Life & Death of a Single Photon Observed

Michel Brune (photo courtesy: l'Ecole Normale Supérieure in Paris)

Nearly one hundred years after such a quantum phenomenon was predicted, a team of French physicists led by Michel Brune at the l'Ecole Normale Superieure in Paris has been successful in watching single photons appearing spontaneously from vacuum, live a brief life, and then vanish into the vacuum. The experiment is the best realization so far of "quantum non-demolition" (QND) measurements on single photons, whereby the presence of a photon is determined without destroying it.

The team used atoms to make a QND measurement of the quantum state of a system containing one photon. They used a microwave cavity cooled to 0.8K. In such a low temperature state, there is about a 5% chance that the cavity will be devoid of microwave photons and a 50% chance that a single photon can be found inside the cavity. The single photon can spontaneously appear from the vacuum and may vanish in less than one second.

A stream of rubidium atoms is passed though the cavity. These have an electron in a highly excited state (so called Rydberg atoms) and are very sensitive to external perturbations, such as electric fields. The atoms are prepared such that they can exist in one of two quantum states (“g” and “e”). The atoms are flipped between g and e by a non-resonant interaction with the cavity field. If the atoms cross an empty microwave cavity, most of them will emerge in state 'g', whereas if they encounter a photon the majority will emerge in state 'e'.

The photon, when it gets absorbed, displaces the position of the atomic energy levels of such a Rubidium atom. As the atoms emerge from the cavity, their state is determined using a high resolution spectroscopy. The team was thus able to make hundreds of such measurements on a single photon without destroying it. And that's the way they succeeded in watching a single photon emerge from the vacuum, live a short life of less than one second, and then disappear.

The researchers are hopeful that their technique can also find a way into potential applications in quantum information systems, in which the cavity can assume the role of a logic gate that switches the quantum state of the atoms depending on the presence or absence of a single photon.

Reference
"Quantum jumps of light recording the birth and death of a photon in a cavity",
Sébastien Gleyzes, Stefan Kuhr, Christine Guerlin, Julien Bernu, Samuel Deléglise, Ulrich Busk Hoff, Michel Brune, Jean-Michel Raimond and Serge Haroche,
Nature, Volume 446, Number 7133, p297 (15 March, 2007)