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IEEE Journal of Selected Topics in Quantum Electronics

High Fidelity Distribution of Telecom Polarization Entangled Photons through a 7.7 Km Antiresonant Hollow-Core Fiber

Alessandro Trenti, Costin Luchian, Francesco Poletti, Radan Slavı́k, Periklis Petropoulos, Obada Alia, George Theodore Kanellos, Hannes Hübel

Published Apr 22, 2024• 11 citations
Publisher

Abstract

Antiresonant hollow-core fibers are a novel type of optical fiber with unparalleled characteristics, unattainable with solid-core fibers. In particular, antiresonant hollow-core fibers are characterized by vacuum-like propagation speed, suppressed dispersion and nonlinear optical effects and, recently, even ultra-low propagation loss. In recent years, the development of a type of antiresonant hollow-core fiber, called Nested Antiresonant Nodeless Fiber (NANF) has seen tremendous improvements, and applications in both classical and quantum communications have been suggested. While encouraging results over meter long distances have been shown, the distribution of polarized entangled photons over inner-city distances through antiresonant hollow-core fibers has not been demonstrated yet. This could be a game changer for the development of quantum networks, which leverage entanglement for the distribution of secret keys, and more in the long term, in a quantum internet scenario. In this work, an experimental investigation of entanglement distribution through a NANF with an overall length of 7.72 km is presented. Remarkably, substantial reduction of latency (about 13μs) and suppressed chromatic dispersion (about one order of magnitude) of the studied NANF compared to a telecom single-mode fiber (Corning's SMF28) of equal length are measured for different bandwidths of the distributed entangled photons. Moreover, by encoding entanglement in polarization, high fidelity (>95%) distribution of narrow-bandwidth entangled photons is demonstrated. This result paves the way to the exploitation of NANF as a superior transmitting medium for quantum technology applications relying on the distribution of entanglement encoded in polarization over inner-city distances.

DOI
10.1109/jstqe.2024.3392416

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Antiresonant hollow-core fibers are a novel type of optical fiber with unparalleled characteristics, unattainable with solid-core fibers. In particular, antiresonant hollow-core fibers are characterized by vacuum-like propagation speed, suppressed dispersion and nonlinear optical effects and, recently, even ultra-low propagation loss. In recent years, the development of a type of antiresonant hollow-core fiber, called Nested Antiresonant Nodeless Fiber (NANF) has seen tremendous improvements, and applications in both classical and quantum communications have been suggested. While encouraging results over meter long distances have been shown, the distribution of polarized entangled photons over inner-city distances through antiresonant hollow-core fibers has not been demonstrated yet. This could be a game changer for the development of quantum networks, which leverage entanglement for the distribution of secret keys, and more in the long term, in a quantum internet scenario. In this work, an experimental investigation of entanglement distribution through a NANF with an overall length of 7.72 km is presented. Remarkably, substantial reduction of latency (about 13μs) and suppressed chromatic dispersion (about one order of magnitude) of the studied NANF compared to a telecom single-mode fiber (Corning's SMF28) of equal length are measured for different bandwidths of the distributed entangled photons. Moreover, by encoding entanglement in polarization, high fidelity (>95%) distribution of narrow-bandwidth entangled photons is demonstrated. This result paves the way to the exploitation of NANF as a superior transmitting medium for quantum technology applications relying on the distribution of entanglement encoded in polarization over inner-city distances.
State of the art classical and quantum communications rely on standard optical fibers with solid cores to transmit light over long distances. However, recent advances have led to the emergence of antiresonant hollow-core optical fibers (AR-HCFs), which, due to the novel fiber geometry, show remarkable optical guiding properties, which are not as limited by the material properties as solid-core fibers. In this paper, we explore the transmission of entangled photons through a novel 7.7 km AR-HCF in a laboratory environment at 1550 nm, presenting the first successful demonstration of entanglement distribution via a long AR-HCF. In addition to showing these novel fibers are compatible with long distance quantum communication, we highlight the low latency and low chromatic dispersion intrinsic to AR-HCF, which can increase the secure key rate in time-bin-based quantum key distribution protocols.
Advancements in quantum communication and sensing require improved optical transmission that ensures excellent state purity and reduced losses. While free-space optical communication is often preferred, its use becomes challenging over long distances due to beam divergence, atmospheric absorption, scattering, and turbulence, among other factors. In the case of polarization encoding, traditional silica-core optical fibers, though commonly used, struggle with maintaining state purity due to stress-induced birefringence. Hollow core fibers, and in particular nested antiresonant nodeless fibers (NANF), have recently been shown to possess unparalleled polarization purity with minimal birefringence in the telecom wavelength range using continuous-wave (CW) laser light. Here, we investigate a 1-km NANF designed for wavelengths up to the 2-μm waveband. Our results show a polarization extinction ratio between ~-30 dB and ~-70 dB across the 1520 to 1620 nm range in CW operation, peaking at ~-60 dB at the 2-μm design wavelength. Our study also includes the pulsed regime, providing insights beyond previous CW studies, e.g., on the propagation of broadband quantum states of light in NANF at 2 μm, and corresponding extinction-ratio-limited quantum bit error rates (QBER) for prepare-measure and entanglement-based quantum key distribution (QKD) protocols. Our findings highlight the potential of these fibers in emerging applications such as QKD, pointing towards a new standard in optical quantum technologies.
Coexistence of classical and quantum light in optical fiber is a key requirement for scalable quantum networks. However, it is fundamentally limited by nonlinear crosstalk from intense classical signals. Hollow-core fiber (HCF), with its intrinsically low optical nonlinearity, offers a promising solution. Here, we experimentally demonstrate the coexistence of polarization-entangled photons with dense wavelength-division multiplexed (DWDM) classical signals with a total launch power of 11.2 dBm, and perform a direct comparison between equal-length single-mode fiber (SMF) and photonic-bandgap-based HCF deployed in a campus network. We observe that HCF suppresses background noise by approximately two orders of magnitude compared to SMF, while preserving high-fidelity entanglement distribution. Our results provide clear experimental evidence of the advantage of HCF and establish its potential as a transmission medium.
The coexistence of quantum information and classical signals in a single fiber is essential for future quantum networks that leverage the well-established optical fiber infrastructure. Although multiplexing technologies can separate quantum and classical signals, pure silica core fibers (PSCFs) remain fundamentally limited by the high nonlinearity, which generates substantial Raman scattering and four-wave mixing noise. Hollow-core fibers (HCFs), guiding light predominantly in air, offer an attractive solution with intrinsically ultra-low nonlinearity and strongly suppressed nonlinear noise. In this work, we demonstrate the entanglement-based key coexisting with data over an 18-km HCF link. We achieve time-encoded high-dimensional quantum key distribution (HD-QKD) carrying 0 dBm of bidirectional received power, corresponding to a theoretical data capacity of up to 2.3 Tbps. During 24 hours of continuous operation, an average secret key rate (SKR) of 10.56 kbps is obtained. Theoretical analysis further predicts SKRs above 135 kbps over transmission distances exceeding 200 km using state-of-the-art low-loss HCFs. These results show significantly improved performance compared with PSCF-based systems and highlight the potential of HCFs for scalable quantum-classical coexistence compatible with the architectures of established fiber-optic networks.