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Optics Express

Entanglement distribution using a biphoton frequency comb compatible with DWDM technology

Rintaro Fujimoto, Tomohiro Yamazaki, Toshiki Kobayashi, Shigehito Miki, Fumihiro China, Hirotaka Terai, Rikizo Ikuta, Takashi Yamamoto

Published Sep 13, 2022• 7 citations
Publisher

Abstract

We demonstrate a distribution of frequency-multiplexed polarization-entangled photon pairs over 16 frequency channels using demultiplexers for the signal and idler photons with a frequency spacing of 25 GHz, which is compatible with dense wavelength division multiplexing (DWDM) technology. Unlike conventional frequency-multiplexed photon-pair distribution by a broadband spontaneous parametric down-conversion (SPDC) process, we use photon pairs produced as a biphoton frequency comb by SPDC inside a cavity where one of the paired photons is confined. Owing to the free spectral range of 12.5 GHz and the finesse of over 10 of the cavity, the generated photons having a narrow linewidth in one channel are separated well from those in the other channels, which minimizes channel cross-talk in advance. The observed fidelities of the photon pairs range from 81 % to 96 % in the 16 channels. The results show the usefulness of the polarization-entangled biphoton frequency comb for frequency-multiplexed entanglement distribution via a DWDM system.

DOI
10.1364/oe.469344

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We demonstrate a distribution of frequency-multiplexed polarization-entangled photon pairs over 16 frequency channels using demultiplexers for the signal and idler photons with a frequency spacing of 25 GHz, which is compatible with dense wavelength division multiplexing (DWDM) technology. Unlike conventional frequency-multiplexed photon-pair distribution by a broadband spontaneous parametric down-conversion (SPDC) process, we use photon pairs produced as a biphoton frequency comb by SPDC inside a cavity where one of the paired photons is confined. Owing to the free spectral range of 12.5 GHz and the finesse of over 10 of the cavity, the generated photons having a narrow linewidth in one channel are separated well from those in the other channels, which minimizes channel cross-talk in advance. The observed fidelities of the photon pairs range from 81 % to 96 % in the 16 channels. The results show the usefulness of the polarization-entangled biphoton frequency comb for frequency-multiplexed entanglement distribution via a DWDM system.
Photonic entanglement distribution with wavelength division multiplexing (WDM) technology is a promising method for increasing capacity in quantum communication. So far, various experiments [1], [2] have been actively conducted by using broadband entangled photon pairs based on spontaneous parametric down-conversion (SPDC) with WDM-compatible spectral filters. However, the separation of broadband photon pairs over the channels inevitably wastes a large amount of photon energy for sufficient suppression of channel crosstalk, which causes photon pairs to have a low brightness. The cavity-enhanced SPDC can be used to overcome this problem. Because the generated photon pairs are gathered into resonant peaks with a spectral spacing determined by the cavity, the joint spectrum of the photon pairs becomes a comb-like structure called a biphoton frequency comb [3–5]. Consequently, by designing the free spectral range (FSR) of cavity to be the same as the channel spacing of the WDM channels, efficient and precise extraction of photon pairs from a biphoton frequency comb to each channel of the WDM system can be achieved [6].
We report the fully telecom compliant generation of photon pairs on a silicon micro-ring resonator chip. The ring cavity is designed such as to demultiplex the emitted frequency-comb like spectrum using standard off-the-shelves telecom 100 GHz dense wavelength division multiplexers (DWDM). We measure energy-time entanglement in different channel pairs with raw visibilities exceeding 97%, which paves the way for silicon photonics based quantum key distribution (QKD) with either increased bit rates, or more users.
We have demonstrated experimentally a nondegenerate polarization-entangled photon-pair distribution in a commercial telecom dense wave-division multiplexing device (DWDM) with eight channels. A promising point of this experiment is that an entangled photon pair is obtained via spontaneous parametric down conversion in a single type-II periodically poled KTiOPO4{}_{4}4​ crystal without postselection. Another promising advantage is that we can actively switch the distribution of the photon pair between different channel pairs in DWDM at will. There is no crosstalk between different channel pairs because of a limited emission bandwidth of the source. Maximum raw visibility of 97.8897.88%\ifmmode\pm\else\textpm\fi{}0.86%97.88 obtained in a Bell-type interference experiment and a Clauser-Horne-Shimony-Holt (CHSH) inequality SSS parameter of $2.63\ifmmode\pm\else\textpm\fi{}0.08$ calculated prove high entanglement of our source. Our work is helpful for building quantum communication networks.
Abstract A frequency-multiplexed entangled photon pair and a high-dimensional hyperentangled photon pair are useful to realize a high-capacity quantum communication. A biphoton frequency comb (BFC) with entanglement can be used to prepare both states. We demonstrate polarization entangled BFCs with over 1400 frequency modes, which is approximately two orders of magnitude larger than those of earlier entangled BFCs, by placing a singly resonant periodically poled LiNbO3 waveguide resonator within a Sagnac loop. The BFCs are demonstrated by measuring the joint spectral intensity, cross-correlation, and autocorrelation. Moreover, the polarization entanglement at representative groups of frequency modes is verified by quantum state tomography, where each fidelity is over 0.7. The efficient generation of a massive-mode entangled BFC is expected to accelerate the increase of capacity in quantum communication.