Detecting weak coupling in mesoscopic systems with a nonequilibrium Fano resonance

TL;DR

This paper demonstrates a method to detect weak coupling in mesoscopic systems by observing nonequilibrium Fano resonances in a quantum point contact interferometer, combining experimental results with a theoretical model.

Contribution

It introduces a nonequilibrium Fano resonance technique using coupled quantum point contacts to detect weak coupling in mesoscopic systems, supported by experimental and theoretical analysis.

Findings

Distortions in Fano resonance indicate weak coupling presence.

Experimental results align with the theoretical model.

NEFR provides a sensitive detection method for weak coupling.

Abstract

A critical aspect of quantum mechanics is the nonlocal nature of the wavefunction, a characteristic that may yield unexpected coupling of nominally-isolated systems. The capacity to detect this coupling can be vital in many situations, especially those in which its strength is weak. In this work we address this problem in the context of mesoscopic physics, by implementing an electron-wave realization of a Fano interferometer using pairs of coupled quantum point contacts (QPCs). Within this scheme, the discrete level required for a Fano resonance is provided by pinching off one of the QPCs, thereby inducing the formation of a quasi-bound state at the center of its self-consistent potential barrier. Using this system, we demonstrate a form of \textit{nonequilibrium} Fano resonance (NEFR), in which nonlinear electrical biasing of the interferometer gives rise to pronounced distortions of its Fano resonance. Our experimental results are captured well by a quantitative theoretical model, which considers a system in which a standard two-path Fano interferometer is coupled to an additional, \textit{intruder}, continuum. According to this theory, the observed distortions in the Fano resonance arise \textit{only} in the presence of coupling to the intruder, indicating that the NEFR provides a sensitive means to infer the presence of weak coupling between mesoscopic systems.