Droplet Epitaxy Quantum Dots on GaAs (111)A substrates for Quantum Information Applications

Among different research areas connected to widespread use of quantum technology, Quantum Information and Communication is one of the most advanced. The term “Quantum Information” is used for any physical information that is encoded in quantum systems. Quantum Information Processing (QIP) deals with the manipulation of quantum information, to perform tasks, which are unachievable in a classical context, for instance, an absolute secure transmission of information, known as quantum cryptography. Nowadays, widely developed optical fiber networks provided an availability of commercial quantum cryptographic systems, where secure keys are encoded by quantum bits (qubits) – the basic unit of Quantum Information, which refers to the state space of a two-level quantum system. So, the long-term goal of the QIP is a creation of a large quantum network (the Quantum Internet), over which photons will transport quantum information to specific quantum nodes (Quantum Computers), where it would be processed and stored. Coming back to a simpler task of QIP, that is quantum cryptography, there are two main approaches for a quantum key distribution (QKD). The first based on the coding of the quantum state of a single particle and on the principle of the impossibility to distinguish reliably two non-orthogonal quantum states. Thus, security of this approach is based on the theorem prohibiting the cloning of an unknown quantum state. Due to the unitarity and linearity of quantum mechanics, it is impossible to create an exact copy of an unknown quantum state without affecting an initial state. There are many QKD protocols based on the approach, and the general one is BB84 (developed by Charles Bennet and Gilles Brassard in 1984), exploiting quantum states of single photons (for instance, polarization of photons). The second approach is based on the quantum entanglement. Two particles (including those separated in the space) can be in a correlation state, so the measurement of the selected value carried out on one of the particles will determine the result of the measurement of the value on the second particle. Basic QKD protocol exploiting, for instance, pair of polarization-entangled photons is E91 (developed by Artur Ekert in 1991). For example, spherically symmetrical atom emits two photons in opposite directions. These photons have an indefinite circular polarization, but due to the symmetry their polarizations are opposite. The polarizations became known only after the measurement. An interception of one photon of an entangled pair does not give any information, but it is a signal that someone is trying to hack the secure line. At the present time, there are bunch of solid-state sources for generation of single and/or entangled photons. For instance, entangled photon pair sources can be achieved by spontaneous parametric down-conversion (SPDC) in nonlinear crystals, intracavity atomic ensembles, and biexciton (XX) – exciton (X) cascade emission of quantum dots (QDs). Among all these sources, QDs are considered as an ideal one and can be used for both QKD protocols. As a source of single photons, QDs emit highly indistinguishable photons with gigahertz repletion rate on demand. The present work is aimed on the fabrication of QDs on GaAs(111)A substrates. The main focus lies on the popular InAs/InAl(Ga)As QDs obtained by Droplet Epitaxy (DE) technique on vicinal GaAs(111)A, exploiting C3v symmetry of the surface to obtain highly symmetric QDs and using step-flow growth mode of the vicinal surface to have possibility to grow thick buffer layers, especially thick layers of distributed Bragg reflectors (DBRs), for entangled photon emission at telecom band (1.31 – 1.55 μm).