Photon-pair production through Spontaneous parametric down-conversion, which is a nonlinear mechanism where a high energy photon decays into 2 low-energy photons, has been identified as an appropriate method for realizing heralded single-photon sources or quantum entanglement. This is particularly important for photonic quantum-information applications. Spontaneous parametric down-conversion in periodically poled nonlinear media allows for collinear propagation.
This method has been commonly referred to as quasi phase matching. Potassium titanyl phosphate, lithium tantalite, and lithium niobate are the popular nonlinear crystals that are suited for periodic poling. Several photon-pair generation applications call for a high spectral quantum purity that is comparable to frequency-uncorrelated photon pairs. Generally, Spontaneous parametric down-conversion states contain frequency entanglement, where when one of the sub-states is traced out, puts the other one into a mixed state.
In a bid to eliminate these correlations in the joint spectral distribution, a number of researchers have opted to use narrow bandpass filters, which transmit only the uncorrelated parts of the spectrum. However, bandpass filtering leads to a significant drop in count rates and heralding efficiency considering that a huge amount of the generated photons is discarded. An alternative method has been adopted where the Spontaneous parametric down-conversion process can be designed carefully such that the generated Spontaneous parametric down-conversion state is a priori frequency uncorrelated.
This approach is only practical in a closed set of cases, that is, particular crystals allow for intrinsically pure Spontaneous parametric down-conversion states only at selected wavelength and polarization configurations. Recently, a number of research works have been implemented where the joint spectral distribution is shaped by a selected custom fabrication of the poled crystals. Reference to the early development stages, this technique is restricted to the very few configurations that are known to be supported by the common nonlinear materials.
Researchers at AIT Austrian Institute of Technology GmbH and University of Vienna in collaboration with Wuhan Institute of Technology presented a comprehensive numerical investigation of five nonlinear materials as well as their features with regards to photon-pair creation via parametric down-conversion. Their research work is published in the journal, Physical Review Applied.
Periodic poling of ferromagnetic nonlinear materials has been identified as a suitable way of generating collinearly propagating photon pairs. Most researchers have adopted potassium titanyl phosphate and lithium niobate crystals for this application. In the current research, the authors provided a comprehensive discussion on the family of potassium titanyl phosphate-isomorphic nonlinear materials, which also included less common CsTiOAsO4, KTiOAsO4, RbTiOAsO4, and RbTiOPO4.
The authors observed that CsTiOAsO4 allowed for the generation of collinear photon pairs with a degenerate telecom wavelength of 1550nm, high spectral purity, and orthogonal polarization, all without ferroelectric periodic poling. The possibility of CsTiOAsO4 being used for the efficient and compact generation of polarization and frequency-entangled photon pairs with superior visibility in the telecom regime was another remarkable feature recorded by the authors.
The study by Fabian Laudenbach and colleagues will be an excellent resource for researchers who use (periodically poled and bulk) nonlinear crystals so as to generate highly performing photon pairs. It will also be helpful in the investigation and manufacture of more exotic non-linear materials.
Fabian Laudenbach, Rui-Bo Jin, Chiara Greganti, Michael Hentschel, Philip Walther, and Hannes Hübel. Numerical Investigation of Photon-Pair Generation in Periodically Poled MTiOXO4 (M=K, Rb, Cs; X=P, As). Physical Review Applied, volume 8,024035 (2017).
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