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Tuesday, 05 March, 2019

Happy about noise

Stochastic resonance stresses signals in the quantum world

White noise signal (Wikipedia)

White noise signal (Wikipedia)

Noise for physicists means nothing but trouble: unspecific signals, interfering frequencies and low measurement sensitivity. Scientists from the Universities of Augsburg and Hannover now discovered that noise can be extremely useful – also in quantum mechanics.

For physicists the term „noise” is associated with problems as unspecific signals, interfering frequencies and low measurement sensitivity. But optimally dosed noise can even lead to new findings which would not have been possible otherwise. This fact was demonstrated by a joint study of Prof. Peter Hänggi and Prof. Peter Talkner from the University of Augsburg (Theoretical Physics) with the group of Prof. Rolf Haug from the University of Hannover (Experimental Solid State Physics). They present their results in the current issue of the scientific journal Nature Physics.

The scientists combined experiments and theory and were able to amplify and disclose extremely weak signals below the detecting threshold only by means of noise. One example for such weak signals can be found in processing information in neurons. Their resting potential lies just below the threshold where the action potential and thereby an impulse is triggered. Often a small impulse is sufficient to exceed the threshold.

Optimally dosed noise for best results

Recent findings demonstrate that this impulse can also arise from noise. The technical term is stochastic resonance. Similar to the ideal excitation frequency for classical resonance phenomena there exists a special noise intensity which amplifies a signal optimally. So best results can be obtained not by silence but optimally dosed noise. With his group Prof. Hänggi could prove the generality of the underlying theory for various physical and biological systems. Recently the scientists discovered that stochastic resonance even occurs in the world of quantum mechanic.

Experiments in the Quantum World

The physicists from Hannover now verified these theoretical results via experiments. A prominent example of quantum physics is quantum tunneling, where a quantum particle is able to pass through a potential barrier without requiring the classically needed energy. Using a single-electron transistor they could demonstrate the effect of stochastic resonance phenomena on the time-resolved quantum tunneling of single electrons.

The laws of quantum mechanics can only be revealed at very low temperatures when every thermal movement and thereby the thermal noise are frozen. For this reason the physicists performed their experiments at temperatures close to absolute zero by exploiting the intrinsic noise of quantum mechanics. They applied a minimal gate voltage to a quantum dot, a three-dimensional structure of only a few nanometers of size. This voltage could be modulated periodically in time and allowed to generate various intensities of noise.

Electrons tunnel to the beat

In general the number of electrons tunneling ON and OFF the quantum dot fluctuates. But at a special intensity of noise this variance was suppressed significantly. Thus the ratio of the variance to the average value – the so-called Fano-factor – dropped down on a minimum. Conversely the result correlates with a maximum of the signal-noise ratio, as can be seen for stochastic resonance outside the quantum world.

By exploiting intrinsic quantum noise the physicists could not only influence the number of electrons tunneling per time across the quantum dot. In addition they managed also to synchronize their residence times on the quantum dot via a periodic modulation of the gate voltage. This can be demonstrated by characteristic maxima  of the time-dependend probability density at which the electrons rest on the quantum dot. Such maxima occur at odd-numbered multiples of half the driving period, being the typical characteristic of a quantum synchronisation. (BZ)



Quantum stochastic resonance in an a.c.-driven single electron quantum dot. T. Wagner, P. Talkner, J.C. Bayer, E.P. Rugeramigabo, P. Hänggi. R.J. Haug. Nature Physics (2019), doi.org/10.1038/s41567-018-0412-5


Prof. Dr. Dr. h.c. mult. Peter Hänggi, Prof. Dr. Peter Talkner
Institut für Physik
86135 Augsburg

Opens window for sending emailhanggi(at)physik.uni-augsburg.de



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