Lab Tricks

Posted on June 27, 2008 by swansont

No, not that kind of trick, you pervert*.

I was doing some homemade wiring, and whether it’s power or signal, you generally want to use twisted-pair (or triplet, or quad, etc.). It’s faster than running all of the single-wire, so there’s a labor-saving aspect to it, but there’s a data quality aspect to it as well.

Any time you have a pair of wires that completes a circuit, you have to worry about ground loops and other signal pickup. If the wires are separated, and the magnetic field that runs through them changes, Faraday’s law tells you that you’ll add some current or a potential difference to the loop. The bigger the loop, the more flux you’ll be capturing. If you write this onto the common ground for your experiment, you will be putting this signal onto all of your equipment. And this is not just the earth’s field — everything radiates. A loop is an antenna for picking up 50 or 60 Hz power and also any other frequency equipment you use in the lab. (Early on in my current job, in the dark days before I had a CD-burner, much less an iPod, I tried listening to the radio in the lab. At one point we added an Acousto-optic modulator and started driving it a smidge above 100 MHz, which was almost the same frequency as the local oldies station, and I couldn’t get that station anymore because of the interference). This will get written on to your signal lines, and will get picked up by power lines, which then writes it on to all of that precision equipment you soldered together, and forgot to add bypass capacitors everyplace you needed them) . Chasing down ground loops is a big pain, as is filtering out noise. Twisting the wires means that the net current flow of any power signal is zero, as current input is as close as it can be to the return path, so the far-field radiation — basically anything further away than the diameter of the wire bundle — is nonexistent. If it’s a data line, it doesn’t look like an antenna, except perhaps for extremely high frequencies — the area for magnetic flux is vanishingly small, so it has no opportunity to pick up a signal.

So you want twisted-pair, but the commercial pickings can be slim for the exact wire type you might want to use, and besides, you want to color-code what you’re doing. So here’s the trick: use a drill. Clamp on to the wires with the chuck, pull taught and squeeze the trigger. Wind up to a reasonable pitch and — I cannot emphasize this enough — release the chuck before lessening the linear tension on the wires. You’ve added a lot of “twist” to the wires, and they will untwist. If you release the linear tension first —trust me on this — it will jumble up like a telephone cord. (If you’re under 30 and don’t understand the phrase “telephone cord,” it’s the phone you’ve seen at your grandparents’ house, perhaps in the basement. The phone might even have a round disc on the front, with ten holes in it around the perimeter)

*my conclusion after perusing the somewhat disquieting search-engine stats. Suffice to say that using “animal robo-p*rn” in a title isn’t leading to searches that are attracting science-minded folk to the post.

Ghostly Visages

Posted on June 25, 2008 by swansont

No, not of an alien at the window.

Here’s a little movie showing atoms being trapped ~~and mistreated~~. What you’re seeing is a video of the monitor that’s hooked up to a little IR camera on the vacuum chamber. The really bright spot that’s squirming around a little are the atoms, or technically, the fluorescence from the atoms. There are probably more than a Sagan of them (i.e. biilliyuns) at about a milliKelvin or so in temperature (which is considered warm!) because it hasn’t been fine-tuned yet. The bluish circle is reflected light off of a flange, and there’s some other scattered light visible.

About 10 seconds in, the trap’s magnetic field is turned off, and the atoms squirt off to the side. A second or two later the field is turned on again, and the trap fills up. If the lasers were properly aligned and balanced, and the earth’s magnetic field were either shielded or zeroed out with trim coils, then what you would see is a nice uniform expansion into a much colder (microKelvin-ish) optical molasses, but the earth’s field is still present here, so that gives rise to an imbalanced residual force which is small compared to the trapping force, so you only really notice it when the trapping field is turned off. The molasses impedes the motion of the atoms, but doesn’t technically trap them, i.e. it doesn’t define a point where the atoms should be, so when only the lasers are there, the atoms would normally just drift through, very slowly. But here they’re being shoved a little bit.

The accelerations involved here are large — these atoms can scatter a million photons a second, give or take, depending on the exact laser frequency, so even though an individual scatter changes the atom’s speed by about 6 mm/sec, when you scatter them that rapidly you can get accelerations of hundreds of g’s. But in the situation here, where the atoms are almost at rest, that’s balanced by an equally large acceleration from an opposing laser.

The Long and Winding Coil

Posted on May 22, 2008 by swansont

One project over the last several weeks has been winding coils for the atomic fountains. There are two different requirements, one is the so-called “C-field” coils and the second is the MOT (magneto-optic trap) coil pair.

The “C-field” is the bias field in an atomic clock that essentially tells the atoms which way is up, i.e. it defines the quantization axis. It also shifts the frequency of the transition, so in a frequency standard you need to know what the field is. In a clock (there is a subtle difference) you care about the stability, i.e. you don’t want it to change, so it’s enough to feed this with a precision current source to give a bias field of a milligauss or so. Two layers, up and back, so the pitch on each layer should tend to cancel and leave you with a vertical field, and about 600 turns per layer. There are also extra shim windings at each end to better simulate an infinite solenoid — a real solenoid’s field drops off at the ends, so we boost it back up a little. The drift region, where the atoms oscillate between the two hyperfine states (the “tick” of an atomic clock), sees a very stable field.

Pretty easy, but time-consuming (as it were); the basic winding took more than four hours. What you see is the jig I used, which has a stepper motor and a home-built feed system that wets the wire with alcohol to activate the bonding material. Square wire is used so it doesn’t have any gaps.

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BBC Seven! BBC Heaven!

Posted on May 17, 2008 by swansont

BBC was in the lab yesterday, filming . . . something. I mostly stayed out of the way, except to utter, “I’m putting my secret stash in here!” within earshot of one of the crew, and then making sure everyone understood it was my secret stash of titanium bolts. (Titanium bolts will get you stoned to the bejeezus-belt)

I have no idea what program (excuse me, programme) was involved. Just rumors that they wanted shiny stuff to film. Last time they were in town was around Y2k, or maybe the millenium, when it seemed that everybody was filming in the lab, and we turned them down because we were running the Cesium fountain, and the room lights had to be off for that. No flash photography, and people in the first six rows will get wet.

Igor … Throw the Main Switch!

Posted on April 7, 2008 by swansont

I’ve always wanted a huge double-pole, double-throw knife switch in the lab, because you can’t utter that phrase without it, but it’s probably a bad idea.

So you settle (just a little)

Because sometimes you just need to run a few hundred amps through something …

Lab Pix: All (Well, Some) Things, Great and Small

Posted on March 16, 2008 by swansont

One thing about R & D is that the project eventually moves you from physics (the “R” part) over to something that’s more engineering in nature (the “D” part). Here’s a quick example. Here’s the laser system for the caesium fountain, which is a research device. It’s spread out to be optimized for getting your hands in and tweaking on knobs, since you don’t know ahead of time what is going to work best. Lots of mirrors and other optics that need to be adjusted, and there has to be room to change things around and/or add things that might work better. You have to generate six beams for trapping (this is done at the table under the big cylinder, splitting two beams from the main table), plus a beam for optical pumping and another for probing the atoms. That’s four different beams on the table, at various (and for the MOT, adjustable) frequencies. The long paths meant that the beams would only stay aligned into the fiber couplers for, at best, weeks at a time.

Eventually you decide on a design that works, and since our production devices need to fit into a smaller space, and aren’t meant to be adjusted much (ideally, not at all) after the initial setup, you make everything smaller. Pretty much everything except the lasers (the 2 blue boxes on the left and 3 black boxes; one center-front and two right-rear) and the spectroscopy (which ends up on a different table) are compacted to fit on a much smaller table.

Everything is fiber-coupled, so it’s modular — it doesn’t matter which particular laser you use, or what you do with the beams after they leave the box. As you can see, it’s rack-mounted, and about 4u high. (You can just see another table at the top of the picture; this houses the spectroscopy/locking hardware)

The Relativistic Van

Posted on January 28, 2008 by swansont

Who cares about gas mileage? This sucker warps time!

When relativity is discussed in popular literature it’s often couched in terms of affecting objects moving at a significant fraction of the speed of light, and that’s a true statement: kinematic time dilation cannot generally be ignored in that situation. But the implication that the opposite is true — that you can ignore these effects under other circumstances — doesn’t hold. At least, it doesn’t hold if you have some expensive toys at your disposal.

Let’s say you were going to drive across the US and back, and you had the aforementioned expensive toys. Maybe you wanted to calibrate clocks and check on the reliability of a satellite time-transfer system, and you have a mobile system that would do time transfer at the source and at the target site, allowing you to check on that calibration. Or something like that.

The time dilation in question gives a fractional frequency shift that goes with the square of the speed, as compared to the square of speed of light. That’s normally very small, and has to be under this approximation (c is big, v/c is small, (v/c) squared is reeaaally small), so you can usually ignore it, right? Not everyone can. The famous Hafele-Keating experiment that used airplanes and around–the–world travel was able to measure kinematic dilations. A trip across the US is ~2700 miles, and at 600 mph you’d get a frequency shift of 4 parts in 10^13 and a dilation of about 13 nanoseconds on your round-trip due to traveling at that speed. (one thing to note is that I’m using a different coordinate system than is used in the H-K writeup, in case you want to play along at home. The answer will be the same, but the east vs west contributions are accounted for differently, and I’m not showing that detail)