A team of German physicists at the Max Planck Institute of Quantum Optics has built a single-photon server with just one atom. They've trapped ultra cold atoms of rubidium in a vacuum chamber and applied laser pulses from one side. The generated photons were of 'high quality,' meaning their energy was very similar from one test to another, and that their properties could be controlled. The researchers think this new way to generate single photons will help the field of quantum information processing.
But let's look at what these researchers have done. Below is a picture showing how "a single atom trapped in a cavity generates a single photon after being triggered by a laser pulse. After the source is characterized, the subsequent photons can be distributed to a user." (Credit: Max Planck Institute of Quantum Optics) And here is a link to a larger version
Now, how was the experiment conducted?
The experiment uses a magneto-optical trap to prepare ultracold Rubidium atoms inside a vacuum chamber. These atoms are then trapped inside the cavity in the dipole potential of a focused laser beam. By applying a sequence of laser pulses from the side, a stream of single photons is emitted from the cavity. Between each emission of a single photon the atom is cooled, preventing it from leaving the trap. To show that not more than one photon was produced per pulse, the photon stream was directed onto a beam splitter, which directed 50% of the photons to a detector, and the other 50% to a second detector. A single photon will be detected either by detector 1 or by detector 2. If detections of both detectors coincide, more than one photon must have been present in the pulse. It is thus the absence of these coincidences that proves that one and not more than one photon is produced at the same time, which is demonstrated convincingly in the work presented.
This research work has been published as an advance publication by Nature Physics under the name "A single-photon server with just one atom" on March 11, 2007. Here is a link to the abstract. You also can read the full paper (thanks to arXiv.org).
The researchers say that "quantum information processing with photons has come one step closer." But I bet you've already read this somewhere else.
Sources: Max Planck Society Press Release, March 12, 2007; and various websites
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