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 today’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.
The 2015 NASA Roadmap for Communications, Navigation, and Orbital Debris Tracking and Characterization identifies Quantum Key Distribution and Quantum Communications as Revolutionary Concepts with high payoff if successful. Photon pair sources with narrow spectral profiles address NASA’s roadmap goals by extending the operational range of quantum communication. In addition, a recent NASA workshop on quantum networks identified the need for narrow-band entangled photon sources for coupling to matter-based qubits for quantum repeaters.
The project will be of interest to non-NASA initiatives. The National Quantum Initiative is dedicated to US superiority in quantum information sciences. The Defense Optical Communications Program is a tri-service effort that includes quantum communication as a primary thrust. The Department of Energy’s Quantum Networks for Open Science has a large-scale quantum network as its goal.