Category Archives: Experiments

Stop! In the Name of … Physics

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Stopping and Freezing a Bullet

Well, not so much a bullet as pretty much any paramagnetic atom (which means most atoms). Reminiscent of Sisyphus cooling only with really big magnetic pulses instead of polarization gradients.

The principle is similar to coilguns being developed by the military to launch projectiles, only “in reverse,” Raizen says. To those atoms with their dipoles aligned opposite the beam’s direction, the pulses are like hills they have to climb, except that each hill disappears before the atoms have a chance to slide “downhill.”

A Look Back at Laser Cooling

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Physical Review Letters is celebrating its Z=79 anniversary, and highlighting important letters. This week is:

Letters from the Past — A PRL Retrospective: This week’s Milestone Letter was originally published in 1970

Acceleration and Trapping of Particles by Radiation Pressure
A. Ashkin Phys. Rev. Lett. 24, 156 (1970)

This was a description of radiation pressure on transparent latex spheres, which felt a force when placed in laser light that had a gradient — the refraction gives rise to a force, or pressure, because the power is asymmetric across the sphere — the light changes direction, so the sphere must recoil, and the amount of recoil doesn’t balance. This is a precursor to a dipole force trap, which traps atoms at a field maximum (or minimum) of a light field, e.g. from focused laser. It also lays out radiation pressure by near-resonant scattering

The absorption and isotropic reradiation by spontaneous emission of resonance radiation striking an atom results in an average driving force or pressure in the direction of the incident light

which was the idea that led to laser cooling and optical molasses.

There is also a brief summary of the laser cooling history there this month, Landmarks: Laser Cooling of Atoms that takes you through the milestones up until the Nobel prizes for laser cooling and Bose-Einstein condensates.

You Say Potato, I Say Potahto

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You say MRI, I say NMR. These sound almost like William Steig would have used them, but not quite.

Chad over at Uncertain Principles give the lowdown on the phenomenon of Nuclear Magnetic Resonance

[…] Nuclear Magnetic Resonance (NMR), which is the “M” and the “R” in “Magnetic Resonance Imaging”– they ditched the “N” because “nuclear” is scaaaary, and doctors are wusses.

Yeah, but tell your doctor that when he’s got a sharp instrument or weird probe-thingy in his hand.

Carly Simon Physics

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I was poking around the blogdom — with the rise of science-y, i.e. non-diary (and, I suppose non-dairy) blogs, surfing the web has become interesting again — and ran across a link to How to Build a Cloud Chamber, and that reminded of the person that built the cloud chamber using a Starbucks cup. (Not sure if he was so vain, however.)

And that reminded me of the question I had back then — TRIUMF had a large, continuously-running chamber in the lobby of the visitors’ entrance last time I was there, and though nobody does it better, I’m sure there are other facilities with similar setups. Why not run a webcam showing it? I haven’t found one.

Second-best is video. Here’s one that shows the construction steps of a good one, and some tracks. That’s at the end, so there will be some anticipation.

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Dog Ballistics

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I guess it’s a dog-day. No, not dogs as projectiles — what a horrible thought. (I used cats in my physics examples when I was teaching. Or smurfs, if I had blue chalk)

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And a wiener dog, no less. Very Gary Larson.

Notice how the dog takes off as soon as the launcher draws back, making a distinctive sound. Pavlovian ballistics.

via Respectful Insolence

Ponch Makes a BEC

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AMO Physics meet chips.

I recently had the pleasure of attending a small workshop on the topic of doing atomic physics on chip-scale apparatus. The presentations and discussions were very interesting, but unfortunately do not lend themselves to a blog report for a couple of reasons. This kind of get-together discusses ongoing projects, some or all of which are not-ready-for-primetime, i.e. things haven’t been written up and published, so I wouldn’t be at liberty to discuss details, and a lot of the really cool stuff was shown in the pictures, some of which also haven’t been published. So while I can’t go into certain details, I can give an overview, and provide some links to representative work that has been published or is otherwise available to the public.

There’s quite a bit of physics that is described as being “tabletop” physics — in part to distinguish it from the physics that requires large collaborations and trips to an accelerator lab, or access to a big telescope or a satellite, etc. However, “tabletop” in atomic physics usually refers to an optical table, which can be a behemoth — an 8′ x 4′ table can weigh 1000 lbs or more. Add to that the requisite optics, optomechanics, vacuum systems and the support electronics, and you still have quite a lot of equipment, and it’s not going to be very portable.

As I had mentioned before, research layouts tend to be bigger than they strictly need to be, in order to provide some flexibility, and you can transition to more compact layouts if you have just one application in mind. If you take the concept a little further, and start thinking of the possibility that you might want an AMO experiment (e.g. a MOT or BEC, or trapped ions) to be applied to some specific problem, and require portability, you have to shrink not only the laser system, but the trap, and everything mentioned above — the vacuum pumps, power supplies and control electronics. Fortunately, a lot of this is coupled: if the trapping volume is decreased you don’t need as much pumping capacity or laser power, so your power demand is also smaller. If you can survive the decrease in signal, you can still go pretty good measurements, though new challenges tend to crop up working close to surfaces rather than far away. Instead of using general-purpose power supplies and electronics, you use devices specifically built for the application. You mount components together, rather than spaced apart, and use fibers rather than mirrors to transport light.

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