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arXiv:2511.16085 · Optics Express

Channel-selective frequency up-conversion for frequency-multiplexed quantum network

Shoichi Murakami, Shunsuke Hiraoka, Toshiki Kobayashi, Takashi Yamamoto, Rikizo Ikuta

quant-phSubmitted Nov 20, 2025• Announced Nov 21, 2025• 1 citation• Revised Nov 21, 2025 (v2)
v1 · Nov 20, 2025v2 · Nov 21, 2025
arXivHTMLPDF

Abstract

We demonstrate channel-selective frequency up-conversion from telecom wavelengths around 1540 nm for optical fiber communication to visible wavelengths around 780 nm, based on second-order optical nonlinearity in a cavity of the converted modes. In our experiment, we selectively convert a light from any frequency mode within frequency-multiplexed telecom signals to a desired output mode, determined by the cavity resonances. Based on the experimental results of the frequency up-conversion, we derive the signal-to-noise ratio of the process at the single-photon level, and discuss its applicability to channel-selective quantum frequency conversion (CS-QFC) in the context of frequency-multiplexed quantum networks. Finally, we describe specific use cases of the CS-QFC, which show its utility as a reconfigurable switching element in frequency-multiplexed networks, particularly for selectively performing Bell-state measurements between two photons originating from different frequencies.

Journal reference
Optics Express 34, 8724-8739 (2026)
DOI
10.1364/OE.585432

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We demonstrate channel-selective frequency up-conversion from telecom wavelengths around 1540 nm for optical fiber communication to visible wavelengths around 780 nm, based on second-order optical nonlinearity in a cavity of the converted modes. In our experiment, we selectively convert a light from any frequency mode within frequency-multiplexed telecom signals to a desired output mode, determined by the cavity resonances. Based on the experimental results of the frequency up-conversion, we derive the signal-to-noise ratio of the process at the single-photon level, and discuss its applicability to channel-selective quantum frequency conversion (CS-QFC) in the context of frequency-multiplexed quantum networks. Finally, we describe specific use cases of the CS-QFC, which show its utility as a reconfigurable switching element in frequency-multiplexed networks, particularly for selectively performing Bell-state measurements between two photons originating from different frequencies.
Dense wavelength division multiplexing (DWDM) is a key technology for realizing high-capacity and flexible quantum communication networks. To enable the emerging quantum internet, quantum frequency conversion is also essential for bridging different quantum systems over optical fiber networks. In this work, aiming at a frequency-multiplexed quantum internet, we present a channel-selective quantum frequency conversion (CS-QFC) based on difference frequency generation in a PPLN waveguide, which has been widely used in conventional QFC experiments. The CS-QFC converts a frequency of a photon at a single frequency to any frequency in the DWDM channels by dynamically selecting one of the pump lights. In the experiment, we demonstrated the CS-QFC from 780 nm to around 1540 nm employing one of the pump lights around 1580 nm detuned with a spacing of 25 GHz. From the experiment using the laser light, we showed that the output-frequency tunability of our CS-QFC reaches 2.5 THz, which corresponds to the ability to establish a 100-ch DWDM dynamic link from a single quantum system. Using photon pairs, we performed an experiment in which photons were routed into seven of the DWDM channels. As a result, photons were converted to the channels corresponding to the pump frequencies, while keeping the nonclassical cross-correlation functions, and no crosstalk to the other channels was observed.
Quantum frequency conversion (QFC) which converts the frequencies of photons while preserving the quantum state is an essential technology for realizing the quantum internet and quantum interconnect. For the QFC based on the frequency downconversion from visible to the telecom wavelengths around 1500 nm, it is widely known that noise photons produced by the strong pump light used for QFC contaminate the frequency-converted photon, which degrades the quality of the quantum property of the photon after QFC. In conventional QFC experiments, noise photons are removed using external narrowband frequency filter systems. In contrast, in this study, we implement a compact QFC device integrating the cavity structure only for the converted mode. While the cavity structure can enhance not only the desired QFC efficiency but also the noise photon generation rate, we show that the cavity-enhanced QFC followed by a relatively wide bandpass filter achieves the signal-to-noise ratio comparable to the QFCs with external narrowband filters. We experimentally demonstrate the cavity-enhanced QFC using a single photon at 780 nm to 1540 nm, in which the non-classical photon statistics is clearly observed after QFC.
High-speed long-range quantum communication requires combining frequency multiplexed photonic channels with quantum memories. We experimentally demonstrate an integrated quantum frequency conversion protocol that can convert between wavelength division multiplexing channels in the telecom range with an efficiency of 55±8% and a noise subtracted Hong-Ou-Mandel (HOM) dip visibility of 84.5%. This protocol is based on a cascaded second order nonlinear interaction and can be used to interface a broad spectrum of frequencies with narrowband quantum memories, or alternatively as a quantum optical transponder, efficiently interfacing different regions of a frequency-multiplexed spectrum.
Interconnecting heterogeneous quantum systems is an important step toward realizing the quantum internet. We propose a quantum network hub that interfaces local quantum devices with dense wavelength-division multiplexing (DWDM) networks in the telecom band via quantum frequency conversion (QFC) with frequency-channel selectivity. We show that standard periodically poled lithium niobate waveguides used for QFC exhibit a dispersion sweet spot, for example around the 780 nm band, enabling wide tunability of the pump wavelength while maintaining phase matching. Experimentally, we demonstrate the network hub by implementing a channel-selective and polarization-insensitive QFC from 780 nm to telecom wavelengths around 1540 nm. We achieve a pump tuning range of 2 THz and successfully distribute polarization-encoded single photons into 16 frequency channels on the ITU-T DWDM grid with 25 GHz channel spacing, while preserving the quantum information. These results position the QFC-based hub as a versatile backbone for connecting a wide range of quantum devices, spanning both photonic and matter-based systems, across frequency-multiplexed telecom networks.
In this work we experimentally and theoretically characterize the converted signal and the noise level of a quantum frequency conversion (QFC) device implemented for frequency down conversion from the visible to telecommunication wavelengths. This frequency converter is a useful device able to interface quantum memories operating at different wavelengths. We observe that the main noise source is directly induced by the pump via spontaneous parametric down-conversion (SPDC), enhanced by imperfections in the periodic poling structure of the waveguide. We also measure conversion with high signal to noise ratio for microsecond long weak coherent states at the single photon level. Finally, we show theoretically that the converted Stokes/Anti-Stokes non-classical correlations of photon pairs emitted by quantum memories could be largely preserved for pulse durations lower than 4µs.
Signal photons emitted by quantum nodes typically fall outside the low-loss telecom window of optical fibers, leading to severe transmission losses. Quantum frequency conversion (QFC) offers an effective optical interface that bridges quantum nodes with telecom-band channels, enabling long-distance quantum communication. In this work, we demonstrate a compact, fiber-integrated QFC system with low noise and a high signal-to-noise ratio (SNR). Using a periodically poled lithium niobate (PPLN) waveguide, input photons at 637.2 nm are down-converted to telecom photons at 1588.3 nm. Our system achieves a total conversion efficiency of approximately 9%, with pump-induced noise suppressed to 154 Hz. For input photon rates of 32.7, 118.0, and 327.7 kHz, the corresponding SNRs are 12.3, 43.9, and 117.8, respectively. We further develop a theoretical model to simulate the entanglement fidelity between nitrogen-vacancy (NV) center spins and the frequency-converted telecom photons. At the emission rate of an NV center, our QFC system maintains an expected fidelity exceeding 52% over a transmission distance of 100 km. These findings highlight the potential of our QFC system for scalable, long-distance quantum networking.