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arXiv:2501.04049 · Optics Letters

Frequency conversion in a hydrogen-filled hollow-core fiber: power scaling, background, and bandwidth

Anica Hamer, Frank Vewinger, Michael H. Frosz, Simon Stellmer

Published Jun 4, 2025• 2 citations
arXivHTMLPDF

Abstract

Large-area quantum networks based on optical fibers allow photons at near-infrared wavelengths to travel with minimal loss. Quantum frequency conversion is a method to alter the wavelength of a single photon while maintaining its quantum state. Most commonly, nonlinear crystals are employed for this conversion process, where near-unity conversion efficiency at high fidelity has been demonstrated. Still, the crystal-based conversion process is plagued by strong background noise, very limited spectral bandwidth, and inhomogeneous temperature profiles at strong pump fields. In the previous work, we have demonstrated frequency conversion in hydrogen-filled hollow-core fibers and claimed that this conversion process does not compromise performance at strong pump fields, is essentially free of background noise, and is intrinsically broadband. Here, we demonstrate that these three claims are justified: we demonstrate the quadratic scaling with pump field intensity, quantify the background level, and present coarse tuning over a range of 10 nm.

DOI
10.1364/ol.553732

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Large-area quantum networks based on optical fibers allow photons at near-infrared wavelengths to travel with minimal loss. Quantum frequency conversion is a method to alter the wavelength of a single photon while maintaining its quantum state. Most commonly, nonlinear crystals are employed for this conversion process, where near-unity conversion efficiency at high fidelity has been demonstrated. Still, the crystal-based conversion process is plagued by strong background noise, very limited spectral bandwidth, and inhomogeneous temperature profiles at strong pump fields. In the previous work, we have demonstrated frequency conversion in hydrogen-filled hollow-core fibers and claimed that this conversion process does not compromise performance at strong pump fields, is essentially free of background noise, and is intrinsically broadband. Here, we demonstrate that these three claims are justified: we demonstrate the quadratic scaling with pump field intensity, quantify the background level, and present coarse tuning over a range of 10 nm.
In large-area quantum networks based on optical fibers, photons are the fundamental carriers of information as so-called flying qubits. They may also serve as the interconnect between different components of a hybrid architecture, which might comprise atomic and solid-state platforms operating at visible or near-infrared wavelengths, as well as optical links in the telecom band. Quantum frequency conversion is the pathway to change the color of a single photon while preserving its quantum state. Currently, nonlinear crystals are utilized for this process. However, their performance is limited by their acceptance bandwidth, tunability, polarization sensitivity, and undesired background emission. A promising alternative is based on stimulated Raman scattering (SRS) in gases. Here, we demonstrate polarization-preserving frequency conversion in a hydrogen-filled antiresonant hollow-core fiber. This approach holds promises for seamless integration into optical fiber networks and interfaces to single emitters. Disparate from related experiments that employ a pulsed pump field, we here take advantage of two coherent continuous-wave pump fields.
Practical implementations of quantum information networks require frequency conversion of individual photons. Approaches based on a molecular gas as the nonlinear medium cover a wide range of the optical spectrum and promise high efficiency at negligible background. We present polarization-preserving frequency conversion in a hydrogen-loaded hollow-core fiber using continuous-wave pump fields. We demonstrate conversion efficiency at the level of a few per mille, discuss various limitations and loss mechanisms, and present a route to increase conversion efficiency to near unity.
Practical implementations of quantum information networks require frequency conversion of individual photons. Approaches based on a molecular gas as the nonlinear medium cover a wide range of the optical spectrum and promise high efficiency at negligible background. We present polarization-preserving frequency conversion in a hydrogen-loaded hollow core fiber using continuous-wave pump fields. We demonstrate conversion efficiency at the level of a few per mille, discuss various limitations and loss mechanisms, and present a route to increase conversion efficiency to near unity.
Preserving the underlying statistical properties of light during frequency transduction is essential for interfacing spectrally disparate quantum systems. To do this successfully, the process needs to be highly efficient, broadband and noiseless. Despite significant progress in this field, satisfying all these conditions remains challenging. Recently, molecular modulation-based transduction in hollow-core fibers has emerged as a promising approach, where biphoton correlations were proven to be preserved during frequency up-conversion. However, beyond this previous demonstration of correlation-maintaining conversion, whether molecular modulation can preserve more general statistical properties of light remains experimentally unexplored. Here, we demonstrate the conservation of photon-number statistics during selective frequency transduction by Raman molecular modulation in a hydrogen-filled hollow-core fiber. As a proof of concept, weak 585-nm signals are selectively converted to 773 nm in the near-infrared and to 471 nm in the blue spectral region with near-unity efficiencies. Phase matching between the generated coherence waves and the transitions of interest is achieved through the effective dispersion of the system, enabling independent selection of the frequency up- and down-conversion processes. The preservation of the photon-number statistics of the shifted signals, along with the large frequency shifts, broad tunability and compatibility with quantum-optical fields, provides a versatile fiber-based platform for quantum frequency transduction. In particular, the demonstrated interfacing between visible and near-infrared wavelengths using low pump energies and low gas pressures offers a route towards connecting spectrally distinct quantum systems, including quantum emitters, sensors and atomic quantum memories, within hybrid quantum 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.
Efficient quantum frequency conversion (QFC) across large spectral gaps remains a challenge for quantum networking. We theoretically investigate a cascaded QFC in hydrogen-filled hollow-core photonic crystal fiber (HC-PCF) to convert telecom photons (1550 nm) to the visible spectrum (676 nm) via two anti-Stokes Raman shifts. Although simultaneous phase matching is impractical across such a wide window, operating in a high-pressure regime overcomes phase mismatches because the high-gain Raman coherence wave has a shorter spatial extent than the nonlinear coherence length. Maxwell-Bloch simulations show conversion efficiencies exceeding 50%. This approach offers a flexible, low-loss path for integrating disparate quantum technologies in a single fiber platform.