Single-Photon Server Created From an Atom

Physicists have succeeded in turning a rubidium atom into a single-photon server, which could allow scientists to generate high-quality photons with consistent energy levels for use in quantum computing.

Finding photons is easy: Every time you switch on a light bulb, 10 to the power of 15 (a million times a billion) visible photons, the elementary particles of light, are illuminating the room in every second. But if you just want one, and not more than one, photon every time you press the switch, you have to work a little harder.

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. (Image: Max Planck Institute of Quantum Optics)

A team of physicists led by professor Gerhard Rempe at the Max Planck Institute of Quantum Optics (MPQ) in Garching, near Munich, have now built a single-photon server based on a single trapped neutral atom. The high quality of the single photons and their ready availability are important for future quantum information processing experiments with single photons. In the relatively new field of quantum information processing, the goal is to use quantum mechanics to compute certain tasks much more efficiently than with traditional computers.

A single atom, by its nature, can only emit one photon at a time. A single photon can be generated at will by applying a laser pulse to a trapped atom. By putting a single atom between two highly reflective mirrors -- a so-called cavity -- all of these photons are sent in the same direction.

The scientists said that, compared with other methods of single-photon generation, their photons are of a very high quality, i.e. their energy varies very little, and the properties of the photons can be controlled. They can, for instance, be made indistinguishable, a property necessary for quantum computation. Until now, they said, it was not possible to trap a neutral atom in a cavity and at the same time generate single photons for a long enough period of time to make practical use of them.

In 2005 Rempe's team was able to increase the trapping times of single atoms in a cavity significantly by using 3-D cavity cooling. In their new article, appearing in the March 11 edition of Nature Physics online, they report they have been able to combine this cavity cooling with the generation of single photons in a way that a single atom can generate up to 300,000 photons. In their current system, the time the atom is available is much longer than the time needed to cool and trap the atom. Because the system can therefore run with a large duty cycle, distribution of the photons to a user has become possible: The system operates as a single-photon server.

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 beamsplitter, which directed half of the photons to one detector and the other 50 percent to another. A single photon will be revealed by one of the detectors.

If the findings 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 they believe has been convincingly demonstrated in their work, the researchers said.

Rempe and his team said their progress means quantum information processing with photons has come one step closer to reality. With the single-photon server operating, they are now ready to take on new challenges, such as deterministic atom-photon and atom-atom entanglement experiments.

For more information, visit: www.mpq.mpg.de/mpq.html