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arXiv:2405.02993 · Optics Express

Polarization purity and dispersion characteristics of nested antiresonant nodeless hollow-core optical fiber at near- and short-wave-IR wavelengths for quantum communications

Ivi Afxenti, Lijun Yu, Taylor Shields, Daniele Faccio, Thomas D. Bradley, Lucia Caspani, Matteo Clerici, Adetunmise C. Dada

Published Aug 27, 2024• 5 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1364/oe.529331

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One of the major challenges in the deployment of quantum communications (QC) over solid-core silica optical fiber is the performance degradation due to the optical noise generated with co-propagating classical optical signals. To reduce the impact of the optical noise, research teams are turning to new and novel architectures of solid-core and hollow-core optical fiber. We studied the impact when co-propagating a single-photon level (850 nm) and two classical optical signals (940 nm and 1550 nm) while utilizing a nested antiresonant nodeless fiber (NANF) with two low-loss windows. The 940 nm signal was shown to impact the single-photon measurement due to the silicon detector technology implemented; however, multiplexing techniques and filtering could reduce the impact. The 1550 nm signal was shown to have no detrimental impact. The results highlight that both high bandwidth optical traffic at 1550 nm and a QC channel at 850 nm could co-propagate without degradation to the QC channel.
Quantum dot single-photon sources are promising for quantum communication. Yet, the most advanced devices operate near 900 nm, where standard single-mode fibers experience significant losses. We address this by employing a hollow-core fiber engineered for low-loss transmission at quantum dot wavelengths, with measured loss of 0.65 dB/km and potentially as low as 0.12 dB/km near 934 nm. The fiber also supports strong classical signals at 1550 nm without adding Raman noise. Using this platform, we transmit all four BB84 polarization states from an InAs quantum dot over 340 m with 0.1% QBER, preserving single-photon purity and indistinguishability even in the presence of a strong classical signal. These results highlight how tailored transmission media enable quantum networks beyond the limits of standard telecom fibers.
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.
Recent comparisons of quantum repeater protocols have highlighted the strong near-term potential of multiplexed two-way architectures for long-distance quantum communication. At the same time, advances in hollow-core fiber (HCF) technology motivate a re-examination of the physical transmission medium as an architectural lever in quantum network design. In this work, we compare emerging anti-resonant HCFs against conventional silica single-mode fibers (SMFs) in multiplexed two-way quantum repeater networks. We evaluate their performance under both telecom and memory-native transmission, accounting for frequency-conversion overheads, coupling efficiencies, memory decoherence, and operational noise. We find that HCF significantly outperforms SMF across a wide range of regimes. With memory-native transmission, HCF yields up to an order of magnitude improvement in secret-key rate per channel use under realistic conversion efficiencies. Even at telecom wavelengths, HCF enables larger optimal repeater spacing, improving rate--cost tradeoffs and reducing repeater requirements. We further quantify the role of memory quality, hardware efficiency, detector and conversion losses, and two-qubit gate noise in shaping these gains. These results show that recent advances in HCF materially expand the design space of practical terrestrial quantum repeater networks.