One of the problems with photons is that detecting their presence tends to destroy them because a photon is detected by absorbing it. In very special circumstances, however, measurements on quantum systems that don't destroy the quantum properties have become possible. These so-called quantum non-demolition measurements measure the superposition state of a quantum object without disturbing it. A superposition of two states means that the object is simultaneously in both states; a measurement will always reveal a particular state. Repeated measurements on identical objects will establish the relative probability of each state. A quantum non-demolition measurement evaluates these probabilities and how they change with time without forcing the object to choose a particular state.
A multinational team of researchers have used this technique to witness the birth, life, and death of a photon. They constructed a very special matter interferometer, which was used to measure the amount of light in an optical cavity. The optical cavity consisted of two superconducting metallic films, which are very highly reflective. Those films were placed in vacuum and the whole apparatus cooled to a little above absolute zero. Every now and again the mirrors would thermally emit a photon of light, which could be captured between the two mirrors. This photon would then bounce back and forth between the mirrors for about an eighth of a second before being absorbed by a mirror. The temperature was such that, statistically speaking, there were never more than two photons in the cavity at any time. Without making a measurement, the cavity is in a superposition of being empty, having one photon, or having two photons. The presence or absence of a photon in the cavity was monitored by its influence on a stream of atoms crossing the photon path.
The wavelength of the photons in the cavity sits on the edge of a transition between two states for Rubidium atoms. This imperfect match between the light rubidium wants to absorb and the light available means that it cannot absorb a photon but can, under special circumstances, be influenced by it. To achieve this, the Rubidium atoms are prepared in a superposition of the two states. They were then passed sequentially through the cavity, where the relative probability of each state was modified by the presence or absence of a photon. Finally, the superposition is modified by a known radiation source which will maximize the probability of one state assuming that it didn't encounter a photon during transit.
