The generation efficiency of correlated photon pairs from a positive-negative index material heterostructure has been investigated by using a rigorous quantum model of spontaneous parametric down-conversion. The mean number of output photon pairs and the signal-field energy spectrum have been calculated. It is shown that the strong confinements of both the pump and signal fields around the resonance state result in a giant enhancement of the correlated photon-pair generation. The generation rate of the correlated photon pair can be improved by several orders of magnitude in the present structure in comparison with those in the corresponding conventional resonant cavity. This means that the present structure can be applied as a highly efficient potential source for entangled photon pairs.
The generation efficiency of correlated photon pairs from a positive-negative index material heterostructure has been investigated by using a rigorous quantum model of spontaneous parametric down-conversion. The mean number of output photon pairs and the signal-field energy spectrum have been calculated. It is shown that the strong confinements of both the pump and signal fields around the resonance state result in a giant enhancement of the correlated photon-pair generation. The generation rate of the correlated photon pair can be improved by several orders of magnitude in the present structure in comparison with those in the corresponding conventional resonant cavity. This means that the present structure can be applied as a highly efficient potential source for entangled photon pairs.
Efficient Generation of Entangled Photon-Pairs The first experiments with correlated photons have been performed in the context of EPR-Bell experiments on the realistic and local properties of quantum mechanics. The source used there produced pairs of polarization entangled photons from a 2-photon decay of Calcium atoms. The technical requirements of these experiments were high (vacuum systems, stronge dye-lasers, etc.) whereas the efficiency of the source was quite low. An important step forward was the introduction of spontaneous parametric down conversion (SPDC), which has become the most common source in quantum optics for generating correlated or entangled photon pairs. In this process photons of an intense pump laser convert to photon pairs in an optical nonlinear crystal. Conservation of energy and momentum leads to strong correlations between the generated photons. With this kind of two-photon source it was possible to realize or improve many experiments on the foundations of quantum mechanics addressing the EPR-Paradoxon and in the new field of quantum information. But again, more advanced experiments and applications suffer from the limited ef- ficiency of the fluorescence process. Many photon pairs are lost by spatial and spectral filtering, which is necessary to achieve polarization entanglement and long coherence times. Different techniques have been implemented to increase the number of photon pairs using two-crystal arrangements, focusing techniques or periodically poled crystals. Most of these methods have the disadvantage that no entangled photons have been observed. It is the aim of this work to increase the yield and to improve the mode definition of entangled photon pairs generated by resonant enhancement of the pump mode and the fluorescence modes. As a first step a linear cavity for the pump mode was realized. Since the conversion probability is proportional to the pump power it was possible to increase the photon pair count rate by factor of 7 over the previous source. Besides the possibility of further improvement on already established pair correlation experiments, such an enhancement allows to build a compact source for photon pairs, in which an expensive argon-ion laser is replaced by a cheap diode laser. Among other applications such sources are of strong interest for quantum cryptography. 3 In many quantum information experiments optical fibers are use to carry the photons over long distance. Therefore, light from the parametric down-conversion source has to be efficiently coupled into fibers. In the second part we report on a new method to optimize collection efficiency by matching the angular distribution of the parametric fluorescence to the spatial mode of an optical fiber. By using this technique, we detected 366500 polarization-entangled photon pairs per second in the near-infrared region in single-mode optical fibers for 465 mW pump power (at 351.1 nm) with a 2 mm BBOcrystal. The entanglement of the photon pairs was verified by measuring polarization correlations of more than 96% in a HV-basis and in a ±45◦-basis. To our knowledge, such enormous count rates of highly entangled photon pairs have not been reached yet with any other technique. In the third part of this thesis we investigated the process of parametric downconversion in a cavity which is resonant to certain longitudinal down-conversion modes only. The idea of placing the parametric down-conversion source inside a cavity is not new. Such a device is usually referred to as a single or double resonant optical parametric oscillator (OPO) and is mainly used to generate squeezed quantum states. In that kind of application the system is operating close to but still under the threshold of oscillation. In our application the situation is quite different. The system is operating far below threshold so that mainly spontaneous emission occurs. In that mode correlations between single photons can still be observed. But bouncing the light back and forth inside the cavity increases the interaction length and hence enhances the signal levels of the down-conversion fields. Further, by resonating two certain modes only, the bandwidth is reduced by orders of magnitude and the coherence time is found to be inverse proportional to the bandwidth. A similar experiment has already been realized with a type-I parametric down-converter in the resonator. We have tried to realize a compact double resonant OPO far below threshold with a type-II parametric down-converter in a high-finesse cavity to realize a bright source of entangled photon pairs with extremely narrow bandwidth.
The ability to engineer pairs of entangled photons is essential to quantum information science, and generating these states using spontaneous parametric down-conversion (SPDC) in nano- and micrometer-scale materials offers numerous advantages. To properly engineer such sources, a reliable model describing nano- and micrometer-scale SPDC is necessary; however, such a theoretical description remains a challenge. Here, we propose and derive a simplified model to describe SPDC in resonant structures, which considers the generation of photon pairs and the resonant enhancement of spectral bands to be separate processes, even though they actually occur simultaneously. We compare our simplified model to both the rigorous theory of SPDC in an etalon - a simple example of a resonant structure - and our experiments on SPDC in etalons and find agreement for low-gain SPDC. By simplifying the calculations required to generate photon pairs, our model promises to make designing complex resonant structures easier, and it promises to hasten the iteration of designs across the field of quantum state engineering.
Entangled photons are the fundamental resource in quantum information processing. How to produce them efficiently has always been a matter of concern. Here we propose a new way to produce correlated photons efficiently from monolayer WS2 based on bound states in the continuum (BICs). The BICs of radiation modes in the monolayer WS2 are realized by designing the photonic crystal slab-WS2-slab structure. The generation efficiency of correlated photon pairs from such a structure has been studied by using a rigorous quantum model of spontaneous parametric down-conversion with the plane wave expansion method. It is found that the generation efficiency of correlated photon pairs is greatly improved if the signal and idler fields are located at the BICs determined by the inverse scattering matrix of the structure. This is in contrast to the parametric down-conversion process for the enhanced generation of nonlinear waves if the pump field is located at the BICs determined by the scattering matrix of the structure. The generation rate of the correlated photon pairs can be improved by 7 orders of magnitude in some designed structures. The generated quantum signals are sensitive to the wavelength and exhibit narrowed relative line width, which is very beneficial for quantum information processing.
We study the process of spontaneous parametric downconversion in a one-dimensional defective quadratic nonlinear photonic crystal. It is shown that the strong confinements of both the pump and signal waves around the defective layer result in a giant enhancement of the entangled photon-pair generation. An enhancement factor as high as 3.4×106 is obtained in our defective structure based on the dual-localized modes. Furthermore, the linewidth of the downconverted fields is only 0.1 nm. Such a photonic crystal structure can be applied as a highly efficient source of entangled photon pairs for highly integrated all-optical circuits.
Correlated photon pairs generated through spontaneous parametric downconversion have become a key resource for research into enhanced optical measurement and quantum optical information processing. A pump-enhanced χ(2) source of correlated photons in the telecom band is presented. An investigation of the pair photon coincidence detection rate measured from the source, together with the evaluation of system losses, indicated a single spatial fibre-optic mode pair production rate of 6.12 × 104 s−1 mW−1 pump power, an order of magnitude greater than was measured without enhancement. A Hong-Ou-Mandel interference experiment confirmed that the enhancement did not have any adverse effect on the spatial and frequency indistinguishability of the emitted pairs. The source comprises a symmetric and linear enhancement cavity for the pump, making it particularly suitable for generating polarisation entangled photon pairs without the need for compensating crystals 1 Thomas, PJ, Chunnilall, CJ, Stothard, DJM, Walsh, DA and Dunn, MH. 2010. Opt. Express, 18: 26600–26612. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar].
We have theoretically investigated photon-pair generation from transition-metal dichalcogenide (TMDC) monolayers embedded in photonic crystals (PCs) by a rigorous quantum model of spontaneous parametric downconversion. The mean number of output photon pairs and the signal-field energy spectrum have been elaborated. Two different structures are studied: one is a monolayer WS2 embedded in the defective PC, and the other is a stack of WS2 layers in the dielectric multilayer structure. It is clearly shown that an obvious improvement of the generation rate of the correlated photon pairs from the monolayer WS2 can be realized in these two structures, and the enhancement can reach as much as 14 orders of magnitude. Such an effective improvement utilizes the improved field localization at the defect state or the band-edge state, the propagation of photons in the PCs, and coherent superposition; similar phenomena can occur for other TMDC monolayers. These phenomena are very beneficial for the design of optical devices.