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.
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.
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.
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.
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.
We report on the efficient, tunable, and selective frequency up-conversion of a supercontinuum spectrum via molecular modulation in a hydrogen-filled hollow-core photonic crystal fiber. The vibrational Q(1) Raman transition of hydrogen is excited in the fiber by a pump pre-pulse, enabling the excitation of a synchronous, collective oscillation of the molecules. This coherence wave is then used to up-shift the frequency of an arbitrarily weak, delayed probe pulse. Perfect phase-matching for this process is achieved by using higher order fiber modes and adjusting the pressure of the filling gas. Conversion efficiencies of ~50% are obtained within a tuning range of 25 THz.
Quantum frequency conversion (QFC) plays a pivotal role in enabling coherent photonic interfaces across disparate quantum systems, serving as a foundation for scalable and interoperable quantum networks. Among various approaches, four-wave mixing (FWM) offers a promising path toward efficient, broadband QFC. In this work, we investigate the feasibility of phase-preserving frequency conversion, a necessary condition for coherent quantum state transfer, by characterizing the spectral phase mapping from input to output fields. We model nonlinear propagation through a gas-filled hollow-core capillary fiber, to demonstrate highly phase-coherent frequency conversion across a broad spectral range, including the infrared to ultraviolet, such as telecom (1550 nm) to visible (516 nm) and deep-UV (308 nm). We further examine how phase-mapping quality and conversion efficiency depend on key system parameters, including the intermediate pulse bandwidth and input pulse energy. Our results reveal optimal operating conditions for achieving future efficient and coherent QFC and provide valuable insights into the nonlinear dynamics underlying FWM-based transduction. These findings lay the groundwork for integrating heterogeneous quantum platforms across widely separated optical bands.
We present a comprehensive numerical investigation of entangled photon pair generation via spontaneous four-wave mixing (SFWM) in hollow-core photonic crystal fibers (HC-PCFs). By analyzing various fiber geometries, dispersion profiles, and anti-resonant characteristics, we identify optimal configurations that enhance photon-pair generation efficiency. Our study highlights the crucial roles of group velocity dispersion engineering, pump parameters, and phase-matching conditions in shaping the joint spectral amplitude (JSA) of the emitted photons. Utilizing gas-filled or evacuated HC-PCFs significantly reduces Raman noise by minimizing field overlap with the glass, while maintaining strong nonlinear interactions. The results reveal practical regimes for improving spectral purity and heralding efficiency. These findings provide practical design guidelines for scalable, low-noise quantum light sources suitable for quantum communication technologies.