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Journal of the Korean Physical Society

Dispersive Broadening of Two-photon Wave Packets Generated via Type-I and Type-II Spontaneous Parametric Down-conversion

Kang-Hee Hong, So-Young Baek, Osung Kwon, Yoon-Ho Kim

Published Dec 1, 2018• 4 citations
Publisher

Abstract

Photons generated via spontaneous parametric down-conversion (SPDC) have broad spectrums and suffer from dispersive broadening of the temporal wave packets when they are transmitted through dispersive media. In this paper we theoretically and experimentally study the detailed amount of the temporal broadening of the two-photon wave packets generated via both type-I and type-II SPDC with β -BaB 2 O 4 of various lengths, by transmitting them through optical fibers. We interpret the results with respect to the spectral properties of the two-photon wave packets. We believe that our results will contribute to implementing protocols involving long-range distributions of photon pairs.

DOI
10.3938/jkps.73.1650

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Photons generated via spontaneous parametric down-conversion (SPDC) have broad spectrums and suffer from dispersive broadening of the temporal wave packets when they are transmitted through dispersive media. In this paper we theoretically and experimentally study the detailed amount of the temporal broadening of the two-photon wave packets generated via both type-I and type-II SPDC with β -BaB 2 O 4 of various lengths, by transmitting them through optical fibers. We interpret the results with respect to the spectral properties of the two-photon wave packets. We believe that our results will contribute to implementing protocols involving long-range distributions of photon pairs.
The generation of two-photon wave packets, produced by spontaneous parametric down-conversion in crystals with linearly chirped quasi-phase-matching grating, is analyzed. Although they are spectrally broad, two-photon wave packets produced in this way are not Fourier-transform limited. We discuss the temporal compression of the wave packets, exploiting the insertion of a standard optical fiber in the path of one of the two photons. The effect is analyzed by means of full numerical calculation, and the exact dispersion dependencies in both the crystal and the fiber are considered. The study paves the way for practical realization of this idea.
One-photon and two-photon wave packets of entangled two-photon states in spontaneous parametric downconversion (SPDC) fields are calculated and measured experimentally. For type II SPDC, measured one-photon and two-photon wave packets agree well with theory. For type I SPDC, the measured one-photon wave packet agrees with the theory. However, the two-photon wave packet is much bigger than the expected value, and the visibility of interference is low. We identify the sources of this discrepancy as the spatial filtering of the two-photon bandwidth and nonpair detection events caused by the detector apertures and the tuning-curve characteristics of the type I SPDC.
The generation of biphoton wave packets with a broad spectrum owing to quasi-phase-matched spontaneous parametric down-conversion in a crystal with an aperiodic poling has been considered. Biphotons at the exit of such a sample are not Fourier limited; i.e., their correlation time is much larger than the inverse width of the spectrum. It has been shown that if the inverse grating vector varies linearly along the sample length, the generated biphoton wave packets can be compressed in time by transmitting two-photon light through an optical fiber.
Single-photon sources are crucial components for the implementation of quantum communication protocols. However, photons emitted by some of the most popular types of realistic sources are spectrally broadband. Due to this drawback, the signal emitted from such sources is typically affected by the effect of temporal broadening during its propagation through telecommunication fibers which exhibit chromatic dispersion. Such problem can be observed e.g. when using sources based on the process of spontaneous parametric down-conversion (SPDC). In the case of long-distance quantum communication temporal broadening can significantly limit the efficiency of temporal filtering. It is a popular method, which is based on the reduction of the duration time of the detection window, used for decreasing the number of registered errors. In this work we analyzed the impact of the type of spectral correlation within a pair of photons produced by the SPDC source on the temporal width of those photons during their propagation in dispersive media. We found out that in some situations the width can be decreased by changing the typical negative spectral correlation into positive one or by reducing its strength. This idea can be used to increase the efficiency of the temporal filtering method. Therefore it can be applied in various implementations of quantum communication protocols. As an example of the application we subsequently analyzed the security of a quantum key distribution (QKD) scheme based on single photons. It was realized in the configuration with the source of photons located in the middle between the legitimate participants of a QKD protocol (called typically Alice and Bob). We demonstrated that when the information about the emission time of the photons produced by the SPDC source is not distributed to Alice and Bob, the maximal security distance can be considerably extended by using positively correlated photons, while in the opposite case strongly (no matter positively or negatively) correlated photons are optimal. We also found out that the results of our calculation may be very sensitive to the spectral widths of the photons produced by the SPDC source. In addition, we concluded that in realistic situation Alice and Bob would have to optimize their source over both the spectral widths of the generated photons and the type of spectral correlation in order to maximally extend the security distance. The results of our work are, in particular, important for the QKD systems utilizing commercial telecom fibers populated by strong classical signals. In those systems temporal filtering method can be used to reduce not only the dark counts registered by the detection system, but also the channel noise originating from the process of Raman scattering, which is the main factor limiting their performance.