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CRC Press eBooks

Beamlike Photon Pairs Entangled by a 2 × 2 Fiber

Takayoshi Kobayashi

Published Oct 15, 2025• 0 citations
Publisher

Abstract

Polarization-entangled photon pairs have been widely used as a light source for quantum communication. The polarization-entangled photon pairs are generally obtained at the crossing points of the light cones that are generated from a type-II nonlinear crystal. However, it is hard to pick up the photon pairs coming out from the crossing points because of their invisible wavelength and low intensity. In our previous work, we succeeded in generating polarization-entangled photon pairs by overlapping two light paths for the photon-pair generation. The photon pairs could be entangled in all of the generated photon pairs without clipping the crossing points, even with some difficulty in their alignment to overlap the two light paths. In this paper, we have developed an optical system that generates polarization-entangled photon pairs using a beamlike photon pair, without the difficulty in alignment. The measured results show that the photon pairs generated in the system are entangled in their polarizations.

DOI
10.1201/9781003618997-25

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Polarization-entangled photon pairs have been widely used as a light source for quantum communication. The polarization-entangled photon pairs are generally obtained at the crossing points of the light cones that are generated from a type-II nonlinear crystal. However, it is hard to pick up the photon pairs coming out from the crossing points because of their invisible wavelength and low intensity. In our previous work, we succeeded in generating polarization-entangled photon pairs by overlapping two light paths for the photon-pair generation. The photon pairs could be entangled in all of the generated photon pairs without clipping the crossing points, even with some difficulty in their alignment to overlap the two light paths. In this paper, we have developed an optical system that generates polarization-entangled photon pairs using a beamlike photon pair, without the difficulty in alignment. The measured results show that the photon pairs generated in the system are entangled in their polarizations.
Polarization-entangled photon pairs have been widely used as a light source of quantum communication. The polarization-entangled photon pairs are generally obtained at the crossing points of the light cones that are generated from a type-II nonlinear crystal. However, it is hard to pick up the photon pairs coming out from the crossing points because of their invisible wavelength and low intensity. In our previous work, we succeeded in generating polarization-entangled photon pairs by overlapping two light paths for the photon-pair generation. The photon pairs could be entangled in all of the generated photon pairs without clipping the crossing points, even with some difficulty in its alignment to overlap the two light paths. In this paper, we have developed an optical system which generates polarization-entangled photon pairs using a beamlike photon pair, without the difficulty in alignment. The measured results show that the photon pairs generated in the system are entangled in their polarizations.
Beamlike photon pairs were generated by spontaneous parametric down-conversion using a type-II β-BaB 2 O 4 crystal. A pump laser generated photon pairs when it was transmitted through the crystal and was reflected back into the crystal by a mirror to generate more photon pairs. The photon pairs generated when the pump laser first transmitted through the crystal (first photon pairs) were also reflected back into the crystal to overlap with the light path of the photon pairs generated in the second transmission of the pump laser through the crystal (second photon pairs). We observed interference between the first and second photon pairs modulated with a half period of the wavelength of the photon pairs, which demonstrates two-photon interference using the beamlike photon pairs. The fringe period confirms that the observed interference is not classical interference but quantum two-photon interference. Through rotating the angles of quarter-wave plates in the light paths of the photon pairs, we generated beamlike photon pairs with entangled polarization. The phase between the first and second photon pairs could be tuned by changing the position of mirrors reflecting the pump pulses and photon pairs. The fringes of coincidence counts showed that the beamlike photon pairs have polarization entanglement.
Beamlike photon pairs were generated by spontaneous parametric down-conversion using a type-II $\ensuremath{\beta}$-BaB2{}_{2}2​O4{}_{4}4​ crystal. A pump laser generated photon pairs when it transmitted through the crystal and was reflected back into the crystal by a mirror to generate more photon pairs. The photon pairs generated when the pump laser first transmitted through the crystal (first photon pairs) were also reflected back into the crystal to overlap with the light path of the photon pairs generated in the second transmission of the pump laser through the crystal (second photon pairs). We observed interference between the first and second photon pairs modulated with a half period of the wavelength of the photon pairs, which demonstrates two-photon interference using the beamlike photon pairs. The fringe period confirms that the observed interference is not classical interference but quantum two-photon interference. Through rotating the angles of quarter-wave plates in the light paths of the photon pairs, we generated beamlike photon pairs with entangled polarization. The phase between the first and second photon pairs could be tuned by changing the position of mirrors reflecting the pump pulses and photon pairs. The fringes of coincidence counts showed that the beamlike photon pairs have polarization entanglement.
Parametric down-conversion in a nonlinear crystal-generated photon pair in which photons have orthogonal polarization. The pair was emitted in a cone shape for each polarization and collimated by a lens to be doughnut shaped. The doughnutlike photon pair runs collinear to the pump laser. The group delay and walk-off between the orthogonal polarization photons in the pair were compensated passing through a half-wave plate and a nonlinear crystal. Then, the photon doughnut pair was coupled into a $1\ifmmode\times\else\texttimes\fi{}2$ fiber together with a pump laser, i.e., the intense pump laser was used as a guide for alignment. After the alignment was finished, a cut filter was inserted in the optical path to cut the pump laser transmitting the photon doughnut pair. The coincidence measurement of the output of the $1\ifmmode\times\else\texttimes\fi{}2$ fiber showed that the photon pairs are entangled in polarization.