta ex1.html