Narrow-Spectrum Two-Photon Sources for Space-Based Quantum Communication

Entangled photons exhibit the unique property of non-locality ndash; meaning the behavior of one photon is ldquo;dependentrdquo; upon the behavior of another photon, even when the photons are separated by vast differences. Originally created through fragile laboratory experiments, quantum entangled photon sources have matured recently into commercial sources that are rugged and much easier to use. With the growing availability of these commercial sources, new applications have begun to emerge. One such application, quantum key distribution, is the use of quantum entangled photons to create a never-ending supply of unbreakable cryptographic keys for secure satellite-to-satellite communications (see figure at right).To date, commercial entangled photon sources have been used primarily for terrestrial, indoor applications - where unwanted ambient light can be easily controlled. Space-based applications, however, require sources that must operate in the presence of abundant sunlight.One of the major constraints limiting the effectiveness of filtering techniques is the large spectral bandwidth of the entangled photon sources themselves (~1-2nm). This broad spectral output limits the ability of spectral filters to isolate the quantum signal, since the filter must have a comparable bandwidth to avoid reducing the intensity of the source. By developing a source with a narrowed spectral output, much greater isolation of the quantum signal is possible. Toward this goal, we propose to design a bright entangled photon source with a spectral bandwidth 100x narrower (lt;10pm) than todayrsquo;s commercial sources. Evaluating the performance of four different design approaches, we will down-select the design that achieves the greatest improvement. Finally, we will show that this improvement, when combined with existing spatial and temporal filters, can achieve the isolation of quantum photons from natural solar photons required to conduct quantum key distribution between satellites. nbsp;nbsp;