  Strong amplification in integrated photonics is one of the most desired
optical functionalities for computing, communications, sensing, and quantum
information processing. Semiconductor gain and cubic nonlinearities, such as
four-wave mixing and stimulated Raman and Brillouin scattering, have been among
the most studied amplification mechanisms on chip. Alternatively, material
platforms with strong quadratic nonlinearities promise numerous advantages with
respect to gain and bandwidth, among which nanophotonic lithium niobate is one
of the most promising candidates. Here, we combine quasi-phase matching with
dispersion engineering in nanophotonic lithium niobate waveguides and achieve
intense optical parametric amplification. We measure a broadband
phase-sensitive on-chip amplification larger than 45 dB/cm in a 2.5-mm-long
waveguide. We further confirm high gain operation in the degenerate and
non-degenerate regimes by amplifying vacuum fluctuations to macroscopic levels
in a 6-mm-long waveguide, with on-chip gains exceeding 100 dB/cm over 600 nm of
bandwidth around 2 $\mu$m. Our results unlock new possibilities for on-chip
few-cycle nonlinear optics, mid-infrared photonics, and quantum photonics.
