Birth, life and death of a photon

Two days ago, I told you that German physicists had built a single-photon server. But French researchers also have used ultra cold atoms of rubidium to record the full life of a photon.

Two days ago, I told you that German physicists had built a single-photon server. But French researchers also have used ultra cold atoms of rubidium to record the full life of a photon. This was one of Einstein's dreams, but it was thought as an impossible one before. In fact, a photon disappears when it delivers its information. But with what has been described as an "experimental masterwork," the physicists have observed the "quantum jumps" done by single photons for as long as half a second. This discovery should lead to important developments for future high performance computers and quantum computing -- and certainly to the field of physics.

This brilliant research project has been conducted at the Laboratoire Kastler Brossel, part of France's National Centre for Scientific Research (CNRS), in its Cavity Quantum Electrodynamics lab. Here are two more links to the research team and to their project intended to see a photon without destroying it.

Before going further, below is a picture showing the experimental set-up used by the scientists (Credit: Laboratoire Kastler Brossel). "Samples of circular Rydberg atoms are prepared in the circular state g in box B, out of a thermal beam of rubidium atoms, velocity-selected by laser optical pumping. The atoms cross the cavity C sandwiched between the Ramsey cavities R1 and R2 fed by the classical microwave source S, before being detected in the state selective field ionization detector D. The R1–C–R2 interferometric arrangement, represented here cut by a vertical plane containing the atomic beam, is enclosed in a box at 0.8 K (not shown) that shields it from thermal radiation and static magnetic fields."

Recording the life ofa photon

Here are more details about the 2.7 cm box built by the researchers.

The box comprises a cavity with walls made from ultra-reflective, superconducting mirrors able to trap a photon for about a seventh of a second. That may not seem much but it is worth considering that, in the same time, a free photon would travel about a tenth of the distance from the Earth to the Moon.

Still, counting photons destroys them by absorbing their energy -- simply by watching them. So what was the discovery of these French researchers?

The French team say they found the answer in a stream of rubidium atoms, which cross the box in which the photon is trapped. Photons have an electrical field that slightly changes the energy levels of the atom, but in this case, not enough to let the atom absorb energy from the field. When an atom crosses the photon's electrical field, this causes a tiny delay in the electrons that orbit the atom's nucleus. The delay is measurable, using the technique of modern atomic clocks, which use electrons' orbit as a "pendulum" to provide a precise time.

In their trap, the French team was able to identify an "off" or "0" state when no photon is present, or an "on" or "1" state when there is one. This opens new ways to work with qubits -- or quantum bits -- if the state of these photons can really be controlled.

This research work has been published by Nature under the name "Quantum jumps of light recording the birth and death of a photon in a cavity" (Volume 446, Number 7133, Pages 297-300, March 15, 2007). Here are two links to the abstract and to the full paper, from which the above figure has been extracted (PDF format, 5 pages, 297 KB, via

But don't be too excited: these experiments only work in the lab, and at temperatures that are too cold for us.

Sources: AFP, via, March 16, 2007; and various websites

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