Polariton Entanglement

Quantum Entanglement is a fascinating and little understood phenomenon that gives quantum physics its peculiar features and empowers quantum information processing. It lies at the heart of current quantum technologies like quantum key distribution as well as future applications in quantum repeaters or quantum enhanced measurement. Entanglement research was instrumental in creating the whole field of quantum information and since then the development of new and better sources of entangled photon pairs has sparked many breakthroughs in quantum communication and beyond. Traditionally, sources of entanglement have been based nonlinear optics in dielectrics. In this project we will exploit a semiconductor effect, the microcavity exciton-polariton (MEP) to facilitate the creation of entanglement. MEPs are half-light half-matter quasiparticles in a microscopic resonator in which light and matter interact very strongly. Due to their ancestry they inherit a peculiar mix of properties, among them a very light effective mass and relatively strong interactions. It is the strong interactions that we can use to create entanglement by arranging the scattering of MEPs that are created by optical excitation into the desired output channel. In our project, we will design and create the appropriate semiconductor nanostructures, characterize the properties of the created MEP pairs and test for their entanglement. MEP entanglement will be useful in a variety of disciplines with one of the most exciting goals being the interaction of entangled photons created from such a source with an MEP Bose-Einstein-Condensate. On the more applied side, MEP based entanglement sources could be developed into electrically pumped, miniaturized devices for the future of quantum communication. These are just two examples of the new and fascinating research directions that such a source of entangled polaritons will be able to unlock.

In our project Entanglement of polaritons we were concerned with so-called exciton-polaritons polaritons for short and their application in creating special states of light. In free space light consists of particles, so-called photons. Traditional light sources, including lasers, emit photons more or less randomly. Various applications in measurement and communication can be improved by using ordered, quantum mechanical states of light instead, for example single photons or entangled photon pairs. When light propagates inside matter it happens that only a fraction of the energy is in the actual light field, while the rest is an excitation of the matter. We can achieve a half-light / half-matter splitting of the energy if we confine the light field between mirrors into a so-called resonator. We use semiconductors for this purpose, which are structured so that both light and the matter particles (electrons) are tightly confined and interact strongly. With the strong interaction we cannot distinguish anymore what is light and what is matter, therefore these new types of particles are called polaritons.In free space two beams of light will cross without any interaction, just as in the case of two flashlight beams. Polaritons behave differently, they inherit from their matter component the ability to collide like billiard balls. We can now exploit these collisions to create the special quantum states of polaritons that were mentioned above. Once the polaritons leave the resonator they become ordinary photons, passing on their special quantum states.In our project we succeeded for the first time world-wide in observing these collisions directly by proving that colliding polaritons are emitted as pairs into particular directions from the semiconductor. It was difficult to measure the effect, because we had to investigate a series of semiconductor structures at temperatures close to the absolute zero and to apply a variety of filtering techniques. Further results include a new and flexible method for controlling the excitation and control light beams, new semiconductor structures based on coupled resonators, which were designed and fabricated, and a new, improved description of the collision processes.Based on our results more work will be required, to actually create entangled polaritons. In all the samples we had available, there were too many unwanted processes. With our newly gained knowledge we are now designing new structures, which should suppress these background processes enough to create entanglement.