Augsburg / Hannover / NIM - Usually, noise in physics means nothing but annoyance to scientists: non-specific signals, interference frequencies and reduced sensitivity. However, physicists at the Universities of Augsburg and Hanover have now discovered, through their studies funded by the Nanosystems Initiative Munich (NIM) that noise, among other things, can be of grand beneficial use in quantum mechanics. 

Physicists usually associate the term noise with problems such as non-specific signals, interference frequencies and reduced sensitivity. Optimally metered noise, however, can even lead to new, otherwise impossible findings. This is shown in a joint study by Prof. Dr. Dr. h. c. mult.Peter Hänggi and Prof. Dr. Peter Talkner from the University of Augsburg (Theoretical Physics) together with the experimental group of Prof. Dr. Rolf Haug from the University of Hannover (Experimental Solid State Physics). Their findings are presented in a recent issue of the professional journal "Nature Physics". 

As part of an experiment-theory collaboration, the physicists have succeeded in amplifying tiny, subthreshold signals with the help of quantum noise so that they become detectable in the first place. An example of ubiquitous subthreshold signals occur in nerve cells. The resting potential then lies just below threshold for triggering a subthreshold stimulus to be processed. Often an assisting tiny impulse is then sufficient to cross the threshold. 

Well-metered noise leads to success 

Recent findings show that this impulse can also come from ambient noise. The technical term for this is stochastic resonance. Similar to the optimal excitation frequency of the classical resonance phenomenon, there exists a certain regime of noise intensity for which the weak signal is amplified optimally. Consequently, not complete silence, but rather well metered noise leads to best measurement results. 

With his group, Hänggi has confirmed the universality of the underlying stochastic resonance theory for various physical and biological systems. Recently, the Augsburg physicists were even able to show cases of stochastic resonance within the world of quantum mechanics possessing their very own laws. 

Experiments in the quantum world 

The Augsburg-Hannover team has now been able to validate the salient theoretical results with the help of Haug and his Hanover team through detailed quantum experiments. A prominent example of quantum physics is "quantum tunnelling," in which a particle overcomes a barrier without applying the classically necessary energy. Using a single-electron tunnelling transistor, they were able to show how the phenomenon of stochastic resonance affects the time-resolved quantum tunnelling of individual electrons. 

The laws of quantum mechanics are only noticeable at very low temperatures, where thermal movement and thus thermal noise are frozen. Accordingly, the physicists carried out their experiments close to absolute zero temperature and took advantage of the noise being intrinsic in quantum mechanics. For this purpose, they applied a minimum gate voltage to a quantum dot, being a three-dimensional structure only a few nanometers in size. They modulated it periodically and could thus generate different noise intensities. 

Electrons tunnel in synchrony 

Normally, the number of electrons that tunnel onto a quantum dot and leave it again fluctuates. However, at an optimally dosed noise intensity the corresponding statistical variance undergoes a significant suppression. The ratio of the variance to the mean value, termed the Fano factor, thus drops to a minimum. Conversely formulated, the result corresponds to a maximum in the signal-to-noise ratio, as observed in stochastic resonance outside the quantum world. 

The scientists were able to influence not only the number of electrons tunnelling per time unit by the intrinsic quantum noise but also their sojourn statistics. Their residence time on the quantum dot could also be synchronised by the periodic gate voltage modulation. This behaviour exhibits itself via characteristic maxima of the time-dependent probability density with which electrons dwell the quantum dot. Such a maxima occur at odd multiples of the half-driving period and constitutes the characteristic feature of a quantum-synchronisation. 
____________________________________ 

Original Publication: 
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 15, 330-334 (2019)
doi.org/10.1038/s41567-018-0412-5 
____________________________________ 

Graphics for this press release: 
http: // idw -online.de/de/image?id=312424 
http://idw-online.de/de/image?id=312425 
____________________________________ 

English version of the corresponding NIM press release: 
http: //www.nano-initiative-munich.de/en/press/press-releases/meldung/n/happy-about-noise/