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Dielectric nanoresonators and metasurfaces for photon pair generation

Subject Area Experimental Condensed Matter Physics
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 407070005
 
Final Report Year 2024

Final Report Abstract

The Project NanoPair aimed at evaluating the potential of nonlinear photonic nanostructures for the generation of entangled two-photon quantum states, so called photon pairs. Photon pairs play an ever-increasing role in new quantum technologies like ultra-secure quantum communication, optical quantum computing or quantum imaging and sensing. Nanostructures are devices with one or more dimensions on the length-scale of the wavelength of light. When fabricated from a material with second-order nonlinearity which allows interaction of three photons, such nanostructures can in principle generate photon pairs by means of spontaneous parametric down-conversion (SPDC). In this process, a high-energy photon spontaneously splits into the sought-after pair of photons, which are potentially quantum entangled. The starting hypothesis of NanoPair was that the ability of nanostructures to control many degrees of freedom of light can be leveraged to generate photon pairs with tailored properties. NanoPair was one of the first projects in the direction of pair-generation in nonlinear nanostructures. Therefore, we had to first develop a fundamental theoretical model describing the quantum process of pair-generation in nanostructures and then proceeded to experimental proofs-ofprinciple. Using our newly developed theoretical framework, we showed that nonlinear nanoresonators can serve as highly tunable source of photon pairs where the degree of entanglement, spectrum and emission direction can be tuned. Notably, we also discovered a second regime of operation where the generation of entangled states in the nanoresonator is protected, leading to the emission of maximally polarization entangled two-photon states over a wide range of pump and photon pair frequencies. Experimentally we demonstrated control of the polarization state from a single gallium arsenide nano-resonator using sum-frequency generation (SFG). Since SFG is the classical reverse process of SPDC, from here the generation of entangled states can be inferred. We then directly measured the generation of maximally entangled quantum states from an ultra-thin 3R-phase molybdenum disulfide (3R-MoS2) crystal. Most interestingly, due to its subwavelength thickness and the particular crystal symmetry of 3R-MoS2, many different entangled states can be generated with the same efficiency. Finally, we designed, fabricated and characterized lithium niobate metasurfaces for SPDC generation. Here, we experimentally showed that resonances formed by nanostructuring the material could shape the SPDC spectrum and enhance the emission. In sum, the findings of NanoPair demonstrate that subwavelength thickness and nanostructured devices are a very promising and versatile new class of quantum state sources. After further optimizations based on these first results, they have the potential to find use as flexibly tunable and very compact photon pair sources in many applications.

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