Multiple images of an object located between two nearly parallel mirrors illustrate the principle of superradiant emission of x-rays. If an ensemble of atoms between two mirrors is resonantly excited by synchrotron radiation, the atoms interact collectively to emit a fast x-ray photon pulse that exhibits a redshift of its frequency, known as the collective Lamb shift.
This experiment´s illustration made it on the Cover of Science Magazine, issue 04 June 2010.

An ensemble of identical atoms submitted to irradiation of light behaves differently than a single atom. Jointly, the atoms emit light at a lower frequency than a single atom would do. This effect, the so-called collective Lamb shift, was recently observed by a research team headed by Dr. Ralf Röhlsberger from the Helmholtz research centre DESY. The scientists from DESY, the European Synchrotron Radiation Facility ESRF (France) and the University of Leuven (Belgium) provided evidence of an effect that theorists predicted already more than 35 years ago, but could not be experimentally proved so far. The results of this experiment carried out at ESRF in Grenoble are published in the current issue of Science.

The Lamb shift is a small difference of the oscillation frequency of electrons in the atom. It becomes visible when light excites atoms to radiate. The frequency shift occurs when the excited atom emits and re-absorbs its light several times before returning to its ground state. The discovery of the Lamb shift in hydrogen in 1947 laid the foundation for the development of quantum electrodynamics (QED) as a unified theory of interaction of light and matter. For this discovery, the physicist Willis Lamb received the Nobel Prize in 1955.

When an ensemble of identical atoms is excited to radiate, it is possible that the emitted light of an atom is not only absorbed and re-emitted by the single atom but also by other atoms of the ensemble. Therefore, the light emitted by these atoms has a lower energy and exhibits a distinct red shift compared to the light that a single isolated atom would emit.

For their experiments, Röhlsberger and his team of researchers developed a new measurement method. They placed an ensemble of 57Fe atoms between two platinum mirrors separated only by a few nanometres and irradiated this array with X-ray radiation. And in fact, the predicted collective frequency shift could be measured in this way, even though it was believed for a long time that the atoms must not be separated by more than a wavelength. The researchers’ group took advantage of the fact that that the radiation of 57Fe atoms is enormously intensified, making the collective Lamb shift clearly visible. With the help of Mössbauer spectroscopy, the shift could be determined very precisely. The measured values are in excellent agreement with theoretical predictions.

This experimental method also offers new possibilities to study collective effects in the interaction of light and matter. Thus, the researchers observed that the light from the ensemble of atoms was emitted almost 100 times faster than from a single isolated atom. This phenomenon is called superradiance. Superradiance enables a very efficient energy transfer between light and matter and it may play an important role for designing more efficient solar cells and in fast optical information processing.