[Paper Review] Distribution of Telecom Entangled Photons through a 7.7 km Antiresonant Hollow-Core Fiber

Michael Antesberger, Carla M. D. Richter+7 more|arXiv (Cornell University)|Aug 2, 2023

Quantum Information and CryptographyComputer Science51 references3 citations

TL;DR

This paper demonstrates the first long-distance distribution of telecom-wavelength time-bin entangled photons through a 7.7 km antiresonant hollow-core fiber (AR-HCF), achieving high-fidelity entanglement preservation. The AR-HCF’s low chromatic dispersion enables smaller time-bin spacings—down to 140 ps—compared to conventional solid-core fiber, significantly enhancing secure key rates in time-bin quantum key distribution protocols.

ABSTRACT

State of the art classical and quantum communication 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.

Motivation & Objective

• To demonstrate long-distance distribution of time-bin entangled photons through a 7.7 km antiresonant hollow-core fiber (AR-HCF), a novel fiber type with superior optical properties.
• To address the limitations of conventional solid-core fibers in quantum communication, particularly the need for frequency shifting of quantum sources to the C-band and the use of expensive detectors.
• To evaluate the performance of AR-HCF in preserving entanglement fidelity under varying time-bin spacings compared to standard SMF28 fiber.
• To quantify the impact of chromatic dispersion and detector jitter on entanglement fidelity in long-haul quantum communication using AR-HCF.
• To establish AR-HCF as a viable backbone for future wideband quantum networks supporting native-wavelength operation of quantum components.

Proposed method

• Generation of a polarization-time-bin entangled Bell state |Ψ⁻⟩ using a spontaneous parametric down-conversion source at 1550 nm.
• Transmission of the time-bin qubit through a 7.72 km AR-HCF with a dispersion parameter of ~2 ps/nm·km at 1550 nm.
• Use of superconducting nanowire single-photon detectors (SNSPDs) with low jitter to measure the time-bin qubit after transmission.
• Two-qubit quantum state tomography to reconstruct the density matrix and quantify entanglement fidelity via concurrence and purity.
• Comparison of entanglement preservation between AR-HCF and standard SMF28 fiber across varying time-bin spacings (Δt).
• Modeling of overlapping time-bin effects as detector error counts to interpret the observed degradation in concurrence and purity at small Δt.

Figure 1: Experimental Apparatus: a) A schematic of the full experimental setup. Panel b) shows a simplified “unfolded” setup with color coded panels corresponding to different sections of Panel a) . See the main text for a detailed explanation of each section of the experiment. Panel c) displays a

Experimental results

Research questions

• RQ1 Can time-bin entangled photons be successfully distributed over 7.7 km through an antiresonant hollow-core fiber without significant fidelity loss?
• RQ2 How does the chromatic dispersion of AR-HCF compare to conventional SMF28 in preserving entanglement at small time-bin spacings?
• RQ3 To what extent does detector jitter limit entanglement fidelity in AR-HCF compared to dispersion effects?
• RQ4 Can AR-HCF support smaller time-bin spacings than SMF28, thereby enabling higher secure key rates in time-bin QKD?
• RQ5 What is the impact of the fiber’s low nonlinearity and near-c group velocity on the scalability of quantum networks?

Key findings

• The experiment achieved successful entanglement distribution over 7.7 km of AR-HCF, with concurrence decreasing from 0.9482 ± 0.0007 to 0.901 ± 0.006 and purity from 0.9493 ± 0.0008 to 0.875 ± 0.006 due to depolarization effects.
• The AR-HCF preserved high entanglement fidelity down to time-bin spacings of 140 ps, whereas SMF28 fiber showed significant fidelity loss at Δt ≈ 300 ps.
• The lower dispersion in AR-HCF allows for smaller time-bin spacings without degradation, enabling higher key rates in time-bin QKD protocols.
• The observed degradation at small Δt is primarily due to detector jitter rather than chromatic dispersion, indicating that dispersion is not the limiting factor in AR-HCF.
• The AR-HCF’s low dispersion (2 ps/nm·km) and near-c group velocity provide intrinsic advantages for low-latency and high-bandwidth quantum communication.
• The results demonstrate that AR-HCFs can support quantum communication at native wavelengths of quantum sources, avoiding the need for frequency shifting and costly InGaAs-APD or SNSPD systems.

Figure 2: Latency Measurements: a) The normalized arrival-time histogram between Photon 1 and 2 after transmission of the entangled time-bin qubit through either $7.7$ km of NANF (blue) or $7.8$ km SMF28 (red) fiber. The photons arrive $13.11\leavevmode\nobreak\ \mu$ s earlier when traversing the NA