Atom foreshadowed: Optical limits

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  • Published: Jul 15, 2012
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Atom foreshadowed: Optical limits

Shadowy events

In an international scientific breakthrough, a Griffith University research team has been able to photograph the shadow of a single atom for the first time.  

Australian researchers have carried out the first absorption imaging of a single atom isolated in a vacuum and clearly reveal a visible shadow cast by the atom. The discovery could revolutionise quantum cryptography allowing systems to eventually span the globe by allowing technologists to construct the required atom-light  interfaces.

An international team headed by scientists at Griffith University, Australia, has for the first time stretched optical microscopy to its absolute limit, the scale of a single atom beyond which the wavelength of visible light is too long to function. They have thus been able to photograph the shadow of a single atom for the first time ever. "We have reached the extreme limit of microscopy; you cannot see anything smaller than an atom using visible light," David Kielpinski of the University's Centre for Quantum Dynamics in Brisbane.

Microscopic nudge

Kielpinski and his colleagues were hoping to nudge downwards the number of atoms that could be rendered visible in optical microscopy and have now gone as far as anyone could possibly go in their efforts. "We wanted to investigate how few atoms are required to cast a shadow and we proved it takes just one," he adds. Writing in a July issue of Nature Communications the team's result builds on half a decade of effort by Kielpinski, Erik Streed and their co-workers.

The key to success is a super high-resolution microscope at Griffith that achieves an amazing degree of contrast in the image it produces making the shadow dark enough to see. No other facility in the world has the capability for such extreme optical imaging, the team says. Of course, another aspect of the work was accessing a technique that allowed the team to hold the single atom steady for long enough to take its photo, something that any photographer struggles with whether snapping photos of children full of energy, delicate flowers twitching in the breeze or brightly coloured fish darting wildly under water. To keep the endlessly oscillating ytterbium atom steady the team used a vacuum chamber and trapped the atom in free space electrostatically.

By shedding light of a specific frequency on the ytterbium atom, the team was able to cast the atom's shadow on to a digital photo sensor. "By using the ultra hi-res microscope we were able to concentrate the image down to a smaller area than has been achieved before, creating a darker image which is easier to see," Kielpinski explains. The frequency of the incident light was also critical to success. "If we change the frequency of the light we shine on the atom by just one part in a billion, the image can no longer be seen," Kielpinski adds.

Atomic implications

Streed posits that the implications of this research are rather far reaching. "Such experiments help confirm our understanding of atomic physics and may be useful for quantum computing," he suggests. However, the work also has more obviously practical applications in biomicroscopy for instance. "Because we are able to predict how dark a single atom should be, as in how much light it should absorb in forming a shadow, we can measure if the microscope is achieving the maximum contrast allowed by physics. This is important if you want to look at very small and fragile biological samples such as DNA strands where exposure to too much ultraviolet light or X-rays will harm the material."

"This work will help us to extend the range of quantum cryptography, a communications technique with security guaranteed by the laws of quantum mechanics," Kielpinski told SpectroscopyNOW. "Present quantum cryptography systems only function over a range of about 100 km. By incorporating quantum processors made of atoms, one could extend these systems to span the globe," he adds. "Our method provides new and exciting ways to set up the atom-light interfaces at the heart of these processor nodes."

Related Links

Nature Commun 2012, online: "Absorption imaging of a single atom"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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