Light's Lifetime

Superconducting mirrors made of copper covered by a thin layer of niobium. These mirrors are able to store microwave photons up to one-tenth of a second (Image: Michel Brune)
Photon's life cycle 'watched' in full
* 18:00 14 March 2007
* NewScientist.com news service
* Amarendra Swarup
For the first time the birth, life and death of a single photon – a particle of light – has been "watched" in real time.
Previously, scientists were restricted to momentary glances because the mere act of measurement absorbed and destroyed the delicate quantum particles.
Now, Serge Haroche and colleagues at the École Normale Supérieure in Paris, France, have succeeded in tracking photons over an average lifetime of 0.13 seconds – long enough for a photon to travel one-tenth of the way to the Moon.
At the heart of their remarkable achievement lies a small box-like cavity, walled with ultra-reflective, superconducting mirrors, which is cooled to just 0.5° above absolute zero (-273.15°C). Photons appear and disappear randomly within the cavity due to tiny energy fluctuations in space that cause quantum particles to blink in and out of existence. However, once there, the photon is trapped, bouncing billions of times between the mirrored walls before it decays.
Trapped and annihilated
To observe the photon, the researchers passed rubidium atoms across the cavity one at a time. A single rubidium atom is unable to absorb a single photon, because the photon is not the correct package of energy to boost the rubidium atom to a different energy state.
However, the photon's electric field slightly shifts the atom’s energy levels by a measurable amount (once the atom has emerged), which the team used to determine whether there were any trapped photons.
“This is not performed at the expense of the photon energy, so if one is detected, it is still there afterwards for successive rubidium atoms, allowing us to track it,” says Haroche. “A typical signal has a sequence of atoms at one energy level, meaning an empty cavity, suddenly interrupted by atoms at another energy level, signalling the photon birth. Later, a jump in the opposite direction signals the photon annihilation.”
“This is a very important fundamental achievement as no one has ever seen a photon a second time,” says Ferdinand Schmidt-Kaler at the University of Ulm in Germany. “It also has significant implications for the rapidly evolving field of quantum computing.”
Quantum computing relies on transferring qubits – quantum bits of information – between different energy states to vastly speed up calculations. According to Schmidt-Kaler, the results demonstrate a stream of atomic qubits can be fully controlled by the qubit state of a trapped photon – a notable achievement, since such operations are fundamental to quantum computers.

Journal reference: Nature (vol 446, p 297)