  Phase sensitive amplifiers (PSAs) have the potential to empower substantial
advances in emerging generations of optical communication systems as well as
classical and quantum on-chip signal processing. The core building block of a
PSA is a nonlinear medium. While the second-order nonlinearity ($\chi^{(2)}$)
is stronger than the third-order nonlinearity ($\chi^{(3)}$), it is used less
often in semiconductors for parametric amplification owing to the challenges of
effectively phase matching the interacting waves as well as two-photon
absorption of the pump. In this work, we demonstrate the successful design,
fabrication, and characterization of the first $\chi^{(2)}$-based semiconductor
PSA using an efficient phase matching approach and a pulsed pump, based on an
aluminium gallium arsenide (AlGaAs) waveguide platform. Non-centrosymmetric
semiconductors such as AlGaAs offer appreciable $\chi^{(2)}$. Such waveguides
also achieve more than one order of magnitude greater pump field confinement
when compared to other materials with large $\chi^{(2)}$ such as Periodically
Poled Lithium Niobate (PPLN). Our AlGaAs PSA achieves an on-chip in-phase gain,
a sensitivity of 0.005 photons per pulse, and approaches theoretical minimal
noise figure (NF) of 0~dB. With the capability of operating on signal states
with sub-single photons per pulse, our PSA could usher in a new era of on-chip
quantum circuits.
