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arXiv:0911.2742

Coupling Remote States through the Continuum: Multi-State Fano Resonances

Y. Yoon, Min-Sung Kang, Takahiro Morimoto, Michio Kida, Nobuyuki Aoki, John L. Reno, Y. Ochiai, Lev G. Mourokh, F. Bird

Published Nov 14, 2009• 0 citations
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Abstract

We demonstrate a fully-tunable multi-state Fano system in which remotely-implemented quantum states interfere with each other through their coupling to a mutual continuum. On tuning these resonances near coincidence a robust avoided crossing is observed, with a distinctive character that confirms the continuum as the source of the coupling. While the continuum often serves as a source of decoherence, our work therefore shows how its presence can instead also be essential to mediate the interaction of quantum states, a result that could allow new approaches to engineer the collective states of nanostructures.

DOI
10.48550/arxiv.0911.2742

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We demonstrate a fully-tunable multi-state Fano system in which remotely-implemented quantum states interfere with each other through their coupling to a mutual continuum. On tuning these resonances near coincidence a robust avoided crossing is observed, with a distinctive character that confirms the continuum as the source of the coupling. While the continuum often serves as a source of decoherence, our work therefore shows how its presence can instead also be essential to mediate the interaction of quantum states, a result that could allow new approaches to engineer the collective states of nanostructures.
Fano resonance is a well-celebrated weak-coupling phenomenon whereby a small interaction between a discrete and a continuum of excited states leads to an interference between the respective two transition amplitudes, resulting in an asymmetric line shape in the excitation spectra. We present the strong-coupling phase of the Fano's model where the interaction is localized in a region $\ensuremath{\xi}$ that is smaller than the collective coupling between the discrete state and the continuum. We explore the behavior of the excitation probability $\ensuremath{\sigma}$ as a function of $\ensuremath{\xi}$, the ratio of the bare transition amplitudes, and the energy of the discrete state. In the strong-coupling phase, the spectral function of the discrete state splits up and produces up to three complete destructive interferences or zeros of $\ensuremath{\sigma}$ along with ultrasharp features. We also find that the discrete state does not have to lie within the continuum to produce the well-known weak-coupling profiles of $\ensuremath{\sigma}$ as long as the collective coupling is strong enough. In addition, we consider a broadened state instead of a continuum and present a method that exactly treats such cases by explicitly including the source of the broadening---a weak coupling to the continuum of environment states---in the model.
Remote coupling between quantum emitters is of great importance for constructing quantum networks. Conventionally, this can be achieved via photon exchange by incorporating the quantum emitters in a waveguide or a large cavity. However, such photonic structures suffer from low quality-factors or large mode volumes, limiting the efficiency of light–matter interaction. Here, we demonstrate remote coupling between two site-controlled semiconductor quantum dot emitters mediated by an optical Fano resonance induced by coupling cavity modes via a continuum waveguide state. Unlike ordinary coupled modes, the Fano mode offers both a spatially extended field and a high local density of optical states at the emitters, enhancing light–matter interaction. This coupling scheme allows the demonstration of mutual excitation between two quantum dots separated in space by > 17 wavelengths. Our approach holds promise for achieving long-distance interaction without compromising interaction efficiency, which is essential for scaling up on-chip integration of quantum networks based on solid-state quantum emitters.
Spectroscopic signatures associated with symmetric Lorentzian and asymmetric Fano line shapes are ubiquitous. Distinct features of Fano resonances in contrast with conventional symmetric resonances have found several applications in photonics such as optical switching, sensing, lasing, and nonlinear and slow-light devices. It is therefore important to effectively generate and control these resonances. In this work, we show through ab initio simulations of coupled light–matter systems that Fano interference phenomena can be realized in a multimode photonic environment by strong coupling to the electromagnetic continuum. Specifically, we show that by effectively enhancing the light–matter coupling strength to the photon continuum in an experimentally feasible way, we can achieve a transition from Lorentzian to Fano lines shapes for both electronic and polaritonic excitations. An important outcome of switching between these spectral signatures is the possibility to control the Purcell enhancement of spontaneous emission alongside electromagnetically induced transparency, which is a special case of Fano resonances. Switching from Fano back to a Lorentzian profile can be achieved by physically reducing the coupling strength to the continuum of modes. Our results hold potential for realizing tunable Fano resonances of molecules and materials interacting with the electromagnetic continuum within multimode photonic environments.
Summary form only given.Fano resonances arise due to quantum mechanical coupling of a discrete state to a degenerate continuum. The line shape in optical absorption spectra is characterized by an intrinsic broadening of the line, strong asymmetry, and a reduced absorption at one side of the peak falling well below the pure continuum level. Intensive theoretical investigations predicted Fano resonances as a general feature of a large variety of low-dimensional semiconductors. We present the observation of Fano resonances in semiconductor superlattices (SL). In SL, discrete excitonic states of the Wannier-Stark ladder are in resonance with subband continua of lower ladder states. The coupling between discrete and continuous states is mediated by Coulomb interaction, which is also responsible for the formation of the discrete excitonic states. As a unique feature of Fano resonances in SL, the Fano coupling strength can be continuously changed in the experiment by tuning the applied electric field. We have also performed time-integrated four-wave-mixing experiments to investigate the influence of Fano coupling on the polarization decay of a Wannier-Stark exciton. We have been able to resolve the polarization decay of a Fano resonance in the time-integrated signal and to follow its changes with field.
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 demonstrate an approach to such detection by making use of one of the most ubiquitous of wave-interference phenomena, namely the Fano resonance. The focus of this joint experimental and theoretical work is on demonstrating a novel form of \textit{nonequilibrium} Fano resonance, that is very different in nature to the usual (quasi-equilibrium) implementations of this phenomenon. The essential idea of our approach is to exploit the strong sensitivity of the Fano resonance to the presence of additional transmission pathways, as a means to identify the coupling of some host system to an \textit{intruder}. Crucially, our nonequilibrium scheme allows us to detect the presence of \textit{hidden} components within some system, even while they remain \textit{invisible} in near-equilibrium transport. For a proof-of-concept demonstration of this phenomenon, we implement an electron-wave interferometer from mesoscopic quantum point contacts. The wave-based origins of the Fano resonance imply, however, that our scheme should be widely realizable across a variety of physical platforms.
The Fano resonance, with a unique asymmetric line shape, is a well-known manifestation of quantum interference with numerous promising applications for light-harvesting devices. Here we report an experimental observation of the Fano effect in an ultracold atom-molecule coupled system realized by photoassociation near a magnetically tunable ddd-wave Feshbach resonance. Our result shows that, compared to ubiquitous single-peak Fano-type line shapes, the coupled Fano resonances feature a two-peak profile. We develop a theory of coupled Fano resonances which agrees very well with the experimental results. This work may open a new perspective for tuning ultracold collisional interaction using the Fano resonance and studying bound states in continuum.