  Nanophotonic entangled-photon sources are a critical building block of
chip-scale quantum photonic architecture and have seen significant development
over the past two decades. These sources generate photon pairs that typically
span over a narrow frequency bandwidth. Generating entanglement over a wide
spectral region has proven to be useful in a wide variety of applications
including quantum metrology, spectroscopy and sensing, and optical
communication. However, generation of broadband photon pairs with temporal
coherence approaching an optical cycle on a chip is yet to be seen. Here we
demonstrate generation of ultra-broadband entangled photons using spontaneous
parametric down-conversion in a periodically-poled lithium niobate nanophotonic
waveguide. We employ dispersion engineering to achieve a bandwidth of 100 THz
(1.2 - 2 $\mu$m), at a high efficiency of 13 GHz/mW. The photons show strong
temporal correlations and purity with the coincidence-to-accidental ratio
exceeding $10^5$ and $>$ 98\% two-photon interference visibility. These
properties together with the piezo-electric and electro-optic control and
reconfigurability, make thin-film lithium niobate an excellent platform for a
controllable entanglement source for quantum communication and computing, and
open a path towards femtosecond metrology and spectroscopy with non-classical
light on a nanophotonic chip.
