Epitaxial GaAs quantum dots: Physics & novel Quantum Devices

Semiconductor quantum dots (QDs) are objects capable of confining electrons in nanometer- sized regions of space. They are therefore also referred to as artificial atoms. QDs may find application in the emerging field of quantum communication and information, where the laws of quantum mechanics are used to speed up certain computation tasks or to encode information in an absolutely secure way. An advantage compared to natural atoms is that QDs can be easily integrated in (quantum) devices, such as light-emitting diodes, which can be mass- produced as current (classical) devices. QDs can be made of different material combinations and most of the research activities have focused on QDs made of Indium-Gallium-Arsenide (shortly InGaAs) material surrounded by a Gallium-Arsenide (GaAs) matrix. The main reason is that the fabrication process for such QDs is well established and their optical quality is very good. However, their performance for some tasks, such as the generation of entangled photons, is still limited. In addition it would be desirable to have entangled photons with wavelengths suitable for transmission in conventional optical fibers and such a goal has not been reached yet with InGaAs QDs. In this project we aim to fabricate and study a novel type of QDs, which consist of GaAs material embedded in Aluminium-Gallium-Arsenide (AlGaAs). In particular we aim to test a new concept to generate entangled photons suitable for transmission in optical fibers. The concept uses an exotic QD-state: a dark-light-hole exciton. This state is difficult to access for InGaAs QDs but can be created in GaAs QDs. As the names suggests, a dark exciton is optically inactive. However - by embedding the QD in an optical resonator (similar to what is usually done for lasers) - we will have the possibility to extract two photons out of the dark exciton. This project will generate new knowledge on the physical properties of GaAs QDs and may lead to new building blocks for future quantum technologies.

Semiconductor quantum dots (QDs) are objects capable of confining electrons in nanometer-sized regions of space. They are therefore also referred to as "artificial atoms". QDs may find application in the emerging field of quantum communication and information, where the laws of quantum mechanics are used to speed up certain computation tasks or to encode information in an absolutely secure way. An advantage compared to natural atoms is that QDs can be easily integrated in (quantum) devices, such as light-emitting diodes, which can be mass-produced as current (classical) devices. QDs can be made of different material combinations and most of the research activities have focused on QDs made of Indium-Gallium-Arsenide (shortly InGaAs) material surrounded by a Gallium-Arsenide (GaAs) matrix. The main reason is that the fabrication process for such QDs is well established and their optical quality is very good. However, their performance for some tasks, such as the generation of entangled photons, is still limited. In this project we have fabricated and studied a relatively new type of QDs, which consist of GaAs material embedded in Aluminium-Gallium-Arsenide (AlGaAs). As envisioned in the original application, we have demonstrated that these dots outperform conventional InGaAs as sources of polarization-entangled photons. To show the potential of GaAs QDs in quantum technologies, we have recently used these semiconductor-based light sources as building blocks for a quantum cryptography line at the campus of the University of Linz.