Faculty of Physics, Hanoi University of Science

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Tales of 1001 Atoms

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" Researchers can now count individual atoms in large ensembles.
Imagine trying to count grains of rice in a cup that are shaking about, and then assume the grains are less than a nanometer long. This is akin to what David Hume and colleagues from the University of Heidelberg in Germany have recently done [1]. As reported in Physical Review Letters, by observing the light emitted from a collection of laser-cooled and trapped atoms, they can distinguish two ensembles differing by just one atom—one grain—and directly observe a small step in signal when one atom is lost from the trap (Fig. 1). You might also wonder why anybody would want to count atoms to one thousand instead of being satisfied with distinguishing, say, one atom from two. The answer is that one day such particle counting techniques could extend precision quantum measurements well beyond current limits, helping to improve the accuracy of atomic clocks [23], or allowing an increase in the capability of atomic magnetometers.

Figure 1

Figure 1 Trapped and continuously laser-cooled atoms emit light that is collected onto a camera. For sufficiently many collected photons, it is possible to count atoms one by one, even for ensembles containing hundreds of atoms.

The most sensitive measurements of various quantities such as time, magnetic field, or motion, are quantum measurements based on interference of waves. Such interferometers, whether they use light or massive particles such as atoms, now almost routinely reach precisions that are not dominated by any technical noise source, but by fluctuations that are inherently quantum mechanical in character. These fluctuations arise directly from the “quantumness” of nature—the granularity associated with particle number or discrete atomic energy levels. A well-known example is shot noise: if a measurement of the average power associated with a stream of photons is repeated multiple times with high resolution, the results will differ because photons arrive randomly. A similar effect limits atomic clocks and other matter-wave interferometers. In atomic clocks, a quantum phase that measures time is estimated by detecting the population difference between two atomic states. Consider an ensemble of atoms in which each atom has equal probability of being in one of two states. In this case, there is quantum noise associated with the distribution of possible measurement outcomes. The corresponding measurement limit, where precision improves as the square root of atom number N, is called the standard quantum limit. So even with 1000 atoms, the best error you could hope for is ±33 atoms."---http://physics.aps.org/articles/v6/137

For full information, see the source: http://physics.aps.org/articles/v6/137

The PRL paperD. B. Hume, I. Stroescu, M. Joos, W. Muessel, H. Strobel, and M. K. Oberthaler, Accurate Atom Counting in Mesoscopic EnsemblesPhys. Rev. Lett. 111, 253001 (2013)

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