
Physics
Demonstration of quantum network protocols over a 14-km urban fiber link
S. Kucera, C. Haen, et al.
This groundbreaking research by Stephan Kucera and colleagues unveils the successful distribution of quantum entanglement and state teleportation over a 14.4 km urban dark-fiber link, showcasing a pioneering leap in quantum communication technology.
Playback language: English
Introduction
Quantum networks necessitate the distribution of entanglement as a fundamental resource. However, deploying quantum networks in urban environments presents significant technical challenges. Existing fiber infrastructure often involves underground cables or cables installed alongside overhead power lines, leading to environmental vulnerabilities. Factors like wind-induced stress, rain, ice, snow, and even birds can introduce fluctuations in polarization mode dispersion (PMD), photon travel times, and polarization-dependent loss (PDL). For classical communication, transmission and PMD are primary fiber metrics. However, for quantum communication using polarization-encoded qubits, a more detailed PMD analysis is crucial, encompassing polarization, phase, and travel time variations. This research explores a 14.4 km urban fiber link in Saarbrücken, Germany, connecting Saarland University and the University of Applied Sciences. The link, comprising both underground and overhead sections along with several patch stations, is characterized extensively. An efficient polarization stabilization scheme is implemented to ensure high-fidelity transmission of photonic polarization qubits. The experimental setup leverages a trapped ⁴⁰Ca⁺ ion as a quantum memory, a high-brightness SPDC source of entangled photon pairs, and polarization-preserving quantum frequency conversion (QFC) to the telecom C-band. This combination makes polarization qubits ideal for the chosen protocols. The study demonstrates photon-photon and ion-photon entanglement distribution, as well as quantum state teleportation, as examples of quantum communication protocols over this challenging urban fiber link.
Literature Review
The paper references numerous studies on quantum networks, entanglement distribution, and quantum communication protocols, highlighting the existing challenges and advancements in the field. It specifically cites works focusing on quantum repeaters, teleportation, clock synchronization using entanglement, distributed quantum sensing, and various implementations of quantum key distribution (QKD) over deployed fiber networks. The review also includes studies on the impact of environmental factors on fiber links and the characterization of such links for quantum communication. These references establish the context of the current work within the broader landscape of quantum networking research and emphasize the novelty of demonstrating these protocols over a complex, real-world urban fiber link.
Methodology
The methodology involved a comprehensive characterization of the 14.4 km fiber link. Optical time-domain reflectometry (OTDR) measured optical loss at 1550 nm. Background noise measurements were made using superconducting nanowire single-photon detectors (SNSPDs), revealing fluctuations attributed to patch station activity and nearby fiber traffic. A broadband-suppression optical filter was implemented to mitigate the background noise. Polarization-dependent loss (PDL) was quantified using a method involving two polarization scramblers and a piezo polarization controller, considering the PDL of the detection setup. The quantum channel's capability to transmit polarization qubits was characterized by a time-dependent matrix and offset vector acting on the input Bloch vector, allowing calculation of process fidelity. A seven-day polarization drift measurement revealed stronger fluctuations during daytime. The fiber-induced time spread (jitter) of transmitted photons was investigated using two methods: one employing the photon pair source and analyzing the arrival time correlation of photon pairs, and another using an auxiliary laser, an acousto-optical modulator (AOM), and a retroreflector to measure changes in optical path length via Doppler shift. Temperature fluctuations in the overhead fiber were identified as a major contributor to this time drift. The experimental setup comprised a photon pair source (with QFC), an ion-trap setup, the fiber link, polarization stabilization units (sender and receiver), and polarization detection setups at both ends. The polarization stabilization employed a gradient descent algorithm to minimize an error function based on the deviation from target polarization states, using piezo elements for voltage control. The quantum communication protocols included entanglement distribution, ion-photon entanglement (using heralded absorption), and quantum state teleportation. Quantum state tomography was used to reconstruct density matrices, enabling fidelity calculations. Background correction was applied to isolate the effects of the fiber link.
Key Findings
The study demonstrated successful quantum communication protocols over a challenging 14.4 km urban fiber link. Key findings include:
1. **Fiber Link Characterization:** The 14.4 km link exhibited a total loss of 10.4 dB, with a polarization-dependent loss (PDL) of only 0.08(9) dB. Background noise was successfully mitigated using a broadband filter. Significant polarization drift was observed, particularly during daytime, necessitating active polarization stabilization.
2. **Polarization Stabilization:** A fully automated polarization stabilization scheme was implemented, achieving a process fidelity of 99% for approximately 100 seconds between stabilization runs. The average stabilization duration was 6.4(17) seconds.
3. **Photon-Photon Entanglement:** High-fidelity photon-photon entanglement distribution was achieved over the link, with fidelities exceeding 98% for up to 60 seconds between stabilization runs. Beyond 60s the influence of the fiber on the generated two-photon state becomes visible.
4. **Ion-Photon Entanglement:** Distant ion-photon entanglement was successfully generated using heralded absorption, achieving a fidelity of 79(2)% (83(2)% with background correction) with the ideal state.
5. **Quantum State Teleportation:** Quantum state teleportation from the trapped ion to a remote photon was demonstrated with fidelities of 80(6)% and 87(5)% (78(6)% and 86(5)% without background correction) depending on the Bell state measurement outcome.
6. **Time Jitter:** The fiber-induced time jitter was bounded to less than 600 ps over an 80 s integration time. The primary cause of the observed drift in the optical path length were temperature fluctuations in the overhead fiber.
These results showcase the robustness of the implemented techniques and the suitability of the fiber link for quantum communication despite its complex urban environment.
Discussion
The successful demonstration of entanglement distribution, ion-photon entanglement, and quantum state teleportation over the 14.4 km urban fiber link highlights the significant progress toward building practical quantum networks. The high fidelities achieved, despite the challenges of a real-world deployed fiber, validates the effectiveness of the active polarization stabilization scheme and the overall experimental setup. The results are particularly significant in demonstrating the feasibility of using existing urban fiber infrastructure for quantum communication. The observed daytime fluctuations in polarization and the impact of environmental factors (temperature) underscore the need for robust, active compensation strategies. The study's findings are relevant to various areas of quantum information science, including quantum key distribution (QKD) and the development of quantum repeaters. Future research should focus on optimizing the system to minimize remaining losses, extending the communication distance, and investigating the origin and compensation of phase fluctuations. The relatively high process fidelities, particularly for the quantum state teleportation, are promising for applications requiring long-distance quantum communication.
Conclusion
This research successfully demonstrated key quantum network protocols – entanglement distribution, ion-photon entanglement, and quantum state teleportation – over a challenging 14.4 km urban fiber link. The high fidelities achieved, despite the link's complexities, validate the effectiveness of the implemented polarization stabilization and experimental techniques. This paves the way for future implementations of QKD and the development of quantum repeaters. Further research will focus on investigating the root causes of phase fluctuations and optimizing the system for longer distances and higher fidelities.
Limitations
The study acknowledges limitations, particularly concerning the origin and time dependence of polarization-dependent loss (PDL) and phase fluctuations in the fiber transmission. These factors, especially phase fluctuations, could impact interference-based quantum communication protocols. While a broadband-suppression filter mitigated background noise, the source and impact of remaining background fluctuations warrant further investigation. Although the study successfully implemented polarization stabilization, the duty cycle between stabilization and data acquisition could be further optimized for enhanced efficiency. The presence of a faulty splice in the fiber limited the overall communication distance achieved in this experiment.
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