  We report a simple scheme for direct generation of frequency-bin entangled
photon pairs via spontaneous parametric downconversion. Our fabricated
nonlinear optical crystal with two different poling periods can simultaneously
satisfy two different, spectrally symmetric nondegenerate quasi-phase-matching
conditions, enabling the direct generation of entanglement in two discrete
frequency-bin modes. Our produced photon pairs exhibited Hong-Ou-Mandel
interference with high-visibility beating oscillations, a signature of two-mode
frequency-bin entanglement. Moreover, we demonstrate deterministic
entanglement-mode conversion from frequency-bin to polarization modes, with
which our source can be more versatile for various quantum applications. Our
scheme can be extended to direct generation of high-dimensional frequency-bin
entanglement, and thus will be a key technology for frequency-multiplexed
optical quantum information processing.
