Entangled photon-pair generation and analysis with lithium niobate-on-insulator quantum photonics

Lithium niobate-on-insulator (LNOI) is an emerging photonics platform with applications in telecommunications, frequency combs and quantum technology. Photon-pairs can be generated in periodically poled LNOI waveguides by spontaneous parametric down-conversion with very high brightness. We fabricate LNOI photonics in-house including periodically poled waveguides, linear optic components such as directional couplers and ring resontators, and phase shifters using the thermo-optic and electro-optic effects.

Here, we generate path and time-bin entangled photon pairs using periodically poled LNOI waveguide. We firstly pump two neighboring SPDC sources to generate a two-photon N00N state on-chip. We verify the purity and fidelity of the state by performing quantum interference with an on-chip phase shifter and Mach-Zehnder interferometer and observe 96.8% visibility and pair generation rates of over 200 MHz/mW.

We next generate time-bin entangled Bell states by pumping a single periodically poled LNOI waveguide with a pair of laser pulses. Time-bin Bell states are an excellent resource for quantum key distribution of fiber networks. We use time separation of 200 ps between the Early and Late time-bins, and we perform quantum state tomography using two tunable Franson interferometers on the same LNOI chip with 200 ps (~3 cm) delay lines. We measure 91.9% fidelity Bell state generation, and quantum interference with 78% visibility, violating Bell’s inequality.