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Issues of radio electronics

PREPARATION OF ENTANGLED PAIRS OF PHOTONS IN BARIUM BETA BORATE CRYSTAL

O. M. Marchenko, N. N. Litvinova, V. K. Prilipko

Published Oct 20, 2018• 0 citations
Publisher

Abstract

For the last two decades, the focus of scientific research in the field of information transmission and processing has been in such sections of quantum optics as quantum computing and quantum cryptography. These areas of optics rely on the discovery of a special type of radiation - beams with pairs of bound or entangled photons. The experimental work in Russia, devoted to the entangled photons is undeservedly little. Therefore, the proposed work is important from a methodical point of view. Moreover, the experiment is performed on domestic equipment, much cheaper than most foreign analogues. The described system can be used as a prototype for setting up educational demonstration and laboratory works. Therefore, the proposed work will be of interest to researchers and universities involved in quantum optics. As a result of the work done, the attempt to create a source of entangled photons based on a barium beta-borate crystal can be considered successful. It should be added that in a second type of crystal pairs of entangled photons arise not only with respect to directions (and frequencies), but also with respect to two orthogonal polarizations. A similar source makes it possible to set up experiments similar to the Einstein-Podolsky-Rosen thought experience in Bohm’s interpretation. Therefore, it is also suitable for testing the Bell inequality. The article considers the practical importance of the phenomenon of generation of entangled photons for the purposes of quantum cryptography, the phenomenon of spontaneous parametric scattering and the creation of a source of entangled pairs of photons based on it, the experimental system and the results of the experiment on generation of correlated photon pairs, tangled with respect to directions and frequencies, the result of the experiment on observing the correlation in the detection times of pairs of entangled photons.

DOI
10.21778/2218-5453-2018-10-50-57

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For the last two decades, the focus of scientific research in the field of information transmission and processing has been in such sections of quantum optics as quantum computing and quantum cryptography. These areas of optics rely on the discovery of a special type of radiation - beams with pairs of bound or entangled photons. The experimental work in Russia, devoted to the entangled photons is undeservedly little. Therefore, the proposed work is important from a methodical point of view. Moreover, the experiment is performed on domestic equipment, much cheaper than most foreign analogues. The described system can be used as a prototype for setting up educational demonstration and laboratory works. Therefore, the proposed work will be of interest to researchers and universities involved in quantum optics. As a result of the work done, the attempt to create a source of entangled photons based on a barium beta-borate crystal can be considered successful. It should be added that in a second type of crystal pairs of entangled photons arise not only with respect to directions (and frequencies), but also with respect to two orthogonal polarizations. A similar source makes it possible to set up experiments similar to the Einstein-Podolsky-Rosen thought experience in Bohm’s interpretation. Therefore, it is also suitable for testing the Bell inequality. The article considers the practical importance of the phenomenon of generation of entangled photons for the purposes of quantum cryptography, the phenomenon of spontaneous parametric scattering and the creation of a source of entangled pairs of photons based on it, the experimental system and the results of the experiment on generation of correlated photon pairs, tangled with respect to directions and frequencies, the result of the experiment on observing the correlation in the detection times of pairs of entangled photons.
The purpose of this project is to set up a system for creating entangled photon pairs for use in various quantum mechanical experiments, including the testing of Bell’s inequalities which demonstrates the fundamentally indeterministic nature of quantum mechanics. The photons are created by pumping a beta barium borate crystal with 405 nm laser light then, due to a process called type 1 spontaneous parametric down conversion, a single pump photon can be converted into two 810 nm photons in an entangled state. In the same lab, a similar set up has already been achieved using a 474 nm pump laser, but by using a shorter wave length, a higher detection rate can be achieved. Two collimators are set up to collect the down converted photons and send them to two photon counting modules. The collimators need to be positioned precisely to detect any down converted photons because they emerge from the crystal in a narrow beam that is at an angle with respect to the pump beam and only a miniscule fraction of pump photons are down converted. The angle at which the down converted photons emerge is a function of how the crystal is prepared and the polarization orientation of the pump beam. It was calculated using a program by National Institute of Standards and Technology.
In recent years, the use of entangled photons in quantum spectroscopy has gained attention, offering new possibilities for studying materials. In this work, we present an entangled photon source developed for future applications in ultrafast quantum spec- troscopy, particularly in biological research. Entangled photons at visible wavelengths are generated using pulsed type-I spontaneous parametric down-conversion (SPDC) in a Beta-Barium Borate (BBO) crystal, pumped by a 1030 nm laser after third and fourth harmonic generation. Photon detection was performed with an intensified sCMOS cam- era, enabling observation and study of spatial and, after integration of a custom-built spectrograph, spatio-energetic correlations between photon pairs. We optimized detec- tion, imaging, and data processing methods. We believe that our results, combined with the improvements we propose, form a solid foundation for integrating entangled photon pairs into time-resolved optical spectroscopy.
We report a Bell state preparation experiment using polarization-entangled photon pairs which are produced by spontaneous parametric down conversion of a 405 nm pulse laser beam inside two orthogonally oriented 0.1 mm beta-barium borate nonlinear crystals with Type-I phased matching. The degree and phase of all four Bell states are easily tunable. With two-photon interference visibilities in excess of 85%, we observe that the Bell parameter S for any of the four Bell states exceeds the classical limit in the Clauser–Horne–Shimony–Holt form of Bell inequality by five standard deviations.
We report on an ultrabright beamlike source of polarization-entangled photon pairs that is suitable for the task of multiphoton quantum information processing. The photon pairs are generated from a beamlike type-II parametric downconversion process in two adjacently located but 180 degrees rotated beta-barium borate crystals. Approximately 30,000 s(-1) entangled photon pairs are recorded experimentally with only 100 mW pump power. The fidelity of the generated entangled state as compared with a Bell state is measured to be 0.974 with the method of quantum state tomography. With this source, we also obtain a violation of Bell's inequality by 61 standard deviations in just a few seconds.
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.
We report a new two-photon polarization correlation experiment for realizing the Einstein-Podolsky-Rosen-Bohm (EPRB) state and for testing Bell-type inequalities. We use the pair of orthogonally-polarized light quanta generated in Type 2 parametric down conversion. Using 1 nm interference filters in front of our detectors, we observe from the output of a 0.5mm beta - BaB2O4 (BBO) crystal the EPRB correlations in coincidence counts, and measure an associated Bell inequality violation of 22 standard deviations. The quantum state of the photon pair is a polarization analog of the spin-1/2 singlet state.