Quantum Entanglement Travels 20 Kilometers in Fiber

A new breakthrough in quantum technology allows entangled photons to travel 20 kilometers through standard optical fibers, bringing the quantum internet closer to reality. This development addresses a major in quantum communication: transmitting fragile quantum states over long distances without losing their unique properties, which is essential for secure data transfer and connecting quantum computers. The research, led by a team from Yokohama National University and other institutions, demonstrates a versatile entanglement source that combines narrow linewidths, high fidelity, and compatibility with quantum memory, all in the telecom wavelength range that minimizes fiber losses.

The key finding is that the researchers successfully generated and transmitted entangled photon pairs over a total of 20 kilometers of fiber, including a wavelength conversion step to make the photons interact with quantum memory. They achieved this using a two-photon comb technique, which produces photons with a very narrow linewidth of about 1 MHz and entanglement fidelities exceeding 95% to Bell states. Even after transmission, the two-photon correlation remained clearly observable, with a normalized correlation coefficient of approximately 3, as shown in Figure 4(c). This indicates that the quantum entanglement survived the journey, a critical requirement for practical quantum networks.

Ology involved creating entangled photons through spontaneous parametric down-conversion in periodically poled lithium niobate crystals placed inside a bow-tie cavity. This setup, detailed in Figure 1, ensured a sub-MHz linewidth and frequency multimodality, meaning the photons could carry information across multiple frequency channels simultaneously. After generation, the photons were sent through 10 kilometers of fiber, and then their wavelength was converted from 1514 nm in the telecom band to 606 nm using sum-frequency generation. This conversion targeted Pr3+:YSO quantum memory, allowing the photons to be stored and retrieved efficiently. The team used superconducting single-photon detectors and time-correlated single-photon counting to measure the correlations, with cavity stabilization maintained via the Pound–Drever–Hall technique to keep the system aligned over time.

Analysis from the paper shows that the two-photon comb had a coherence time of up to 1 microsecond and a free spectral range of 116 MHz, as illustrated in Figure 2(a). The entanglement fidelity to Bell states like |Φ+⟩ and |Ψ+⟩ was around 90%, and the Clauser–Horne–Shimony–Holt parameter reached 2.47, confirming nonlocality. After wavelength conversion and fiber transmission, the correlation coefficient g_s,i^(2)(0) remained strong, demonstrating that the setup could handle real-world imperfections like fiber dispersion and polarization changes. The researchers noted that the wavelength converter's limited bandwidth of 25 GHz did not significantly degrade performance, and they observed a post-selective effect that enhanced the signal-to-noise ratio after conversion.

In context, this work matters because it enables more stable and scalable quantum communication using existing fiber infrastructure, unlike satellite-based s that depend on weather conditions. For everyday readers, this means progress toward unhackable communication networks and distributed quantum computing, where multiple quantum devices can work together over long distances. The ability to transmit entangled photons over 20 kilometers in fiber, as shown here, could lead to advancements in secure financial transactions, confidential data sharing, and scientific collaborations that rely on quantum principles.

Limitations from the study include the need for specialized filters to further reduce noise in the wavelength conversion process and of maintaining polarization insensitivity over long fibers. The paper also points out that the current setup requires careful temperature control and alignment, and the bandwidth of the wavelength converter may limit the number of frequency modes used in multiplexing. Future work aims to integrate time-bin states for better resistance to polarization changes and to develop improved filtering techniques to enhance performance.