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18 pages, 564 KiB

Open AccessArticle

Electrons in Quantum Dots on Helium: From Charge Qubits to Synthetic Color Centers

by Mark I. Dykman and Johannes Pollanen

Entropy 2025, 27(8), 787; https://doi.org/10.3390/e27080787 - 25 Jul 2025

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Electrons trapped above the surface of helium provide a means to study many-body physics free from the randomness that comes from defects in other condensed-matter systems. Localizing an electron in an electrostatic quantum dot makes its energy spectrum discrete, with controlled level spacing. The lowest two states can act as charge qubit states. In this paper, we study how the coupling to the quantum field of capillary waves on helium—known as ripplons—affects electron dynamics. As we show, the coupling can be strong. This bounds the parameter range where electron-based charge qubits can be implemented. The constraint is different from the conventional relaxation time constraint. The electron–ripplon system in a dot is similar to a color center formed by an electron defect coupled to phonons in a solid. In contrast to solids, the coupling in the electron on helium system can be varied from strong to weak. This enables a qualitatively new approach to studying color center physics. We analyze the spectroscopy of the pertinent synthetic color centers in a broad range of the coupling strength. Full article

(This article belongs to the Special Issue From Order to Disorder: Superfluidity, Stochastic Processes, and the Dynamics of Life—Dedicated to Professor Peter McClintock on the Occasion of His 85th Birthday)

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17 pages, 2106 KiB

Open AccessArticle

Informational Entropy Analysis of Artificial Helium Atoms

by Marcilio N. Guimarães, Rafael N. Cordeiro, Wallas S. Nascimento and Frederico V. Prudente

Atoms 2025, 13(5), 42; https://doi.org/10.3390/atoms13050042 - 12 May 2025

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Abstract

We use the Shannon informational entropies as a tool to study the artificial helium atom, namely, two electrons confined in a quantum dot. We adopt configurations with spherical and cylindrical symmetries for the physical system of interest. Using the informational quantities, we analyze the effects of electronic confinement, we validate the entropic uncertainty relation, we identify that the Coulomb interaction potential between the electrons is no longer important for strong confinements, and we indicate/predict the avoided crossing phenomena. Finally, we carried out a density function analysis. When available, the results are compared with those in the literature. Full article

(This article belongs to the Section Atom Based Quantum Technology)

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25 pages, 16826 KiB

Open AccessArticle

Compositional and Structural Modifications by Ion Beam in Graphene Oxide for Radiation Detection Studies

by Mariapompea Cutroneo, Lorenzo Torrisi, Letteria Silipigni, Alena Michalcova, Vladimir Havranek, Anna Mackova, Petr Malinsky, Vasily Lavrentiev, Pavol Noga, Jozef Dobrovodsky, Petr Slepicka, Dominik Fajstavr, Lucio Andò and Vaclav Holy

Int. J. Mol. Sci. 2022, 23(20), 12563; https://doi.org/10.3390/ijms232012563 - 19 Oct 2022

Cited by 6 | Viewed by 2242

Abstract

In the present study, graphene oxide foils 10 μm thick have been irradiated in vacuum using same charge state (one charge state) ions, such as protons, helium and oxygen ions, at the same energies (3 MeV) and fluences (from 5 × 10¹¹ ion/cm² to 5 × 10¹⁴ ion/cm²). The structural changes generated by the ion energy deposition and investigated by X-ray diffraction have suggested the generation of new phases, as reduced GO, GO quantum dots and graphitic nanofibers, carbon nanotubes, amorphous carbon and stacked-cup carbon nanofibers. Further analyses, based on Rutherford Backscattering Spectrometry and Elastic Recoil Detection Analysis, have indicated a reduction of GO connected to the atomic number of implanted ions. The morphological changes in the ion irradiated GO foils have been monitored by Transmission Electron, Atomic Force and Scanning Electron microscopies. The present study aims to better structurally, compositionally and morphologically characterize the GO foils irradiated by different ions at the same conditions and at very low ion fluencies to validate the use of GO for radiation detection and propose it as a promising dosimeter. It has been observed that GO quantum dots are produced on the GO foil when it is irradiated by proton, helium and oxygen ions and their number increases with the atomic number of beam gaseous ion. Full article

(This article belongs to the Special Issue Carbon-Based Nanomaterials 4.0)

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15 pages, 2050 KiB

Open AccessArticle

Luminescence from Droplet-Etched GaAs Quantum Dots at and Close to Room Temperature

by Leonardo Ranasinghe, Christian Heyn, Kristian Deneke, Michael Zocher, Roman Korneev and Wolfgang Hansen

Nanomaterials 2021, 11(3), 690; https://doi.org/10.3390/nano11030690 - 10 Mar 2021

Cited by 5 | Viewed by 3249

Abstract

Epitaxially grown quantum dots (QDs) are established as quantum emitters for quantum information technology, but their operation under ambient conditions remains a challenge. Therefore, we study photoluminescence (PL) emission at and close to room temperature from self-assembled strain-free GaAs quantum dots (QDs) in refilled AlGaAs nanoholes on (001)GaAs substrate. Two major obstacles for room temperature operation are observed. The first is a strong radiative background from the GaAs substrate and the second a significant loss of intensity by more than four orders of magnitude between liquid helium and room temperature. We discuss results obtained on three different sample designs and two excitation wavelengths. The PL measurements are performed at room temperature and at T = 200 K, which is obtained using an inexpensive thermoelectric cooler. An optimized sample with an AlGaAs barrier layer thicker than the penetration depth of the exciting green laser light (532 nm) demonstrates clear QD peaks already at room temperature. Samples with thin AlGaAs layers show room temperature emission from the QDs when a blue laser (405 nm) with a reduced optical penetration depth is used for excitation. A model and a fit to the experimental behavior identify dissociation of excitons in the barrier below T = 100 K and thermal escape of excitons from QDs above T = 160 K as the central processes causing PL-intensity loss. Full article

(This article belongs to the Special Issue Quantum Nanostructures by Droplet Epitaxy for Optoelectronics and Quantum Information Technologies)

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