Abstract
Integrated optics provides the platform for the experimental implementation of highly complex and compact circuits for quantum information applications. In this context integrated waveguide sources represent a powerful resource for the generation of quantum states of light due to their high brightness and stability. However, the confinement of the light in a single spatial mode limits the realization of multi-channel sources. Due to this challenge one of the most adopted sources in quantum information processes, i.e. a source which generates spectrally indistinguishable polarization entangled photons in two different spatial modes, has not yet been realized in a fully integrated platform. Here we overcome this limitation by suitably engineering two periodically poled waveguides and an integrated polarization splitter in lithium niobate. This source produces polarization entangled states with fidelity of {mathcal F} mathrm{=0.973}pm 0.003 and a test of Bell’s inequality results in a violation larger than 14 standard deviations. It can work both in pulsed and continuous wave regime. This device represents a new step toward the implementation of fully integrated circuits for quantum information applications.
Highlights
In the last decade quantum photonics has played a crucial role in the development of quantum information processes
Two periodically poled nonlinear waveguides designed to generate orthogonally polarized photons via type II parametric down conversion are connected to a zerogap directional coupler acting as a polarizing beam splitter (PBS)
We have demonstrated the realization of a completely integrated source of spectrally degenerate polarization entangled photons emitted into different spatial modes
Summary
In the last decade quantum photonics has played a crucial role in the development of quantum information processes. The confinement of light in a single channel makes it challenging to emit single photons into different spatial modes without the need of postselection This is the case of one of the most adopted sources in quantum information experiments as quantum teleportation,[22,23] quantum key distribution[24,25] and quantum secure direct communication,[26] i.e. a source of spectrally degenerate polarization entangled states.[27,28] While the emission into two different spatial modes is intrinsic in bulk crystals, in integrated waveguides this still remains a challenge and the main reason why a fully on chip source of polarization entangled states was still missing.
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