Viennese Experiment: Quantum noise is different.

22. Apr 2008  — Mögliche Grenzen für den Quantencomputer aufgezeigt Bild links unten: Schema zur Messung des Quantenrauschen in eindimesionalen Vieltelchensystemen: Auf dem Atomchip werden zwei ein-dimensionale Quantensysteme bestehend aus Bose-kondensierten Rb Atomen (Rb-BEC) präpariert. Nach dem Abschalten der Atomfallen expandieren die Atomwolken in radialer Richtung und inerferieren. Das so entstandene Interferenmzmuster wird durch Schattenwurf mit Hilfe einer CCD Kamera photgraphiert. Die Quanteneigenschaften der Atome im BEC werden durch die Interferenz sichtbar, das Quantenrauschen im 1d BEC durch die Form der Welligkeit der beobachteten Interferenzstreifen.

Viennese Experiment: Quantum noise is different.

There is never complete silence. The residual noise gets noticed only by the one who listens carefully.

The group of Prof. Jörg Schmiedmayer of the Technical University of Vienna together with colleagues from Harvard University have now tuned into the noise in an interacting many body systems at the quantum level to determine ultimate boundaries of quantum physics.

In their experiments, the Vienna team measured quantum and thermal excitations of the phase field in a one-dimensional chain of neutral atoms. The atoms have been cooled down to 30 nK, only a few billionth of a degree above absolute zero. At these low temperatures, the atoms loose their individual character and form a so-called Bose-Einstein condensate: they behave like a single gigantic quantum system, stretched to a long string like on a harp.

Although as close to absolute zero temperature as technically possible, a minuscule residual temperature is sufficient to set the phase field of the “quantum string” into motion, creating thermal noise. Additionally, the laws of quantum mechanics prohibit that the string is at complete rest. Quantum noise sets the ultimate limit that can not be overcome.

The theory group of Prof. Eugene Demler at Harvard University has shown that thermal and quantum contributions to the noise play a dominating role at different wave lengths of the string vibrations. By carefully analyzing interference images at different length scales, the fundamental quantum noise could be isolated for the first time.

The joined effort of the Vienna and the Harvard group has shown how fundamental phenomena of physics can be studied when going to extreme temperatures and geometries. Identifying and understanding basic limits of quantum systems are imperative for further applications in metrology or quantum information science.



Nature Physics