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AIP conference proceedings

Super-radiance and open quantum systems

Alexander Volya

Published Jan 1, 2005• 19 citations
Publisher

Abstract

Quantum wires, loosely bound nuclei, molecules in chemical reactions and exotic narrow pentaquark states are different examples of open quantum mesoscopic systems. The coupling with and through continuum is their common feature. We discuss general properties of quantum systems in the regime of strong continuum coupling, when the mechanism of Dicke super‐radiance changes intrinsic dynamics, signatures of quantum chaos, lifetime of unstable states and reaction cross sections. The examples are shown for various areas of mesoscopic physics.

DOI
10.1063/1.1996889

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Acta Physica Polonica A•Dec 1, 2015•1 citation
Quantum wires, loosely bound nuclei, molecules in chemical reactions and exotic narrow pentaquark states are different examples of open quantum mesoscopic systems. The coupling with and through continuum is their common feature. We discuss general properties of quantum systems in the regime of strong continuum coupling, when the mechanism of Dicke super‐radiance changes intrinsic dynamics, signatures of quantum chaos, lifetime of unstable states and reaction cross sections. The examples are shown for various areas of mesoscopic physics.
This report on progress explores recent advances in our theoretical and experimental understanding of the physics of open quantum systems (OQSs). The study of such systems represents a core problem in modern physics that has evolved to assume an unprecedented interdisciplinary character. OQSs consist of some localized, microscopic, region that is coupled to an external environment by means of an appropriate interaction. Examples of such systems may be found in numerous areas of physics, including atomic and nuclear physics, photonics, biophysics, and mesoscopic physics. It is the latter area that provides the main focus of this review, an emphasis that is driven by the capacity that exists to subject mesoscopic devices to unprecedented control. We thus provide a detailed discussion of the behavior of mesoscopic devices (and other OQSs) in terms of the projection-operator formalism, according to which the system under study is considered to be comprised of a localized region (Q), embedded into a well-defined environment (P) of scattering wavefunctions (with Q + P = 1). The Q subspace must be treated using the concepts of non-Hermitian physics, and of particular interest here is: the capacity of the environment to mediate a coupling between the different states of Q; the role played by the presence of exceptional points (EPs) in the spectra of OQSs; the influence of EPs on the rigidity of the wavefunction phases, and; the ability of EPs to initiate a dynamical phase transition (DPT). EPs are singular points in the continuum, at which two resonance states coalesce, that is where they exhibit a non-avoided crossing. DPTs occur when the quantum dynamics of the open system causes transitions between non-analytically connected states, as a function of some external control parameter. Much like conventional phase transitions, the behavior of the system on one side of the DPT does not serve as a reliable indicator of that on the other. In addition to discussing experiments on mesoscopic quantum point contacts that provide evidence of the environmentally-mediated coupling of quantum states, we also review manifestations of DPTs in mesoscopic devices and other systems. These experiments include observations of resonance-trapping behavior in microwave cavities and open quantum dots, phase lapses in tunneling through single-electron transistors, and spin swapping in atomic ensembles. Other possible manifestations of this phenomenon are presented, including various superradiant phenomena in low-dimensional semiconductors. From these discussions a generic picture of OQSs emerges in which the environmentally-mediated coupling between different quantum states plays a critical role in governing the system behavior. The ability to control or manipulate this interaction may even lead to new applications in photonics and electronics.
Physical examples of open quantum systems include an atom in the bath of photons or phonons, the (hypothetical) quantum computer in noisy background or any macro- or even mesoscopical system consisting of large number of the microscopical constituents whose behavior we can not explicitly describe. We discuss general issues that arise when studying models of such kind - such as decoherence, dissipation and finding the proper set of physical observables. After a brief introduction into the non-Hermitian quantum mechanics we consider a simple model demonstrating how certain kind of interaction can control the above-mentioned properties. In particular, it is explicitly shown how the interaction determines the irreducible set of observables. We also show that the same quantum system, depending on range of its parameters, may exist in two different regimes: quasi-Hermitian and strongly non-Hermitian.
The coupling of an isolated quantum state to a continuum is typically associated with decoherence and decreased lifetime. For coupling rates larger than the bandwidth of the associated continuum, decoherence can be mitigated, and new stable eigenstates emerge. Here, we laser-couple diatomic molecules of highly excited Rydberg atoms, so-called Rydberg macrodimers, to a continuum of free motional states. Enabled by their small vibrational eigenfrequencies, we achieve the regime of strong continuum couplings and observe the appearance of new resonances. We explain the observed spectroscopic features as molecular states emerging in the presence of the light field using a Fano model. For atoms arranged on a lattice, we predict the strong continuum coupling to even stabilize triatomic molecules and find the first signatures of these by observing three-atom loss correlations using quantum gas microscopy. Our results present a mechanism to control decoherence and bind polyatomic molecules using strong light-matter interactions. Published by the American Physical Society 2024
Various manifestations of coherence properties in quantum and classical dynamics in open and closed systems are studied. Among many different issues and phenomena related to coherence, particular aspects are expounded by models chosen from quantum chaos, quantum optics, mesoscopic and high energy physics, and semiclassical relativity. I show how coherence in quantum and classical systems manifests itself in different forms and is enhanced, altered, and suppressed in the presence of chaos, randomness, boundary, and environment. Author's contributions start from the first time discussion of decoherence in quantum cat map and quantum kicked rotor. Wave propagation in stochastic spacetime is considered as that in random media by extending the analogy of spacetime metric with the refractive index of media...
Quantum many-body chaos is described as a practical (theoretical, experimental, and computational) instrument in physics of mesoscopic systems of interacting particles.Using mainly nuclear physics applications, it is shown that interactions of constituents create stationary states of high complexity with respect to the nean-field basis with observable properties smoothly changing along the spectrum.Both local Gaussian orthogonal ensemble type features and the global evolution along the spectrum are used to understand the many-body physics and define thermodynamic properties of isolated mesoscopic objects.Among the examples discussed, especially interesting is a chaotic enhancement of weak perturbations illustrated by a large parity violation in neutron resonances on heavy nuclei.Artificially introduced chaotic elements are used to explore the nuclear landscape and predict phase transformations.