Abstract
We investigated the high-purity entangled photon-pair generation in five kinds of “non-poled” potassium titanyl phosphate (KTP) isomorphs (i.e., KTiOPO4, RbTiOPO4, KTiOAsO4, RbTiOAsO4, and CsTiOAsO4). The technique is based on the spontaneous parametric down-conversion (SPDC) under Type II extended phase matching (EPM), where the phase matching and the group velocity matching are simultaneously achieved between the interacting photons in non-poled crystals rather than periodically poled (PP) KTPs that are widely used for quantum experiments. We discussed both theoretically and numerically all aspects required to generate photon pairs in non-poled KTP isomorphs, in terms of the range of the beam propagation direction (or the spectral range of photons) and the corresponding effective nonlinearities and beam walk-offs. We showed that the SPDC efficiency can be increased in non-poled KTP isomorphs by 29% to 77% compared to PPKTP cases. The joint spectral analyses showed that photon pairs can be generated with high purities of 0.995–0.997 with proper pump filtering. In contrast to the PPKTP case, where the EPM is achieved only at one specific wavelength, the spectral position of photon pairs in the non-poled KTP isomorphs can be chosen over the wide range of 1883.8–2068.1 nm.
Highlights
Spontaneous parametric down-conversion (SPDC) in second-order (χ(2)) nonlinear optic media has been a practical source of entangled quantum systems, especially due to its high stability [1,2]
The spontaneous parametric down-conversion (SPDC) properties under Type II extended phase matching (EPM) were theoretically investigated in the mm2 biaxial crystal to which the kinds of “non-poled” potassium titanyl phosphate (KTP) isomorphs belong, including PM, group velocity (GV) matchings, effective nonlinearities, and spatial walk-offs between the interacting photons
The heralded-state spectral purities of SPDC output were quantified via joint spectral amplitude (JSA) analyses, and the results show that the purities calculated for Type II EPM with a proper pump filtering are as high as 0.995–0.997
Summary
Spontaneous parametric down-conversion (SPDC) in second-order (χ(2)) nonlinear optic media has been a practical source of entangled quantum systems, especially due to its high stability [1,2]. The PPKTP is still an attractive source of entangled quantum systems with the high spectral purity of photon pairs, Type II SPDC is possible even without PP structures since non-poled KTPs are biaxial birefringent crystals. It is necessary to investigate whether the generation of high-purity photon pairs through Type II EPM is still possible and useful for practical quantum systems, even without the PP structure in KTP. We investigated Type II EPM characteristics of five non-poled KTP isomorphs, in terms of the range of the beam propagation direction (or the spectral range of photon pairs) and the corresponding effective nonlinearities and beam walk-offs. In contrast to the PPKTP case, where the EPM is achieved only at one specific wavelength, the spectral position of a photon pair in the non-poled KTP isomorphs can be chosen over the wide range of 1883.8–2068.1 nm.
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