A new scheme sends large numbers of entangled photons reliably through commercial fiber-optic cables, demonstrating a capability required for secure quantum networks. By Charles Day Large-scale networks that distribute information among quantum processors or that use quantum encryption for the security of a large number of systems will require many pairs of quantum mechanically entangled photons per second.Sending these photons reliably through existing commercial fiber-optic communication lines is a challenge, given the fragility of entanglement.Now researchers have sent 20,000 such photons per second down a 34-km-long section of a New York fiber-optic network with a fidelity of 99% [1].They sent these photons continuously for two weeks without the frequent recalibrations needed in previous systems with lower photon rates.The researchers say that their results are an Quantum cable.A fiber-optic cable under New York City carried large numbers of quantum mechanically entangled photons with high fidelity, thanks to a system that continually compensates for disturbances of the photons' polarizations.Credit: tiero/stock.adobe.comimportant step toward the commercialization of quantum networks.
A new scheme sends large numbers of entangled photons reliably through commercial fiber-optic cables, demonstrating a capability required for secure quantum networks. By Charles Day Large-scale networks that distribute information among quantum processors or that use quantum encryption for the security of a large number of systems will require many pairs of quantum mechanically entangled photons per second.Sending these photons reliably through existing commercial fiber-optic communication lines is a challenge, given the fragility of entanglement.Now researchers have sent 20,000 such photons per second down a 34-km-long section of a New York fiber-optic network with a fidelity of 99% [1].They sent these photons continuously for two weeks without the frequent recalibrations needed in previous systems with lower photon rates.The researchers say that their results are an Quantum cable.A fiber-optic cable under New York City carried large numbers of quantum mechanically entangled photons with high fidelity, thanks to a system that continually compensates for disturbances of the photons' polarizations.Credit: tiero/stock.adobe.comimportant step toward the commercialization of quantum networks.
The distribution of high-fidelity high-rate entanglement over telecommunication infrastructure is one of the main paths toward large-scale quantum networks, enabling applications such as quantum encryption and network protection, blind quantum computing, distributed quantum computing, and distributed quantum sensing. However, the fragile nature of entangled photons operating in real-world fiber infrastructure has historically limited continuous operation of such networks. Here, we present a fully automated system capable of distributing polarization-entangled photons over a 34-km deployed fiber in New York City, achieving high rates of nearly 5 × 10 5 pairs/s. Separately, we demonstrate a high fidelity of approximately 99 % for rates up to 2 × 10 4 pairs/s. Lastly, we achieve 15 days of continuous distribution, with a network up-time of 99.84 % . Our work paves the way for practical deployment of always-on entanglement-based networks with rates and fidelity adequate for many current and future use cases. Published by the American Physical Society 2024
Distributing high-fidelity, high-rate entanglement over telecommunication infrastructure is one of the main paths towards large-scale quantum networks, enabling applications such as quantum encryption and network protection, blind quantum computing, distributed quantum computing, and distributed quantum sensing. However, the fragile nature of entangled photons operating in real-world fiber infrastructure has historically limited continuous operation of such networks. Here, we present a fully automated system capable of distributing polarization entangled photons over a 34 km deployed fiber in New York City. We achieve end-to-end pair rates of nearly 5×105 pairs/s and entanglement fidelity of approximately 99%. Separately, we achieve 15 days of continuous distribution, with a network up-time of 99.84%. Our work paves the way for practical deployment of 24/7 entanglement-based networks with rates and fidelity adequate for many current and future use-cases.
Entangled light sources are considered as core technology for multiple quantum network architectures. Of particular interest are sources that are based on a single quantum system as these offer intrinsic security due to the sub-Poissonian nature of the photon emission process. This is important for applications in quantum communication where multi-pair emission generally compromises performance. A large variety of sources has been developed, but the generated photons remained far from being utilized in established standard fiber networks, mainly due to lack of compatibility with telecommunication wavelengths. In this regard, single semiconductor quantum dots are highly promising photon pair sources as they can be engineered for direct emission at telecom wavelengths. In this work we demonstrate the feasibility of this approach. We report a week-long transmission of polarization-entangled photons from a single InAs/GaAs quantum dot over a metropolitan network fiber. The photons are in the telecommunication O-band, favored for fiber optical communication. We employ a polarization stabilization system overcoming changes of birefringence introduced by 18.23 km of installed fiber. Stable transmission of polarization-encoded entanglement with a high fidelity of 91% is achieved, facilitating the operation of sub-Poissonian quantum light sources over existing fiber networks.
Quantum networks exhibit enhanced performance compared to their classical counterparts, enabling secure communication and enhanced sensing protocols. Scalable quantum networks must transmit photons over distances ranging from kilometers to metropolitan scales while preserving entanglement. The objective of this thesis is to characterize the sources and switches of real-world deployed quantum networks, which require an intimate knowledge of nonlinear concepts and devices that drive these operating regimes. Importantly, quantum networks are deployed alongside existing classical communication infrastructure to ensure scalability and compatibility with established fiber-based systems. As a result, quantum and classical signals must coexist within the same telecommunication network. Because quantum infrastructure relies on optical fiber, these networks are susceptible to instabilities arising from fiber- induced effects. To fully realize a scalable quantum network, we investigated the quantum physical layer to optimize performance and identify limitations in our networking system. We characterized the performance of an entangled-photon source used to produce polarization-entangled photons. We then observe how well a high-speed, low-loss quantum switch preserves entanglement, demonstrating its utility for transmitting photons to different nodes in scalable quantum networks. We then analyze a 30 km deployed fiber located in the DC metropolitan area to examine how fiber-induced effects influence network performance. Quantum networks uniquely leverage quantum phenomena to enable fundamentally secure communication and distributed quantum computing beyond the capabilities of classical systems. To realize these advantages, careful characterization of the quantum physical layer and deployed fiber-based networks is essential, as it ensures the stability, reliability, and high-fidelity operation required for large-scale quantum network applications.
Network operators hold a significant asset in their widespread optical fiber networks and as quantum technologies advance, these operators will be ideally positioned to offer new streams of services in the context of the Quantum Internet. However, establishing a functional Quantum Internet requires diverse technological advancements, particularly in the efficient distribution of entanglement over optical fibers. Entanglement serves as the cornerstone for quantum protocols and technologies such as quantum repeaters, teleportation, sensing, and time synchronization. Nonetheless, as photons propagate through fiber, they encounter various impairments that can degrade entanglement quality. Specifically, polarization-entangled photons suffer from polarization drifts, which exhibit stochastic behavior and can reduce fidelity. In this paper, we examine the prospects of entanglement distribution for the Quantum Internet, presenting experimental campaigns that highlight both the challenges and solutions. We demonstrate the inherent instability of photon fidelity in field-deployed fibers and show how compensation schemes can effectively enhance entanglement transmission in urban fiber networks. Our findings include results from a four-day entanglement distribution experiment conducted over Deutsche Telekom's optical fiber network in Berlin, achieving an average Bell parameter and fidelity of 2.52 and 99.6%, respectively.
Abstract Entanglement-based quantum key distribution (QKD) is an essential ingredient in quantum communication, owing to the property of source-independent security and the potential on constructing large-scale quantum communication networks. However, implementation of entanglement-based QKD over long-distance optical fiber links is still challenging, especially over deployed fibers. In this work, we report an experimental QKD using energy-time entangled photon pairs that transmit over optical fibers of 242 km (including a section of 19 km deployed fibers). The QKD is realized through the protocol of dispersive-optics QKD (DO-QKD) with high-dimensional encoding to utilize coincidence counts more efficiently. A reliable, high-accuracy time synchronization technology for long-distance entanglement-based QKD is developed based on the distribution of optical pulses in quantum channels. Our system operates continuously for more than 7 d without active polarization or phase calibration. We ultimately generate secure keys with secure key rates of 0.22 bps and 0.06 bps in the asymptotic and finite-size regimes, respectively. It shows that entanglement-based DO-QKD is reliable for long-distance realization in the field if its high requirement on time synchronization is satisfied.