Quantum communication via solid-core fiber (SCF) networks is currently limited by signal attenuation, whereas hollow-core fibers (HCFs) offer a promising alternative due to their lower absorption and scattering losses. While gas-filled HCFs are widely used in atomic and molecular optics, their impact on quantum communication remains largely unexplored. This study is an investigation of the interaction between entangled photons and various gas species to assess their relevance for HCF-based quantum communication. Using a type-I SPDC setup, polarization-entangled photon pairs were generated and analyzed within a vacuum/gas cell system. The system demonstrated statistically significant non-classical correlations, with negligible signal attenuation differences across most gas configurations, except for Neon which consistently showed lower correlation values. Future improvements are suggested to enhance measurement variance, enabling deeper analysis and potential applications in quantum communication using gas filled HCFs.
Quantum communication via solid-core fiber (SCF) networks is currently limited by signal attenuation, whereas hollow-core fibers (HCFs) offer a promising alternative due to their lower absorption and scattering losses. While gas-filled HCFs are widely used in atomic and molecular optics, their impact on quantum communication remains largely unexplored. This study is an investigation of the interaction between entangled photons and various gas species to assess their relevance for HCF-based quantum communication. Using a type-I SPDC setup, polarization-entangled photon pairs were generated and analyzed within a vacuum/gas cell system. The system demonstrated statistically significant non-classical correlations, with negligible signal attenuation differences across most gas configurations, except for Neon which consistently showed lower correlation values. Future improvements are suggested to enhance measurement variance, enabling deeper analysis and potential applications in quantum communication using gas filled HCFs.
Quantum communication via solid-core fiber (SCF) networks is currently limited by signal attenuation, whereas hollow-core fibers (HCFs) offer a promising alternative due to their lower absorption and scattering losses. While gas-filled HCFs are widely used in atomic and molecular optics, their impact on quantum communication remains largely unexplored. This study is an investigation of the interaction between entangled photons and various gas species to assess their relevance for HCF-based quantum communication. Using a type-I SPDC setup, polarization-entangled photon pairs were generated and analyzed within a vacuum/gas cell system. The system demonstrated statistically significant non-classical correlations, with negligible signal attenuation differences across most gas configurations, except for Neon which consistently showed lower correlation values. Future improvements are suggested to enhance measurement variance, enabling deeper analysis and potential applications in quantum communication using gas filled HCFs.
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.
Hollow-core fiber (HCF) promises to unify air-borne light propagation and non-line-of-sight transmission, thus holding great potential for versatile photonics-based quantum information applications. The early version of HCF based on photonic-bandgap guidance has not proven itself a reliable quantum channel because of the poor modal purity in both spatial and polarization domains, as well as significant difficulty in fabrication when the wavelength shifts to the visible region. In this work, based on the polarization degree of freedom, we demonstrate high-fidelity ( ∼ 0.98 ) single-photon transmission and distribution of entangled photons over a 36.4 m hollow-core conjoined-tube fiber (CTF) by using commercial silicon single-photon avalanche photodiodes. Our CTF realizes the combined merits of low loss, high spatial modal purity, low polarization degradation, and low chromatic dispersion. We also demonstrate single-photon low-latency ( ∼ 99.96 % speed of light in vacuum) transmission, paving the way for extensive uses of HCF links in versatile photonics-based quantum information processing.
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.
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.
Quantum computers are expected to offer significant advantages over classical computers by using quantum mechanical phenomena such as superposition and entanglement. Potential future quantum networks may rely on hybrid architectures that combine the advantages of different qubit platforms. In such networks, quantum information must be transferred between stationary and flying qubits, typically photons. A major challenge, however, is that different quantum systems operate at vastly different wavelengths, making direct interconnection impossible. This challenge is addressed by quantum frequency conversion (QFC), which enables the transfer of quantum information between different spectral regions while preserving the quantum state. Most current QFC approaches employ the second-order nonlinearity of crystals. An alternative approach is based on the third-order nonlinearity of gases. In particular, coherent Raman scattering processes such as coherent Stokes and anti-Stokes Raman scattering (CSRS/CARS) enable polarization-independent, broadband and tunable frequency conversion through virtual energy levels. Within the Cluster of Excellence Matter and Light for Quantum Computing (ML4Q), a hydrogen-filled hollow-core fiber is used to realize continuous-wave CSRS-based frequency conversion. The goal is to convert photons emitted by InAs/GaAs quantum dots at approximately 865 nm into the telecom O-band around 1350 nm, thereby enabling the use of these quantum dots in fiber-based quantum communication networks. In this thesis, the conversion stage evolved from a short gas-cell configuration to a hollow-core fiber system providing interaction lengths of several meters. The development and optimization of the experimental setup, including the selection of laser wavelengths, hollow-core fibers, and the design of custom hydrogen chambers are discussed. A detailed characterization of the frequency-conversion system is performed, focusing on the dependence of the conversion efficiency on pressure, intensity, and interaction length, as well as on polarization and bandwidth. Special attention is given to conversion efficiency compared to optical losses and background contributions, which ultimately limit the performance of the current implementation. The key challenges are identified and potential strategies for future improvements are discussed. The central achievement of this work is the experimental demonstration of quantum-correlation-preserving frequency conversion. Using entangled photon pairs generated by spontaneous parametric down-conversion, the preservation of quantum correlations is verified by comparing the cross-correlation functions before and after the conversion process. This work lays the foundation for further experiments with continuous-wave CSRS-based quantum frequency conversion, including measurements with InAs/GaAs quantum dots.