Flying qubits and entangled photons

Abstract

Efficient generation of polarized single photons or entangled photon pairs is crucial for the implementation of quantum key distribution (QKD) systems. Self organized semiconductor quantum dots (QDs) are capable of emitting on demand one polarized photon or an entangled photon pair upon current injection. Highly efficient single-photon sources consist of a pin structure inserted into a microcavity where single electrons and holes are funneled into an InAs QD via a submicron AlOx aperture, leading to emission of single polarized photons with record purity of the spectrum and non-classicality of the photons. A new QD site-control technique is based on using the surface strain field of an AlO x current aperture below the QD. GaN/AlN QD based devices are promising to operate at room temperature and reveal a fine-structure splitting (FSS) depending inversely on the QD size. Large GaN/AlN QDs show disappearance of the FSS. Theory also suggests QDs grown on (111)-oriented GaAs substrates as source of entangled photon pairs.

Semiconductor nanostructures for flying q-bits and green photonics

“…A single semiconductor QD with its discrete deltafunction-like excitonic ground state, allowing for non-resonant excitation, has decisive advantages as compared to any other system e.g. isolated atoms [13][14][15]. Being embedded in a suitable p-i-n structure, an electrically driven high frequency pulsed source of single photons, q-bits on demand, can be realized.…”

“…Figure 8 shows traces of an auto-correlation experiment obtained from a trion (charged exciton) emission. The second-order auto-correlation function value g (2) (0) = 0.05 proves to be an almost ideal antibunching of the single polarized photons, the q-bits [13][14][15].…”

“…Controlled growth of epitaxial layers with extremely low densities of QDs of 10 [13][14][15] cm 2 has indeed been realized on top of oxide apertures localizing single QDs (A) Charge carriers are confined in a semiconductor (red) with a smaller bandgap than the cladding semiconductor (blue). The confinement changes the density of states D(E).…”

“…Combining basic principles of quantum mechanics with most modern information theory allows the introduction of quantum communication schemes, like those first proposed by Bennett and Brassard (BB84 protocol) [13][14][15]. The "Quantum Information Science and Technology Road Map", developed on behalf of the Advanced Research and Development agency (ARDA) of the US government, asks for the development of inexpensive practical sources of single photons of well-defined polarization, called q-bits, for the realization of the BB84 and more advanced protocols and/or for sources of entangled photon pairs.…”

“…It is of decisive importance for applications in quantum communication or spintronic [13][14][15] that in a QD of zincblende structure, the energy levels are spin-degenerate only. Figure 2 shows a QD of pyramidal shape whose structure in the base plane has C 4V symmetry.…”