Two-photon comb with wavelength conversion and 20-km distribution for quantum communication

Long-distance quantum communication faces challenges due to optical fiber losses, necessitating quantum repeaters. Satellite-based approaches have shown promise, but are limited by atmospheric conditions. Fiber-based transmission offers stability and leverages existing infrastructure. Quantum repeaters, typically employing Bell-state measurements with or without quantum memory (QM), are crucial for overcoming distance limitations. The ideal entanglement source for long-distance fiber-based quantum internet, a versatile entanglement source (VES), should emit telecom wavelength photons (~1.5 μm) to minimize fiber loss, have a narrow linewidth (<10 MHz) compatible with QM, and achieve high-fidelity quantum entanglement. Current VES implementations struggle to achieve all these specifications simultaneously. This paper addresses this challenge by demonstrating a VES utilizing a two-photon comb (TPC) and wavelength conversion (WC). Previous research explored cavity-enhanced spontaneous parametric down conversion (SPDC) for various applications, including bright single modes and narrow linewidths. Wavelength conversion techniques, using lasers or single photons, enable compatibility between telecom wavelengths and QM absorption lines but face challenges in efficiency and noise.

The development of quantum technologies, including quantum computing and communication, is rapidly advancing. Google's demonstration of a 53-qubit quantum processor highlights the potential for quantum computers to break current encryption methods like RSA. Quantum communication is vital for unconditionally secure communication and efficient quantum computing via quantum internet networks. Quantum internet applications include quantum networks of clocks, secure cloud quantum computing, and distributed quantum computing. Quantum entanglement, essential for quantum internet, is used in teleportation, entanglement swapping, and dense coding. However, long-distance quantum communication faces the challenge of optical fiber loss, necessitating quantum repeaters. While satellite-based experiments have shown long-distance entanglement generation and quantum key distribution (QKD), they are susceptible to atmospheric conditions. Fiber-based transmission provides more stable operation. The maximum distance for QKD without repeaters is estimated at ~550 km, with experimental achievements up to 421 km. For more advanced applications beyond QKD, like connecting quantum nodes (simulators/computers), quantum memory (QM) is needed to assist repeaters and preserve quantum information. The need for a QM-compatible versatile entanglement source (VES) emitting telecom wavelengths, having narrow linewidths, and achieving high-fidelity entanglement is crucial for developing a fiber-based quantum internet. Existing VES demonstrations remain limited in achieving all these characteristics simultaneously.

This study utilizes a two-photon comb (TPC) technique generating entangled photon pairs with a narrow sub-MHz linewidth in the telecom band (~1514 nm). Wavelength conversion (WC) is achieved using sum-frequency generation (SFG) in a periodically poled lithium niobite (PPLN) waveguide with a 1010 nm auxiliary laser, targeting the Pr³⁺:YSO QM absorption line (~606 nm). The TPC consists of degenerate 1514-nm SPDC crystals in a mutually orthogonal arrangement within a bow-tie cavity, generating polarization entanglement. This setup offers advantages in brightness, alignment insensitivity, and path/phase compensation. The cavity, approximately 2.5 meters long, provides a sub-MHz linewidth and multiple frequency modes, suited for frequency-multiplexed quantum communication. Cavity locking is achieved via the Pound-Drever-Hall technique. Two-photon statistics are measured using a Hanbury-Brown-Twiss-type setup with superconducting single photon detectors (SSPDs) and silicon avalanche photodiodes (SiAPDs). Photonic-state tomography, employing the maximum-likelihood method, is performed to characterize the entangled state. To generate Bell states, zero-order half-wave plates are used. Wavelength conversion uses a PPLN waveguide with a strong auxiliary laser at 1010 nm to convert 1514 nm photons to 606 nm. Two-photon correlation is characterized by the normalized second-order correlation coefficient g⁽²⁾(0). Experiments are conducted with and without 10 km of fiber transmission, evaluating the effects of fiber loss, polarization rotation, and pulse broadening on g⁽²⁾(0). Single-photon wavelength conversion experiments are also performed for comparison.

The generated TPC exhibits a wide spectral range (~1 THz), a narrow linewidth (~1 MHz), and high-fidelity polarization entanglement (~90% fidelity to an arbitrary Bell state). A long coherence time and high-speed modulation capability are demonstrated. The narrow linewidth is maintained even with a reduced finesse cavity. Photonic-state tomography confirms high fidelity (96.1%) and concurrence (93.0%) in the multimode regime. Four Bell states are successfully generated with fidelities above 88%. Wavelength conversion of the TPC (WC-TPC) is demonstrated, yielding a clear comb structure. Two-photon correlation is observed after 10 km of fiber transmission for each photon (20 km total) with a g⁽²⁾(0) value of approximately 3. The increase in g⁽²⁾(0) after WC is attributed to a correlation-filtering effect; the wavelength conversion selectively picks up strongly correlated photon pairs. This effect is explained by considering the frequency and time domains, focusing on how the conversion process reduces noise photons. Although the wavelength converter has a limited bandwidth (~25 GHz), WC-TPC achieves an adequate signal-to-noise ratio (SNR).

The success of this study lies in the combination of TPC and WC. The TPC's narrow linewidth and long coherence time, even with multiple frequency modes, are crucial for compatibility with quantum memories. The narrow linewidth is achieved by careful design, temperature control of the PPLN crystals, and the use of a long cavity. The high fidelity and generation of Bell states demonstrate the effectiveness of the approach for frequency-multiplexed entanglement. The observation of two-photon correlation after 20 km of fiber distribution highlights the potential for long-distance quantum communication. The correlation-filtering effect of the wavelength conversion enhances the SNR of the correlated photon pairs. The study's findings address the challenge of creating a versatile entanglement source suitable for long-distance quantum communication by combining a high-fidelity, narrow-linewidth, multimode entanglement source with a wavelength conversion process that enhances the signal and is compatible with quantum memory systems.

This work successfully demonstrates a versatile entanglement source for long-distance quantum communication using a two-photon comb and wavelength conversion. The source achieves high fidelity, a narrow linewidth, and is compatible with quantum memory systems. Future work will focus on developing a specialized filter to further enhance the SNR after wavelength conversion, exploring polarization-insensitive wavelength conversion, and converting the polarization basis to a time-bin basis for improved robustness in long-distance transmission. Optimizing the TPC to increase the distribution rate and investigating Bell-state measurements for multimodes are also important next steps.

The limited bandwidth of the wavelength converter (~25 GHz) affects the overall efficiency and SNR. Improving this bandwidth is crucial for enhancing the performance. The study uses a single-polarization type of wavelength converter, which could be improved by developing a polarization-insensitive type. Furthermore, the effects of dispersion and wavelength-dependent retardance on tomographic reconstruction need further investigation. The g⁽²⁾(0) values are affected by various factors (fiber losses, polarization rotation, pulse broadening) which could be reduced by optimizing the experimental setup and using specialized filtering.