Most metallic elements have a crystal structure that is either body-centered cubic (bcc), face-centered close packed, or hexagonal close packed. If the bcc lattice is the thermodynamically most stable structure, the close-packed structures usually are dynamically unstable, i.e., have elastic constants violating the Born stability conditions or, more generally, have phonons with imaginary frequencies. Conversely, the bcc lattice tends to be dynamically unstable if the equilibrium structure is close packed. This striking regularity essentially went unnoticed until ab initio total-energy calculations in the 1990s became accurate enough to model dynamical properties of solids in hypothetical lattice structures. After a review of stability criteria, thermodynamic functions in the vicinity of an instability, Bain paths, and how instabilities may arise or disappear when pressure, temperature, and/or chemical composition is varied are discussed. The role of dynamical instabilities in the ideal strength of solids and in metallurgical phase diagrams is then considered, and comments are made on amorphization, melting, and low-dimensional systems. The review concludes with extensive references to theoretical work on the stability properties of metallic elements.

https://perssongroup.lbl.gov/papers/rmp2012-instabilities.pdf

Lattice instabilities in metallic elements

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization- entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices.

https://iopscience.iop.org/article/10.1088/1367-2630/9/9/315/pdf

Triggered polarization-entangled photon pairs from a single quantum dot up to 30 K

We present an overview on approaches currently employed to fabricate advanced quantum dot configurations by epitaxial growth. Widely investigated self-assembled quantum dots, i.e. In(Ga)As/GaAs and (Si)Ge/Si, are first introduced. Different quantum dot structures can be derived from In(Ga)As quantum dots by combining them with in situ etching, by layer stacking or by using them as stressors. Other fabrication methods include droplet epitaxy and multilayer deposition on hole patterned substrates.The combination of bottom–up and top–down methods results in absolute position control of self-assembled quantum dots. We review these 'seeded quantum dot crystals' in detail. Finally, we discuss a promising approach to realize quantum dot crystals with controlled spatial and optical properties.

https://iopscience.iop.org/article/10.1088/0034-4885/72/4/046502/pdf

Advanced quantum dot configurations

A proposed device—an optical analogue of the superconducting Josephson interferometer—might enable detailed studies of the role that dissipation has in strongly correlated quantum-optical systems.

/pdf/the-quantum-optical-josephson-interferometer-2d383p5r5g.pdf

The quantum-optical Josephson interferometer

The recent progress in integrated quantum optics has set the stage for the development of an integrated platform for quantum information processing with photons, with potential applications in quantum simulation. Among the different material platforms being investigated, direct-bandgap semiconductors and particularly gallium arsenide (GaAs) offer the widest range of functionalities, including single- and entangled-photon generation by radiative recombination, low-loss routing, electro-optic modulation and single-photon detection. This paper reviews the recent progress in the development of the key building blocks for GaAs quantum photonics and the perspectives for their full integration in a fully-functional and densely integrated quantum photonic circuit.

/pdf/gaas-integrated-quantum-photonics-towards-compact-and-multi-49y7dv7up5.pdf

GaAs integrated quantum photonics: Towards compact and multi-functional quantum photonic integrated circuits

Lateral quantum coupling between two self-assembled (In,Ga)As quantum dots has been observed Photon statistics measurements between the various excitonic and biexcitonic transitions of these lateral quantum dot molecules display strong antibunching confirming the presence of coupling Furthermore, we observe an anomalous exciton Stark shift with respect to static electric field A simple model indicates that the lateral coupling is due to electron tunneling between the dots when the ground states are in resonance The electron probability can then be shifted to either dot and the system can be used to create a wavelength-tunable single-photon emitter by simply applying a voltage

Quantum light emission of two lateral tunnel-coupled (In,Ga)As/GaAs quantum dots controlled by a tunable static electric field.

Semiconductor quantum dot molecules (QDMs) are systems composed of two or more closely spaced and interacting QDs. QDMs are receiving much attention both as playground for studying coupling and energy transfer processes between "artificial atoms" and as new systems, which substantially extend the range of possible applications of QDs. QDMs can be conveniently fabricated by self-assembly either through chemical synthesis or epitaxial growth. Although QDMs relying on the random occurrence of nearby QDs can be used for fundamental studies, special fabrication protocols must be used to create QDMs with well-defined properties. In this article, we focus on self-assembled QDMs obtained by epitaxial growth and embedded in a semiconductor matrix, which are appealing for the possible realization of quantum gates based on two-level systems defined in QDs. We provide a comprehensive overview of the development and current stage of the research on QDMs composed of vertically (in the growth direction) or laterally (in the growth plane) aligned QDs. The review highlights some recent milestone works and points out the challenges and future directions in the field.

Self-Assembled Quantum Dot Molecules

One of the biggest challenges of nanotechnology is the fabrication of nano-objects with perfectly controlled properties. Here we employ a focused laser beam both to characterize and to {\it in-situ} modify single semiconductor structures by heating them from cryogenic to high temperatures. The heat treatment allows us to blue-shift, in a broad range and with resolution-limited accuracy, the quantized energy levels of light and charge carriers confined in optical microcavities and self-assembled quantum dots (QDs). We demonstrate the approach by tuning an optical mode into resonance with the emission of a single QD and by bringing different QDs in mutual resonance. This processing method may open the way to a full control of nanostructures at the quantum level.

https://arxiv.org/pdf/cond-mat/0610074.pdf

{\it In-situ} Laser Microprocessing at the Quantum Level

Epitaxial self-assembled quantum dots (QDs) are commonly obtained by the Stranski-Krastanow (SK) growth mode, in which QDs form on top of a thin two-dimensional (2D) wetting layer (WL) In SK QDs, the properties of the WL such as thickness and composition are hard to control independently of those of the overlying QDs We investigate here strain-free GaAs/AlGaAs QDs located under a GaAs quantum well (QW), analogous to the WL in SK QDs The thickness of such a QW can be arbitrarily controlled, allowing the optical properties of the QDs to be tuned without modifying the QD morphology and/or composition By means of single-QD photoluminescence spectroscopy, we observe well-resolved excited-state shell structures with intershell spacing increasing monotonically with decreasing QW thickness This behavior is well reproduced by eight-band $k\ensuremath{\cdot}p$ calculations combined with the configuration-interaction model taking the realistic QD morphology as input Furthermore, for the thinnest GaAs layer investigated here, no QW emission is detected, indicating that it is possible to suppress the two-dimensional layer usually connecting QDs Finally, we find that all recombination involving an electron-hole pair in the ground state, including the positive trion, occurs at the low-energy side of the neutral exciton emission This behavior, previously observed for GaAs/AlGaAs QWs, is a consequence of the large lateral extent of the QDs, and hence of pronounced self-consistency and correlation effects

Self-assembled quantum dots with tunable thickness of the wetting layer: Role of vertical confinement on interlevel spacing

We tune the emission energy of self-assembled InAs∕GaAs(001) quantum dots (QDs) by partial GaAs capping and annealing. During the annealing step, the surface above the QDs flattens substantially. The existence of In-rich cores below the thin GaAs cap is directly probed by using in situ selective etching. The blueshift of the QD emission energy, resulting from the reduction of the QD height, is tuned by increasing the annealing time or by reducing the capping layer thickness. For long annealing times, the ensemble photoluminescence (PL) displays a multipeak behavior which is attributed to monolayer fluctuations of the QD height. Single-QD micro-PL shows that the quality of the QDs can be improved by desorbing the In accumulated around the QDs by performing the annealing at high substrate temperatures.

/pdf/structural-and-optical-properties-of-in-ga-as-gaas-quantum-ryg7jz0vq5.pdf

Lijuan Wang

Papers

Quantum light emission of two lateral tunnel-coupled (In,Ga)As/GaAs quantum dots controlled by a tunable static electric field.

Self-Assembled Quantum Dot Molecules

{\it In-situ} Laser Microprocessing at the Quantum Level

Self-assembled quantum dots with tunable thickness of the wetting layer: Role of vertical confinement on interlevel spacing

Structural and optical properties of In(Ga)As∕GaAs quantum dots treated by partial capping and annealing