The phase matching for frequency-degenerated parametric fluorescence in tunneling- coupled channel waveguides is analyzed. Type II down conversion with individual photons in each pair appearing in different output channels allows for generation of polarization entangled states well suited for quantum information processing applications.
As optical quantum information processing protocols and experiments become increasingly more complex, integrated optics provide a small and robust alternative to traditional bulk optics. Specifically, waveguide technology allows for the creation of bright single-photon sources based on the fact that photon pairs can be created at any location along the waveguide. For our goals, we are working on the characterization of a highly nondegenerate Spontaneous Parametric Down-Conversion (SPDC) waveguide source on a periodically poled KTP (PPKTP) crystal. Our current waveguide source uses type-II phase-matching to create collinear signal and idler photons at 1550 nm and 810 nm, respectively, with the promise of generating simultaneous time-bin and polarization entanglement in future iterations. Our intended source application is for use in quantum key distribution and superdense teleportation protocols between a space platform and collection telescopes on Earth.
We present an integrated source of polarization entangled photon pairs in the telecom regime, which is based on type II-phasematched parametric down-conversion (PDC) in a Ti-indiffused waveguide in periodically poled lithium niobate. The domain grating - consisting of an interlaced bi-periodic structure - is engineered to provide simultaneous phase-matching of two PDC processes, and enables the direct generation of non-degenerate, polarization entangled photon pairs with a brightness of B = 7 × 10(3) pairs/(s×mW×GHz). The spatial separation of the photon pairs is accomplished by a fiber-optical multiplexer facilitating a high compactness of the overall source. Visibilities exceeding 95 % and a violation of the Bell inequality with S = 2.57±0.06 could be demonstrated.
In the last years the increasing complexity of quantum information protocols triggered the development of integrated waveguide technology for quantum applications [1-3] such that stability is intrinsically obtained thanks to the combination of many optical elements on a monolithic chip and scalability is potentially achievable. In this context the realization of integrated sources of quantum light is of fundamental interest. Some experimental implementations using parametric down conversion in nonlinear materials [4], as well as in semiconductors [5] have been reported. However, in the landscape of integrated devices, the realization of one important source of entangled states is still missing: a source of spectrally indistinguishable photon pairs emitted on two different spatial modes, which share entanglement in the polarization degree of freedom without the need of post-selection. This source is widely adopted in bulk setups [6], indeed entanglement is a fundamental requirement for quantum information purposes and the polarization of photons is the most used degree of freedom for quantum information encoding. However the implementation of such a scheme in integrated waveguides is a challenging task due to the combination of orthogonal polarizations and the need of two different spatial modes.