Quantum technologies of the 21st century will have a massive impact on future information processing, communication, sensing, and metrology applications. Key applications include quantum computing and quantum cryptography. A quantum computer exploits the quantum superposition principle and entanglement to solve complex mathematical problems or for simulations intractable for today's supercomputers. Arguably closer to large-scale realization and commercialization is Quantum Key Distribution (QKD), a method to exchange random keys with unconditional security.
Central to QKD is the encoding and transmission of information in individual photons. Due to the Heisenberg uncertainty, the information cannot be readout surreptitiously, and the no-cloning theorem forbids noiseless copying. This, however, limits the maximum communication distance of QKD in fiber networks to several hundreds of kilometers, due to the finite scattering and absorption of photons in optical fibers. Due to the no-cloning theorem, conventional repeater nodes cannot be realized.
Using satellites as relay nodes is a potential way around the distance limitations, as the scattering in the atmosphere above an altitude of 10 kilometers becomes negligible. For such space-to-ground scenarios, where a satellite is equipped with a quantum light source and sending single photons to different ground stations distributed across the world, new quantum technologies need to be developed.
With QUICK³ (which stands for QUantum photonIsChe Komponenten für sichere Kommunikation mit Kleinsatelliten), we develop a single photon source based on a fluorescent defect in the 2D material hexagonal boron nitride and evaluate its functionality in space on a 3U CubeSat. Moreover, the photon source is interfaced with a quantum interferometer with which we can test extended quantum theories in microgravity. As a long-term perspective we are also investigating hybrid systems where we interface the quantum light source with quantum memories.