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

Quantum entanglement is a physical phenomenon that can be harnessed to increase the sensitivity of sensors, create unbreakable communication security, and enable powerful new computers. Space-based applications, however, require entangled photon sources that must operate in the presence of abundant sunlight. Because quantum techniques require the measurement of single photons, background light poses a formidable challenge. One of the major constraints limiting the effectiveness of filtering techniques is the large spectral bandwidth of the entangled photon sources themselves (~1-2nm). To address this problem, Qubitekk proposed to develop narrowband photon pair sources for space-based quantum communication. The Phase I effort revealed two approaches that are feasible with todayrsquo;s technology and these two will form the basis of the Phase II effort. The first approach is to filter the output of a conventional single-pass downconversion source and to compensate for the reduced output by increasing the pump laser power. The second approach is based on cavity-enhanced downconversion, in which the nonlinear crystal is placed in an optical cavity that has the effect of enhancing emission at resonant wavelengths and suppressing emission elsewhere. Although this approach is more complex, much narrower bandwidths can be achieved, not only offering greater background discrimination but also a source capable of coupling to matter-based qubits. The latter feature is an important element of quantum repeater development. These two approaches will be achieved through the execution of four Technical Objectives: 1) Demonstrate filtered downconversion with strong pump; 2) Conduct studies of filtered downconversion with realistic lighting conditions; 3) Demonstrate cavity-enhanced downconversion; and 4)Demonstrate narrowband photon detection using an etalon filter.