Scientists have devised a new technique for real-time detection of freely moving individual neutral atoms that is more than 99.7% accurate and sensitive enough to discern the arrival of a single atom in less than one-millionth of a second, about 20 times faster than the best previous methods.
The system, described in Advance Online Publication at the Nature Physics web site by researchers at the Joint Quantum Institute (JQI) in College Park, MD, and the Universidad de Concepción in Chile, employs a novel means of altering the polarization of laser light trapped between two highly-reflective mirrors, in effect letting the scientists "see" atoms passing through by the individual photons that they scatter.
The ability to detect single atoms and molecules is essential to progress in many areas, including quantum information research, chemical detection and biochemical analysis.
That arrangement is a familiar one for labs studying the interaction of atoms and photons. The JQI system, however, has two distinctively unique features.
First, the researchers use two polarizations of cavity light simultaneously: one (horizontal) which is pumped in to efficiently excite the atoms, and the other (vertical) which only appears when emitted by an atom inside the cavity. Although the descent of the atom through the chamber takes only 5 millionths of a second, that is 200 times longer than it takes for the atom to become excited and shed a photon, so this process can happen multiple times before the atom is gone.
Second, they create a magnetic field inside the cavity, which causes the laser light polarization to rotate slightly when an atom is present. Known as the Faraday effect, this phenomenon is typically very weak when observed with a single atom. However, since the light reflecting between the mirrors passes by the atom about 10,000 times, the result is a much larger rotation of a few degrees. This puts significantly more of the laser light into the vertical polarization, making the atoms easier to "see."
The light eventually escapes from the cavity and is fed through a polarizing beamsplitter which routes photons with horizontal polarization to one detector, and vertical polarization to another. Each arriving photon generates a unique time stamp whenever it triggers its detector.
Although the detector for the vertically polarized light should only be sensitive to light coming from an atom in the cavity, it can be fooled occasionally by stray light in the room. But because there are multiple emissions from each atom, there will be a burst of photons whenever an atom passes between the mirrors. This is the signature that the researchers use to confirm an atom detection.
"The chief difficulty lies in verifying that our detector is really sensitive enough to see single atoms, and not just large groups of them," says team leader Luis A. Orozco of JQI. "Fortunately, the statistics of the light serve as a fingerprint for single-atom emission, and we were able to utilize that information in our system."
The Joint Quantum Institute is a research partnership between the University of Maryland and the National Institute of Standards and Technology, with additional support and participation of the Laboratory for Physical Sciences. This research was conducted with support from the National Science Foundation and the National Institute of Standards and Technology.
* "Photon Burst Detection of Single Atoms in an Optical Cavity," M.L. Terraciano, R. Olson Knell, D.G. Norris, J. Jing, A. Fernandez and L.A. Orozco, http://www.nature.com/nphys/index.html, DOI 10.1038/NPHYS1282.
Luis Orozco | EurekAlert!
http://www.nature.com/nphys/index.html, DOI 10.1038/NPHYS1282
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