As optical quantum information processing protocols and experiments become increasingly more complex, integrated optics provide a small and robust alternative to traditional bulk optics. Specifically, waveguide technology allows for the creation of bright single-photon sources based on the fact that photon pairs can be created at any location along the waveguide. For our goals, we are working on the characterization of a highly nondegenerate Spontaneous Parametric Down-Conversion (SPDC) waveguide source on a periodically poled KTP (PPKTP) crystal. Our current waveguide source uses type-II phase-matching to create collinear signal and idler photons at 1550 nm and 810 nm, respectively, with the promise of generating simultaneous time-bin and polarization entanglement in future iterations. Our intended source application is for use in quantum key distribution and superdense teleportation protocols between a space platform and collection telescopes on Earth.
We can envision an eventual global multi-node quantum network, with hubs located around the planet. This, however, is still a far reach from current state of the art. Here we discuss some of our approaches to bridge the gap. Specifically, we are pursuing airborne and satellite-based free-space quantum communication. Free-space platforms naturally lend themselves to reconfiguration - likely required by a future quantum-secure network -- as nodes may be easily moved/reoriented to target new nodes. We are implementing a multi-copter drone-based quantum cryptography link, including fast, high-resolution optical stabilization; compact, independent sources; and lightweight single-photon detection. Having access to an agile, reconfigurable QKD networking system will enable quantum cryptography to reach applications prohibited by current approaches, such as temporary networks in seaborne, urban, or even battlefield situations. By using transmitters and receivers at higher altitudes, deleterious effects weather events like fog and turbulence can be mitigated. At longer scale, we are pursuing a quantum link from the International Space Station to earth, which will use hyperentanglement to enable a variety of advanced quantum communication protocols, including multi-bit-per-photon key distribution and "superdense" teleportation. With our table-top experiment we have investigated the effects of loss and turbulence, and demonstrated a system to compensate for the otherwise devastating effect of the Doppler effect from the rapidly moving ISS platform.
Superdense Teleportation (SDT) is a suitable protocol to choose for an advanced demonstration of quantum communication in space. We have taken further steps towards the realization of SDT in such an endeavor. Our system uses polarization and time-bin hyperentanglement via non-degenerate spontaneous parametric downconversion to implement SDT of 4-dimensional equimodular states. Previously, we have shown high fidelity (>90%) SDT implementation and the feasibility to perform SDT on an orbiting platform by correcting the Doppler shift. Here we discuss new analysis of the received state reconstruction performance in the presence of high channel loss and multiple pair events. Additionally, initial characterization of a waveguide-based entanglement source intended for space will be presented.