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Posted: March 28, 2007
The life and death of a photon 'filmed' for the first time
(Nanowerk News) Microscopic quantum systems 'jump' from one state to another in unexpected fashion. Physicists have already detected quantum jumps of atoms, electrons, ions and other particles. However, they had never yet seen this for photons, which are usually destroyed as soon as they are detected. This has now been accomplished using an ingenious technique, described in the 15 March issue of the journal Nature ("Quantum jumps of light recording the birth and death of a photon in a cavity"), whereby photons are trapped in a superconducting cavity. Researchers from the Kastler Brossel laboratory (CNRS/ENS/Collège de France/Université Paris 6) observed, in real time, the birth, life and death of a single photon. Einstein's dream of trapping a photon in a box for a period of time of around a second has at last come true.
A photon is an elementary particle of light. In general it can only be observed when it disappears. The eye, like most light receivers, absorbs the photons it detects irreversibly, and the information carried by the light is destroyed as soon it is recorded. We can of course see the same (macroscopic) object as often as we wish, but, each time, the photons which carry its image to our eye are new ones.
However, there is nothing in nature that says that photons have to be destroyed in order to be measured. Researchers at the Kastler Brossel laboratory (CNRS/ENS/Collège de France/Université Paris 6) have thus managed to observe, hundreds of times, a single photon trapped in a box. After a period of time which can be as long as half a second, the particle of light finally escapes, quite suddenly and unpredictably, by making a quantum jump. For the first time, the researchers have observed 'live' the history of the life and death of individual photons.
The 'photon box', in which the researchers recorded the life and death of a single photon, is made up of two semi-conducting mirrors placed opposite each other and 2.7 cm apart. They are cooled to a temperature close to absolute zero. (Image: M. Brune - CNRS 2007)
The key to the experiment is a 'photon box', which is a cavity formed by two superconducting mirrors cooled to a temperature near absolute zero. A photon from the residual thermal radiation bounces back and forth over a billion times between the mirrors, which are placed 2.7 centimeters apart, before it disappears (as opposed to a mere million times for mirrors reflecting photons of visible light). On average, the photon travels a distance equal to the Earth's circumference.
Photons are usually detected by atomic absorption. An atom can exist in different energy states, and it can absorb a photon by moving from one energy state to a higher one. By measuring the variation in energy of absorbing atoms crossing the cavity it would be possible to know whether an atom contains a photon, but this would destroy the photon and it would only therefore be seen once
The researchers found a clever way of getting round this problem by choosing atoms whose transition between two states 0 and 1 corresponds to an energy which is different from that of the photons. In this case, the law of conservation of energy means that the atom cannot absorb the light. However, the presence of the photon slightly alters the frequency of the transition of the atom (which is measured using an auxiliary microwave field outside the cavity). The end result is that the atom changes to state 1 if the cavity contains a photon and remains at state 0 if it is empty, as in the standard method. However, this time the energy absorbed by the atom is taken from the auxiliary field and not from that of the cavity. As a result, the photon is still there after having been seen, and is ready to be measured again.
The researchers recorded a great number of sequences lasting several seconds in which thousands of atoms, crossing the cavity one by one, were detected either in state 0 or in state 1. In a typical signal, the atoms are first detected in state 0: the cavity is empty. Suddenly the atoms appear in state 1, showing that a photon has been trapped between the mirrors. The photon comes from the residual thermal radiation which surrounds the cavity. Generally it remains trapped for about a tenth of a second. In some sequences the photon survives longer, for up to half a second. It then disappears as fast as it appeared, leaving the cavity empty. The moments at which the photons appear and disappear reveal the quantum jumps of light, which occur at random. By observing such jumps over a period of several hours, the researchers directly confirmed the statistical properties of thermal radiation laid down a century ago by Planck and Einstein. In this experiment, the information carried by a quantum of light is transferred hundreds of times to a physical system without being lost. The same photon controls the state of a large number of atoms, which is a big step towards quantum information processing.
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