Category Archives: Experiments

Superman Already Knew This

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Sticky tape generates X-rays

Researchers at the University of California, Los Angeles, have shown that simply peeling ordinary sticky tape in a vacuum can generate enough X-rays to take an image — of one of the scientists’ own fingers

Only when you do it in a vacuum, though. Not clear (the concept, not the tape) if that is because the atmosphere degrades the charge buildup on the tape or if the occasional x-rays just scatter in the atmosphere, or some third option or a combination. I’ve seen bluish light when I’ve peeled adhesives before, so the mechanoluminescent properties aren’t a surprise, but the energy of them is.

Straightening Out the Tangles in Time

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This is timely, as it were. Scientific American has an article on frequency combs that appears to be publicly accessible. Rulers of Light: Using Lasers to Measure Distance and Time

Optical frequency comb applications require exquisite control of light across a broad spectrum of frequencies. This level of control has been available for radio waves for a long time but is only now becoming possible for light. An analogy to music helps in understanding the required level of control. Before the development of combs, lasers could produce a single color, like a single optical tone. They were akin to a violin with only one string and no fingerboard, capable of playing only one note (ignore for the moment that musical notes are much richer than pure tones). To play even a simple piece would require many different instruments, each painstakingly tuned. Each violin would require its own musician, just as every single-frequency laser requires its own operator.

In contrast, one operator can use an optical comb to cover the entire optical spectrum, not merely like a pianist at a piano but like a keyboardist playing an electronic synthesizer that can be programmed to mimic any musical instrument or even an entire orchestra. Comb technology, in effect, enables symphonies of hundreds of thousands of pure optical tones.

As explained in the article, one key for usefulness in timing and frequency is that the comb spans an octave, i.e. a factor of two in frequency, so that a “tine” (spectrum line) of the comb at the low end can be frequency-doubled in a nonlinear crystal, and referenced to a line at the high end, making the comb stable — the frequency of any line is well-known. You can now reference a convenient optical transition to the comb and do clock measurements. Since the frequency is much higher, if a suitable (i.e. narrow) transition can be found the fractional error will be much smaller, and the measurements that much more precise.

One thing that was apparent from the Frequency Standards and Metrology conference is that combs are everywhere. A number of different atoms are being investigated, both neutrals typically trapped in an optical lattice, or ions trapped in, well, ion traps. Once you have a really nice clock, though, you need to have another very nice clock (or clocks) with which you can compare. Multiple clocks can be referenced to a comb, and this is being done in the larger labs. And there are also people investigating better techniques for comparing remote signals using fiber transmission of signals, to overcome the limitations of satellite comparisons.

Try That With Email!

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Return to sender: Artist puts Royal Mail to the test

To put [letter carriers] to the test she concealed the addresses of 130 letters to herself in a series of increasingly complex puzzles and ciphers. Among the disguises she employed were dot-to-dot drawings, anagrams and cartoons. The answer, it seems, was very far indeed. Amazingly, only 10 failed to complete their journey back to her.

Viva Las Vegas

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One of the questions one asks when trapping atoms in a magneto-optic trap (MOT) is “What shall we do with the atoms?” You often have an idea before you do the trapping — it’s not like we’re trophy hunters, trapping just to have something on the wall. Trapping in and of itself hasn’t been the goal for quite some time now, at least in experimental labs; one wants to do some kind of experiment with the atoms. Some of the time that can be done in the trap, but quite often it involves moving the atoms somewhere else. Sometimes you actually wanted an atomic beam of some sort, instead of a collection of atoms just sitting there, suspended in space — the trapping environment involves bright, near-resonant laser light and magnetic fields and those could be undesirable. The atom beam gets you away from this, and if you look at the beam from a perpendicular direction, the Doppler shift is very small. Perhaps you want low-speed collisions, and tuning the speed of the beam allows you to do your experiment. There are also a number of atom-optics experiments that can be done, e.g. sending the atoms through transmission gratings comprising an interferometer. The problem could also be the relatively high vapor pressure of the gas in your vapor cell giving you excessive background signals, or collisions with that background vapor could be the problem, limiting the trap lifetime. So you need to move the atoms, transporting them to a region that is better-suited for the experiment you are doing.

When I was at TRIUMF, the problem was the background and trap lifetime. We were trapping radioactive atoms, and the idea was that when an atom decayed, the beta would go one way and the atom would recoil, and each could be detected. But a vapor-cell MOT captures only the small percentage of atoms stupid enough moving slowly enough to get trapped, leaving the majority of the zipping around in the cell or sticking to the walls (or worse, attaching themselves to detectors). Not only did this mean they would be swamping the signal from the trapped atoms, the signals would be coming from different directions and originating from different points.

About the time we started fretting about this problem (you have to trap them first before you worry about the next step, and nobody had trapped these isotopes before) we got a visit from Zheng-Tian Lu, then at JILA/NIST, and he had come up with an ingenious method of generating a low-velocity atomic beam and shared the details with us (the paper was in the pipeline but had not yet been published at the time)

A typical vapor-cell MOT uses three beams along the cartesian axes, and it’s possible to do this by retroreflecting each of these beams — the vapor is dilute, so with decent mirrors there isn’t a large drop in intensity (any imbalances will push the trap slightly off-center as the effect of the magnetic field compensates). You get the proper polarization of the beams by placing a quarter-wave plate in front of the retroreflection mirror (this changes the circular light to linear and then back to circular of opposite helicity; if you started with linear it would circularize it and change it back to linear, perpendicular to the original. Ah the fun you can have with waveplates)

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Did Jules Verne Write This?

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Journey to the Center of the Neutron

A neutron contains three quarks, and nuclear physicists don’t completely understand how these move within the particle. Last year, an analysis revealed a negative charge at the center of the neutron, and now an article in the Rapid Communications section of the September Physical Review C attributes this negative core to very fast moving “down” quarks. The results elaborate on an emerging three-dimensional view of these fundamental particles and their proton cousins.