Highlights and designs on generation of telecom and VIS/IR degenerate polarization-entangled photons in different platforms

Quantum entanglement is a key resource in a wide range of applications in quantum optics, quantum information and quantum communication (QC). Photonic qubits, where information is encoded in the quantum state of the photons are an ideal choice for some of these applications, due to their speed, robustness and ease of manipulation. In particular, entangled photon pairs, due to their high entanglement purity and relative simplicity of generation, are key elements for many QC protocols. Through spontaneous parametric down-conversion (SPDC), such photon pairs can be generated in all-fiber, wavelength division multiplexed (WDM) waveguide-based sources yielding telecom photons around 1550nm or in free-space-optical (FSO) nonlinear bulk crystals yielding visible/infrared (VIS/IR) photons around 800nm. In this work we will present our experimental highlights for two such sources developed for polarization entanglement. The broadband (>46nm) waveguide-based telecom photon source has a spectral brightness ∼2.5×10⁷ pairs/s/mW/nm with a coincidence-to-accidental ratio (CAR)∼ 1322 that can be employed towards multiuser quantum key distribution. While the narrowband (<0.2nm) free-space, IR photon source has a brightness ~ 2x10⁴ pairs/s/mW with a CAR ~1000 that has potential applications in long-distance FSO-QC systems. Covering these aspects, we will also present on-chip waveguide device designs for direct generation of postselection-free, multiple polarization-entangled biphoton states.