Atto boy!

The shortest time yet measured,

and the most sensitive set of scales.

P EOPLE WHO THINK “NANO” IS THE EPITOME OF SMALLNESS should think again. A nanosomething is but a billionth of that something. Two new pieces of research concern themselves with atto-somethings— billionths of the nano.

Ferenc Krausz, of Vienna University of Technology, and his colleagues, are concerned with attoseconds. They have just published a paper in Nature which describes their measurement of the shortest time interval ever recorded, a mere 100 attoseconds. Meanwhile, across the Atlantic, at Cornell University, Harold Craighead and his colleagues have succeeded in building a set of scales that are sensitive to a fraction of an attogram. Their work is due to appear in a forth-coming paper in the Journal of Applied Physics.

Dr Krausz’s achievement was a by-product of his studies of the orbitals of electrons around atomic nuclei. Quantum theory makes precise predictions about the energy of those orbitals, and Dr Krausz was checking that those predictions are correct . (They are.)

To measure the energy in question, he used two successive pulses of laser light, each 250 attoseconds long. The first knocked electrons free from their orbitals. The second scattered them. The scattering changed the momentum of the electrons in a way that depends on their original orbital position. Momentum is mass times velocity, but all electrons weigh the same, so measuring their velocity was enough. Enough. But tricky. Dr Krausz did it by recording the arrival times of different electrons using a piece of equipment called a multi-channel plate detector. ie most sensitive set of scales. And he needed to be able to tell them apart as precisely as possible, to avoid recording two arrivals as one.

In this case “as precisely as possible” meant within 100 attoseconds. That is the limit imposed by Heisenberg’s famous uncertainty principle (which states that the precision of a time measurement is limited by the precision of a corresponding energy measurement). And by careful crafting of the scattering pulses he was able to go all the way to that limit.

This sort of work is pretty abstruse. Dr Craighead and his colleagues have a more concrete goal in mind: identifying viruses by weighing them. Different sorts of viruses have different weights, but all viruses of a particular type weigh the same amount.

Dr Craighead created his tiny scales from crystals of silicon. Using beams of electrons, he carved a cantilever out of the edge of each crystal (think of a knife held down on a table, with the blade sticking out) . Put an object on the end of such a cantilever and the lever will bend. It will also tend to vibrate at a frequency deter-mined by the weight of the object . It was this vibrational frequency that the team measured.

Instead of actual viruses, Dr Craighead weighed small amounts of gold. The very smallest was 0.39 attograms—about 10,000 atoms. That level of sensitivity should be enough to identify viruses, but it is not enough for Dr Craighead. By refining the system further, he expect s to be able to weigh a zeptogram — a thousandth of an attogram .

Move over nanotech, it seems that zeptotechnology is just around the corner.


                                                               The ECONOMIST Magazine

February 28th, 2004. (Pg.77)

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