  Optical quantum communication technologies are making the prospect of
unconditionally secure and efficient information transfer a reality. The
possibility of generating and reliably detecting quantum states of light, with
the further need of increasing the private data-rate is where most research
efforts are focusing. The physical concept of entanglement is a solution
guaranteeing the highest degree of security in device-independent schemes, yet
its implementation and preservation over long communication links is hard to
achieve. Lithium niobate-on-insulator has emerged as a revolutionising platform
for high-speed classical telecommunication and is equally suited for quantum
information applications owing to the large second-order nonlinearities that
can efficiently produce entangled photon pairs. In this work, we generate
maximally entangled quantum states in the time-bin basis using lithium
niobate-on-insulator photonics at the fibre optics telecommunication
wavelength, and reconstruct the density matrix by quantum tomography on a
single photonic integrated circuit. We use on-chip periodically-poled lithium
niobate as source of entangled qubits with a brightness of 242 MHz/mW and
perform quantum tomography with a fidelity of 91.9+-1.0 %. Our results,
combined with the established large electro-optic bandwidth of lithium niobate,
showcase the platform as perfect candidate to realise fibre-coupled, high-speed
time-bin quantum communication modules that exploit entanglement to achieve
information security.
