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Date: Aug 22, 2018

Echoes from the quantum world

A collaborative study by experimental and theoretical research groups lead by Johann Kroha of the Physikalische Institut at the University of Bonn shows that certain materials emit optical echo pulses that reveal direct information about their quantum-mechanical nature.

When we move from the meter scale of our daily life to the length scale of (sub-) atomic particles, physics change and quantum mechanics take over. Particles no longer bounce off each other like solid billiard balls. Instead, they become fuzzy and can pass through each other without noticing. In fact, it is impossible to say where exactly each particle is and how fast it is moving. Cooling a many-particle system down enhances this quantum-mechanical behavior. In a hot system, particles have a lot of energy and whizz around in a normal way. Towards absolute zero temperature (where zero is -273°C on the Celsius scale), this energy subsides. However, the particles do not come to a standstill. They are now in quantum-mechanical motion and the aforementioned fuzziness takes over.

Under such quantum-mechanical fluctuations, a material may develop rather peculiar properties. For example, its electrical resistivity may drop to zero, a state called superconducting. In a superconductor, electrical currents flow without energy loss and heat waste - a very useful material property. Although quantum mechanics generally occurs in the realm of low temperatures, such a superconducting state can ensue into higher temperatures, as long as quantum motion dominates over normal motion. Thus, the seemingly abstract low-temperature quantum world can become very relevant to real-life device applications.

How can we access these exotic quantum states? One approach is to measure magnetic, electric or other physical properties as function of temperature and use deviations from normal behavior to draw conclusions about processes going on in the quantum world. Another method is to use high-energy radiation and blow the system into pieces, attempting to derive quantum-mechanical properties from the observed debris. Not surprisingly, the indirectness of these approaches may lead to controversial interpretations of the nature of such a quantum state.

In an experimental and theoretical collaboration involving, among others, the Physikalische Institut (Johann Kroha) at the University of Bonn and the Laboratory of Multifunctional Ferroics (Manfred Fiebig) at the Eidgenössische Technische Hochschule (ETH) Zürich, a much more direct look at the quantum world has been developed. They pluck at the quantum state just slightly with a light pulse at extremely low energy, an energy range denoted as terahertz regime. (Terahertz radiation is used in full-body scanners at airports, for example.) The terahertz light pulse jiggles the quantum state, without destroying its quantum mechanical nature. In return, the system emits a time-delayed terahertz pulse, a visual echo, whose shape and time delay bear direct information about the stability and composition of its quantum state. This method is especially sensitive to the interactions between electrons or between atomic-scale magnets (so-called spins) in a material. Until now, the researchers have applied it to a certain class of materials, called heavy-fermion compounds, where they have been able to uncover peculiar properties of quantum-mechanically dominated phase transitions. However, the method may have much wider use. In a bold transfer from the quantum world to every-day life. It is as if we would shine a camera flash on a person, and minutes later the person emits another light pulse which, for example, carries information about the person's state of health. 

You can read more scientific information on this experiment in the current issue of Nature Physics at

Christoph Wetli, Shovon Pal, Johann Kroha, Kristin Kliemt, Cornelius Krellner, Oliver Stockert, Hilbert von Löhneysen, Manfred Fiebig: Time-Resolved Collapse and Revival of the Kondo State Near a Quantum Phase Transition, Nature Physics (2018), DOI 10.1038/s41567-018-0228-3.


Prof. Dr. Johann Kroha
Tel.: 0228/73-2798


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