SciAm 50: Advances in Ultrameasurement

Zeptoliter pipettes and quantum rulers give new meaning to the word "small"

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Scientists use pipettes when they need to dispense well-defined volumes of liquid. Existing pipettes can deliver fluid volumes as small as an attoliter—a quintillionth, or a billionth of a billionth, of a liter.

Physicists Peter W. Sutter and Eli A. Sutter of Brookhaven National Laboratory have broken that lower limit by constructing a pipette that metes out a droplet measured in a unit that is a thousandth as small—a zeptoliter (a sextillionth of a liter). Such a minute volume can contain as little as 10,000 to as much as a million atoms of metal.

The researchers used a germanium nanowire with a solid reservoir of gold-germanium alloy at one end. They encapsulated the two-micron-long assembly in a carbon shell, which constituted the pipette. Inside a vacuum chamber, they heated and melted the alloy and then aimed an electron beam at the shell’s tip. The beam bored an escape hole for the molten metal, which formed a minuscule droplet up to 40 nanometers in diameter and 35 zeptoliters in volume.

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If measuring things in zeptoliters is difficult, consider doing so at a scale where the rules of classical physics cease to prevail. Quantum metrology—the field in which quantum mechanics is used to obtain highly precise measurements—has allowed physicists at Hokkaido University in Japan and the University of Bristol in England to almost double the precision of measurement when using photons to gauge distances.

The scientists have built on previous work that uses photons “entangled” in the same quantum superposition of states. The team directed two photonic pairs into an interferometer—an instrument that creates a circular beam path with mirrors in which light waves interfere with one another. Each photon splits, taking separate paths simultaneously. Four photons in an entangled state circulate around the interferometer in one direction, and another quadruplet traverses the loop in the other. The interference produced by the countercirculating photons reveals tiny differences in how far each quadruplet has traveled.

The precision measurements could, for example, be useful when using lasers to etch ultrathin circuits on computer chips.
—Steven Ashley

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