Another Path to Degeneracy

One of my colleagues mentioned this in group (as in meeting, not therapy) — research that has formed a Bose-Einstein Condensate directly from laser cooling and trapping: Laser cooling to quantum degeneracy

In order to form a BEC, you have to get enough atoms together in a potential well, and with a low enough energy in order to trigger the transition that leaves the bulk of them in the ground state (that’s the condensation). Normally, atoms under such confinement will distribute themselves among the various quantized energy states, but under the proper circumstances the condensation will happen. The tipping point occurs when the phase space density — the number of atoms in a volume defined by the cube of the deBroglie wavelength (which is momentum dependent, so it’s related to the kinetic energy) is above some critical value. The trick, then, is getting enough atoms cold enough and confined such that their deBroglie waves are overlapping.

Up until now, the path to BECs have involved a stage of magnetic trapping, but since magnetic traps are shallow (energetically speaking) you laser-cooled and trapped atoms to load into the magnetic trap. The trap “walls” were then lowered, allowing the most energetic atoms to escape, much like the most energetic molecules leave a hot cup of water, so this is called evaporative cooling. It results in the average energy, and thus temperature, going down. A drawback is that you also lose atoms, but if the phase-space volume decreases faster than atoms are lost it results in the phase space density going up, and a condensate.

So why not simply laser-cool the atoms? The problem with that is that atoms absorbing the photons that are cooling them also re-emit those photons, and when the physical density is large, those emitted photons hit other atoms rather than escaping, and this limits both the density and the temperature. This interaction can’t be present where the condensate is formed.

So the researchers turned it off, in a manner of speaking. They added a second form of trap, a dipole trap, which doesn’t involve the absorption and emission of photons — the light is far from resonance, and you use the electric field gradient of the light to form the trap. In the region where this trap was present, the laser scattering was turned off by shifting the energy of the excited state, which you can do if you add an external field. This was accomplished by the addition of another laser, which gives rise to the AC Stark shift (Johannes, not Tony), or a light shift. The trapping light is no longer near resonance, so the scattering is greatly suppressed. It’s also localized, so it’s only in effect where you shine the laser.

The area where the trapping is still occurring still contains very cold atoms, so the atoms in the dipole trap can still collide and thermalize with them, but once that second trap fills sufficiently, the critical phase-space density is achieved and a BEC forms. Very neat.

The New Phonebook's Here! The New Phonebook's Here!

Oops, I mean Phys Rev A

Tests of local position invariance using continuously running atomic clocks

As the disembodied voice said, “If you build it, he will come.” Meaning, in this case, that if you have a bunch of atomic physicists who have built one type of continuously running atomic clock, which is different than other, commercially available kinds of continuously-running atomic clocks, it means there is an opportunity to do a test of one of the principles of general relativity.

There should be a version up on ArXiv soon, so I’ll post a link when I can, and after the ScienceOnline 2013 conference I intend to write up a blurb, muddling through the general relativity a bit, in the context of what we did. (and by “we” I mean the first author, Steve, who did the heavy lifting)

(Oh, and BTW, this paper’s publication was of course timed to coincide with my 5th Blogoversary, which was yesterday)

That's What it Is

Uncertain principles: Physics Is About Rules, Not Facts

The idea that air resistance forces somehow invalidate Newtonian mechanics is depressingly common, but it’s based on a common misconception of what physics is. Physics is not a collection of facts, it’s a set of rules for understanding the universe– in the specific case of Newtonian physics, rules governing the effect that forces have on the motion of objects. “All objects near the Earth’s surface fall at the same rate” is not a central idea of Newtonian physics, just one of the simplest predictions from it. The central ideas of Newtonian physics are the rules used to quantify the effect of interactions, chiefly the “second law of motion” which says that the rate of change of the momentum of an object is equal to the sum of all the forces acting on it.

And I Say it's All Right

Sun Primer: Why NASA Scientists Observe the Sun in Different Wavelengths

Specialized instruments, either in ground-based or space-based telescopes, however, can observe light far beyond the ranges visible to the naked eye. Different wavelengths convey information about different components of the sun’s surface and atmosphere, so scientists use them to paint a full picture of our constantly changing and varying star.


Just when you thought it was safe to go back in the water.

what if: Spent Fuel Pool

What if I took a swim in a typical spent nuclear fuel pool? Would I need to dive to actually experience a fatal amount of radiation? How long could I stay safely at the surface?

Spoiler alert, but necessary for my comment

In fact, as long as you were underwater, you would be shielded from most of that normal background dose. You may actually receive a lower dose of radiation treading water in a spent fuel pool than walking around on the street.

This jibes with my experience in the navy — the crew had their exposure monitored, and the word was that it wasn’t unusual for someone on a sub not directly being exposed to radioactive/contaminated materials or hanging out near the reactor compartment to have their dose be lower than what one would get on the surface. The benefitted from spending most of their time several tens of meters blow the surface, and all of the attenuation that afforded.

More PITA Than PEW

I align lasers for a living. Or so it seems.

That’s not actually my job description, but if you observed what I do in the lab over a long enough period, it wouldn’t be an unreasonable description of the plurality of my effort. That job doesn’t occupy as much of my time as it once did; because of some engineering advancements we’ve made, once lasers are aligned they stay aligned for much longer, but there’s still a bit of time spent doing this.

And this is the bane of my existence

That’s a multi-axis fiber port. And they are a complete pain in the butt to align.

The reason for using them is the “engineering advancement” I mentioned — if you couple your laser into an optical fiber, it’s very easy to send that light somewhere else. If you sent it through open space, you leave yourself open to all sorts of problems — dust and dirt on your optics, things blocking the beam, and misalignment issues creeping in — if you send the light just 1 meter, each milliradian of error moves your beam a millimeter, and thermal changes can “walk” a mirror mount to steer in a subtly different direction. If you need precision, long-term alignment, fiber is the way to go.

Free-space alignment is relatively easy. This is a mirror pair we call a dog-leg (golfers might observe it’s two dog legs, but this isn’t golf)

I’ve drawn in a laser beam (you wouldn’t see an actual beam unless there was dust or other some particulates to scatter the light, and we try to minimize that) bouncing off of the two mirrors. Each mirror mount has two knobs: one pushes on a corner for horizontal tilting, while the other pushes on the opposite corner for vertical tilting; there is a small ball bearing at the third corner, and a couple of springs to hold it all together. Adjusting a knob will tilt the mirror and deflect the beam, and change the angle of reflection off of the mirror (and also off of the next mirror). You can un-do the angle change with the second mirror, so that the exiting beam is headed in the same direction as you started, but having been translated to one side. The adjustments are orthogonal, so you can “walk” a beam anywhere you want by adjusting pairs of knobs, as long as the beam continues to hit the mirrors.

That’s pretty easy, in principle; in practice it’s a little tougher, because you often do this “blind” — you are looking at the target or a display, telling you how well you are hitting the target, and the lights might be off. But it’s a skill that’s picked up pretty quickly.

The fiber port, though, is tougher. The chuck in the middle is for an optical fiber, and in my case it’s a single-mode optical fiber. The core of such a fiber is a few microns across and is very sensitive to the spatial mode (the shape) of the light it accepts. I’ve previously shown what a poor mode looks like, but assuming you have a nice zeroeth order mode, you need to send the light in with the fiber positioned just right — not only hitting the core of the fiber, but at the proper angle and with the fiber tip just the right distance from the fiber, to match the mode characteristics.

There’s a little divot on the left side of the fiber port that houses the screw for moving the fiber chuck in the x direction, and one at the top for y, and they are both sensitive and subject to a bit of hysteresis (turning through some angle and then getting back to the original position doesn’t reposition the holder exactly), and also have the annoying habit of walking off when you remove the allen key/driver. The small black dots on the face of the holder are the tip/tilt controls, which also suffer a bit from hysteresis. In addition, to translate in the z-direction (moving the fiber closer or further from the lens) you have to change all three tilt controls. In practice this means your coupling efficiency goes down as you change the first two settings, and you have to hope you can recover your signal as you work on the third. If you can’t, it may be because you are moving the wrong way along z, or because you changed one of the tilt settings too much, so you have to try many iterations to find out what’s going on.

Just so you know it’s not all pew, pew, pew when you work with lasers.