Single photons are a fundamental element of most quantum optical technologies. The ideal single-photon source is an on-demand, deterministic, single-photon source delivering light pulses in a well-defined polarization and spatiotemporal mode, and containing exactly one photon. In addition, for many applications, there is a quantum advantage if the single photons are indistinguishable in all their degrees of freedom. Single-photon sources based on parametric down-conversion are currently used, and while excellent in many ways, scaling to large quantum optical systems remains challenging. In 2000, semiconductor quantum dots were shown to emit single photons, opening a path towards integrated single-photon sources. Here, we review the progress achieved in the past few years, and discuss remaining challenges. The latest quantum dot-based single-photon sources are edging closer to the ideal single-photon source, and have opened new possibilities for quantum technologies.

High-performance semiconductor quantum-dot single-photon sources.

A review is given of theoretical concepts and experimental results on spontaneous formation of periodically ordered nanometer-scale structures on crystal surfaces. Thermodynamic theory is reviewed for various classes of spontaneously ordered nanostructures, namely, for periodically faceted surfaces, for periodic surface structures of planar domains, and for ordered arrays of three-dimensional coherently strained islands. All these structures are described as equilibrium structures of elastic domains. Despite the fact that driving forces of the instability of a homogeneous phase are different in each case, the common driving force for the long-range ordering of the inhomogeneous phase is the elastic interaction. The theory of the formation of multisheet structures of islands is reviewed, which is governed by both equilibrium ordering and kinetic-controlled ordering. For the islands of the first sheet, an equilibrium structure is formed, and for the next sheets, the structure of the surface islands meets the equilibrium under the constraint of the fixed structures of the buried islands. The experimental situation for the fabrication technology of ordered arrays of semiconductor quantum dots is analyzed, including a discussion of both single-sheet and multiple-sheet ordered arrays.

Spontaneous ordering of nanostructures on crystal surfaces

An asymmetric pair of coupled InAs quantum dots is tuned into resonance by applying an electric field so that a single hole forms a coherent molecular wave function The optical spectrum shows a rich pattern of level anticrossings and crossings that can be understood as a superposition of charge and spin configurations of the two dots Coulomb interactions shift the molecular resonance of the optically excited state (charged exciton) with respect to the ground state (single charge), enabling light-induced coupling of the quantum dots This result demonstrates the possibility of optically coupling quantum dots for application in quantum information processing

https://science.sciencemag.org/content/sci/311/5761/636.full.pdf

Optical Signatures of Coupled Quantum Dots

The self-organization of pyramidal PbSe islands that spontaneously form during strained-layer epitaxial growth of PbSe/Pb1−xEuxTe (x = 0.05 to 0.1) superlattices results in the formation of three-dimensional quantum-dot crystals. In these crystals, the dots are arranged in a trigonal lattice with a face-centered cubic (fcc)–like A-B-C-A-B-C vertical stacking sequence. The lattice constant of the dot crystal can be tuned continuously by changing the superlattice period. As shown by theoretical calculations, the elastic anisotropy in these artificial dot crystals acts in a manner similar to that of the directed chemical bonds of crystalline solids. The narrow size distribution and excellent control of the dot arrangement may be advantageous for optoelectronic device applications.

https://science.sciencemag.org/content/sci/282/5389/734.full.pdf

Self-Organized Growth of Three- Dimensional Quantum-Dot Crystals with fcc-Like Stacking and a Tunable Lattice Constant

The spontaneous emission from an isolated semiconductor quantum dot state has been coupled with high efficiency to a single, polarization-degenerate cavity mode. The InAs quantum dot is epitaxially formed and embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%.

Single-mode spontaneous emission from a single quantum dot in a three-dimensional microcavity.

For InAs Stranski–Krastanov (SK) island growth on GaAs by molecular‐beam epitaxy, we show that the in‐plane island diameter varies exponentially with the growth temperature over the range of 390–540 °C. A transition region in SK growth between isolated island growth and island coalescing is investigated as functions of growth temperature and equivalent InAs layer‐by‐layer monolayer (ML) coverage in order to extend the isolated island regime for quantum confinement applications. InAs islands of 150 A in diameter have been grown. Growth of In0.5Ga0.5As islands indicates an increased 2D epitaxial region before island nucleation and a decreased island concentration compared to growth of InAs islands.

Substrate temperature and monolayer coverage effects on epitaxial ordering of InAs and InGaAs islands on GaAs

We have studied the effects of monolayer coverage, V/III flux ratio and growth rate on the density of three‐dimensional growth induced isolated InAs islands grown on GaAs by molecular‐beam epitaxy. Within the isolated island growth regime, increasing the monolayer coverage increases the InAs island density with only a small increase in island size. Decreasing the V/III flux ratio or decreasing the growth rate increases the island density without changing the average in‐plane island diameter. We have observed island densities that are 80% of the ideal close‐packed island density. We propose a model explaining the island density increase with monolayer coverage; local variations in accumulated strain in the wetting layer vary the point at which local islanding is initiated. As more material is deposited more islands are nucleated and the island density increases. The island density increases with decreasing V/III flux ratio or growth rate by increasing the adatom surface diffusion in the underlying wetting ...

Effects of monolayer coverage, flux ratio, and growth rate on the island density of inas islands on gaas

An apparatus to accurately hold an optical fiber within a commercial fiber optic connector. The connector has a first high precision slice having multiple holes of a first area and a first alignment opening and a second high precision slice having multiple holes of a second area and a second alignment opening. The holes of the first high precision slice are arranged relative to the first alignment opening so that, when the second high precision slice and the first high precision slice are juxtaposed with one another and the first alignment opening and the second alignment opening are aligned, the holes of the first high precision slice and the holes of the second high precision slice will be offset relative to each other and will define an opening having an area less than a smaller of the first area and second area. The opening is capable of closely constraining an optical fiber inserted therethrough. A method of making a fiber optic connector adapted to receive a fiber bearing unit involves coupling at least two high precision pieces, having misaligned holes relative to each other, together coupling the at least two high precision pieces to a low precision piece to form a unit, inserting optical fibers into the unit, terminating the fibers in the holes, and housing the unit within a fiber optic connector housing.

Multi-piece fiber optic component and manufacturing technique

InAs quantum dots layers produced by growth‐induced islanding are vertically stacked with a variable thickness GaAs spacer layer. Using transmission electron microscopy, dense planar arrays of randomly ordered InAs islands are found to be vertically aligned in columns. We have compared single, 2, 5, and 10 layers of InAs islands and the vertical alignment is maintained. Atomic force microscopy shows the island size is unchanged throughout the layering process. 8 K photoluminescence shows a spectral peak shift and linewidth narrowing that we associate with electronic coupling within the columns. Variations in the GaAs spacer region produce spectral changes that are consistent with this coupling.

Structural and photoluminescence properties of growth‐induced InAs island columns in GaAs

Three‐step asymmetric coupled quantum wells have unique excitonic properties, particularly under bias. We demonstrate these properties through the absorption changes in quantum well optical modulators. The samples consisted of p‐i‐n diodes with an active region of 20 coupled wells, each coupled well containing a 50 A GaAs well and a 20 A In0.2Ga0.8As well separated by a 10 A Al0.33Ga0.67As barrier. Analysis of the structure shows that field‐induced enhancement and suppression of electron and hole envelope wave function overlap can be observed through a corresponding increase or decrease in exciton absorption peaks. Our devices showed suppressed absorption with bias for the electron‐heavy hole 1 exciton and enhanced absorption with bias for the electron‐heavy hole 2 exciton. Stress‐related effects on the electron‐light hole 1 exciton are also observed. Absorption change per applied bias is five times lower than at the zero‐field exciton wavelength in quantum well devices utilizing the conventional quantum‐...

Large, low‐voltage absorption changes and absorption bistability in GaAs/AlGaAs/InGaAs asymmetric quantum wells

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