Single Photon Generation

The  Amann group has long-standing experience in the development of Vertical Cavity Surface Emitting Lasers (VCSELs). This group plans to introduce semiconductor and dielectric nanostructures into such VCSELs in order to manage the resonator properties, to control noise and spontaneous emission and to achieve wavelength-tunable single-mode emission in a predefined polarisation and with widely adjustable beam profiles. Such devices are of interest for information processing and allow basic research on light-matter interaction and fluctuation phenomena in nanophotonic sources. In addition to standard photonic crystal structures particular emphasis will be on annular resonator structures because of their superior waveguiding properties and on sub-wavelength surface profiles. These activities may be extended to develop and characterize resonant cavity structures and to combine the nanophotonic resonators with electronically quantized systems such as QD active zones in order to investigate single-photon emitters and coupling mechanisms between each individual electronic and each (or very few) optical state(s). These activities will strengthen the already existing interdisciplinary cooperation between physics oriented groups (Abstreiter, Finley, Vogl) and the technology-oriented Amann group.

Semiconductor QDs show quantum-optical properties very much analogous to those of atoms in a dilute gas. Unlike atoms however they are imbedded in a solid-state matrix and are therefore spatially localized. This makes their optical addressability straightforward. In addition their charge and spin can be tuned electrically. Both features make QDs prime candidates for designing solid-state quantum-optical devices.

 Karrai’s group will concentrate in particular on novel and challenging optical manipulation of the charge-spin properties It is planned to use microwave pulses in conjunction with high-resolution laser spectroscopy to control and prepare the QD in a particular spin state. Similarly a control of the spin states can be also obtained using two-color laser spectroscopy via optical spontaneous emission with spin-flip. Such an all-optical control of the spin state is an important step towards devices for quantum information processing.

The  Finley group and  Abstreiter group are focusing on solid state-based quantum optics and coherent manipulation of charges, spins and photons using semiconductor nanostructures. One example is the fabrication and optical study of nanoscale photonic devices, aiming at the control of light-matter coupling at the single photon - single emitter level. Individual semiconductor QD nanostructures will be embedded within high finesse, 0D solid state cavities formed by introducing point defects into photonic crystals defined in optically thin semiconductor membranes. The frequency detuning between the QD exciton and cavity mode will be controlled using electro-optical and MEMS techniques to probe the light-matter coupling in both the weak (irreversible) and strong (reversible) coupling regimes. Achieving strong coupling for a single QD in a tunable solid-state optical cavity will provide widespread potential applications in quantum information science and “few photon” photonics. Furthermore, electrons with a well defined spin can be optically or electrically injected into the QD and detected optically with high external efficiency via the polarization of the emitted photon, thus providing the potential to test quantum correlations and spin entanglement in few electron dots.

The  Wixforth group has pioneered the investigation and application of surface acoustic waves (SAW) for controlling photons and charge in semiconductor nanostructures in recent years. This expertise will be used to realize novel single photon sources. Acoustically driven single electron transport through a lateral constriction leads to a quantized acousto electric current. Such constrictions have been successfully realized by a lateral split gate geometry or – more recently also by a semiconducting carbon nanotube. Single electron transport through a carbon nanotube is planned to be combined with a lateral pn junction within the same constriction. Single electrons within the confining potential of the SAW enter the p region at the speed of sound and emit single photons at a rate given by the SAW frequency. A single photon source can also be realized using the dynamic piezoelectric potential of SAW to dissociate optically generated electron-hole pairs followed by a bipolar transport towards a single quantum dot. Recombination of the laterally separated electrons within the quantum dot leads to an acoustically driven photon train. The goal is the demonstration of a SAW driven single photon source.

Another approach to control single photon emission is pursued by the group of  Weinfurter, which has performed pioneering work in the area of quantum cryptography in the past. In this project it is planned to use color centers in diamond to realize on-demand single photon emission. Their high photo-stability at room temperature may allow the achievement of simple and robust devices, ideal for practical applications.