In modern quantum technologies, preservation of the photon statistics of quantum optical states upon frequency conversion holds the key to the viable implementation of quantum networks, which often require interfacing of several subsystems operating in widely different spectral regions. Most current approaches offer only very small frequency shifts and limited tunability, while suffering from high insertion loss and Raman noise originating in the materials used. We introduce a route to quantum-correlation-preserving frequency conversion using hydrogen-filled antiresonant-reflecting photonic crystal fibers. Transient optical phonons generated by stimulated Raman scattering enable selective frequency up-conversion by 125 terahertz of the idler photon of an entangled pair, with efficiencies up to 70%. This threshold-less molecular modulation process preserves quantum correlations, making it ideal for applications in quantum information.
In modern quantum technologies, preservation of the photon statistics of quantum optical states upon frequency conversion holds the key to the viable implementation of quantum networks, which often require interfacing of several subsystems operating in widely different spectral regions. Most current approaches offer only very small frequency shifts and limited tunability, while suffering from high insertion loss and Raman noise originating in the materials used. We introduce a route to quantum-correlation-preserving frequency conversion using hydrogen-filled antiresonant-reflecting photonic crystal fibers. Transient optical phonons generated by stimulated Raman scattering enable selective frequency up-conversion by 125 terahertz of the idler photon of an entangled pair, with efficiencies up to 70%. This threshold-less molecular modulation process preserves quantum correlations, making it ideal for applications in quantum information.
Preservation of the photon statistics of quantum states of light upon frequency conversion is of great importance for modern quantum optics. For example, it holds the key to practical implementation of systems such as quantum networks which often require interfacing of different sub-units that do not operate in the same frequency bands. Although many integrated schemes have been demonstrated [1] , [2] , most offer small frequency shifts, limited tunability, and often suffer from high insertion loss and Raman noise generated in bulk materials.
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.
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.
Large-scale quantum networks rely on optical fiber networks and photons as so-called flying qubits for information transport. While dispersion and absorption of optical fibers are minimum at the infrared telecom wavelengths, most atomic and solid state platforms operate at visible or near-infrared wavelengths. Quantum frequency conversion is required to bridge these two wavelength regimes, and nonlinear crystals are currently employed for this process. Here, we report a novel approach of frequency conversion to the telecom band. This interaction is based on coherent Stokes Raman scattering (CSRS), a four-wave mixing process resonantly enhanced in a dense molecular hydrogen gas. We show the conversion of photons from 863 nm to the telecom O-band and demonstrate that the input polarization state is preserved. This process is intrinsically broadband and can be adapted to any other wavelength.
State-preserving frequency conversion in the optical domain is a necessary component in many configurations of quantum information processing and communication. Thus far, nonlinear crystals are used for this purpose. Here, we report on an approach based on coherent anti-Stokes Raman scattering (CARS) in a dense molecular hydrogen gas. This four-wave mixing process sidesteps the limitations imposed by crystal properties, it is intrinsically broadband and does not generate an undesired background. We demonstrate this method by converting photons from 434 nm to 370 nm and show that their polarization is preserved.