AsianScientist (Dec. 25, 2017) – An international team of physicists has verified a basic prediction of the quantum-mechanical behavior of resonant systems. They published their work in Nature Physics.
Experimental physicist Robert H. Dicke predicted in 1954 that, in the same way that a large collection of bells will act differently from a single bell that is tapped, a group of atoms will emit light in response to excitation at a faster rate than a single atom. He predicted a ‘superradiant’ state, where the decay of a system is much faster when large numbers of photons or quanta are put in a system with many atoms as compared to a system with a single atom in isolation.
Taking the analogy of bells, he suggested that if you have a large number of bells that you excite together, they can ring loudly, but the sound dies out much more quickly than the gentle fading of a single bell. His approach included quantum effects, predicting that the fastest decay occurred when the number of quanta was half the number of atoms. Dicke also predicted that a very strong change in decay rate would occur even when the number of quanta in the system was much less than the number of atoms in the system.
In this study, the researchers from five of the world’s most advanced X-ray sources in Europe, Japan and the US replaced the low-energy quanta envisioned by Dicke with high-energy X-rays, allowing them to follow the decay of the system one quantum—meaning one X-ray—at a time.
However, getting strong pulses of X-rays is much harder than for low energy light, and required using the most modern sources—X-ray free electron lasers. These sources have only become available recently, and of the few operating in the world, only one, SACLA, at the RIKEN SPring-8 Center in Japan, achieves the required high energy.
Using this source, the scientists were able to precisely follow the decay for up to 68 X-ray photons. They observed a dramatic reduction of the time taken to emit the first X-ray as the number of X-rays increased, which is in excellent agreement with Dicke’s prediction. The single-photon decay under the same experimental conditions was studied at the European Synchrotron Radiation Facility (ESRF) in France.
“Through this work, we were able to demonstrate Dicke’s work to be correct, and were also able to offer an alternative picture of the decay properties, based on a statistical approach. This will be valuable for understanding future work,” said Dr. Alfred Baron of the RIKEN SPring-8 Center who co-led the study.
The article can be found at: Chumakov et al. (2017) Superradiance of an Ensemble of Nuclei Excited by a Free Electron Laser.
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Source: RIKEN.
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