Integrated Sources of Entangled and Indistinguishable Photons

The transformative nature of quantum optics and quantum information processing research has already changed the way we think about nature and its relation to information. Moreover, devices for secure communications have been commercialized based on the laws of quantum physics. Many of the most spectacular developments in the field have to do with quantum states of light. This project will take the production of these states to the next level in terms of miniaturization and integration and aims at an in-depth understanding of the underlying physical processes. In a III-V semiconductor system we will develop and study a quantum optics platform of nonlinear optical elements, single quantum emitters and other waveguide optical elements. We will take advantage of the fact that resonantly excited quantum dots can emit very clean single photons where subse- quent photons are indistinguishable from each other, which is crucial for any use in multi-photon protocols, for example in quantum repeaters. Moreover, we will utilize Bragg-reflection waveguides to facilitate efficient nonlinear conversion of light. The desired process is spontaneous parametric down-conversion, which converts a shorter wavelength pump photon to an entangled pair of pho- tons in the telecommunications wavelength band. Within an on-chip approach we will study and optimize the generation of single and entangled pho- ton pairs by resonantly coupling the output of electrically pumped whispering gallery mode micro- lasers to single quantum dots embedded in nearby micropillar cavities and Bragg-reflection wave- guides. While all of these structures and building blocks have been realized before, no one has achieved the level of integration that we are targeting. Besides technological challenges, there ex- ist a number of exciting physical questions such as an in-depth understanding of the non-linear processes involved in the generation of entangled photon pairs which will be tackled by our project. Combined with passive optical elements our work will set the ground for a quantum optics platform that could revolutionize the way we conduct quantum optics experiments and may in the long run become a new quantum technology. Project description Integrated Sources of Entangled and Indistinguishable Photons Page 1 of 1

Quantum optics and its application in quantum communication and quantum computing attract broad interest because they promise to simplify difficult tasks and advance information and communication technologies as well as sensing and metrology to new levels. In this project, the collaborating groups developed the integration of quantum optical functionality into semiconductor photonic chips and were able to achieve significant progress as reported in several scientific publications. In particular, we worked with so called Bragg-waveguides to confine light in a special way in a semiconductor, so that particles of light - photons - can split into pairs of photons by spontaneous parametric down-conversion. By way of their creation, these photon pairs can be entangled in their polarization or time. The quantum mechanical entanglement results in extremely strongly correlated remote measurement results, which can, for example, be used as shared secret random bit strings in secure encrypted communication. Within the project we managed to manifestly improve the efficiency of photon pair creation and the quality of the achievable entanglement. We characterized the quantum state and could, by a new design, reduce the time difference between the two photons of a pair. In another experiment, we were able to show that we can entangle the two photons in time, which happens to be the most robust way for long-distance transmission through optical fibers. Finally, we were able to produce and test a first device that combines the laser and conversion waveguide in one chip. For this the Würzburg team created a laser that uses quantum dots - tiny semiconductor regions of AlInGaAs as the gain medium. By sending an electrical current through the device it will act as a laser at a near-infrared wavelength, and subsequently convert the laser photons to photon pairs in the telecommunication wavelength range. With some improvements in efficiency this brings us closer to an ideal battery-powered source of entangled photon pairs.