Efficient telecom frequency conversion (TFC) in atomic systems is crucial for integrating atom-based quantum nodes into low-loss fiber-optic quantum networks. Here, we demonstrate high-efficiency TFC from 795 nm to 1367 nm in a cold 87Rb ensemble via diamond-type four-wave mixing (FWM), achieving conversion efficiencies of 66% and 80% at optical depths of 75 and 110, respectively, using a weak coherent probe field. These results surpass all previously reported values in atomic systems, enabled by a systematic investigation of the built-in V-type and cascade-type electromagnetically induced transparency spectra that guided the optimization of FWM conditions. Although this work employs coherent fields, our previous theoretical study has shown that quantum states can be preserved with high fidelity during the conversion process, highlighting the promise of diamond-type atomic FWM as a robust interface for long-distance quantum communication.
Efficient telecom frequency conversion (TFC) in atomic systems is crucial for integrating atom-based quantum nodes into low-loss fiber-optic quantum networks. Here, we demonstrate high-efficiency TFC from 795 nm to 1367 nm in a cold 87Rb ensemble via diamond-type four-wave mixing (FWM), achieving conversion efficiencies of 66% and 80% at optical depths of 75 and 110, respectively, using a weak coherent probe field. These results surpass all previously reported values in atomic systems, enabled by a systematic investigation of the built-in V-type and cascade-type electromagnetically induced transparency spectra that guided the optimization of FWM conditions. Although this work employs coherent fields, our previous theoretical study has shown that quantum states can be preserved with high fidelity during the conversion process, highlighting the promise of diamond-type atomic FWM as a robust interface for long-distance quantum communication.
We experimentally investigate the frequency down-conversion through the four-wave mixing (FWM) process in a cold 85 Rb atomic ensemble, with a diamond-level configuration. An atomic cloud with a high optical depth (OD) of 190 is prepared to achieve a high efficiency frequency conversion. Here, we convert a signal pulse field (795 nm) attenuated to a single-photon level, into a telecom light at 1529.3 nm within near C-band range and the frequency-conversion efficiency can reach up to 32%. We find that the OD is an essential factor affecting conversion efficiency and the efficiency may exceed 32% with an improvement in the OD. Moreover, we note the signal-to-noise ratio of the detected telecom field is higher than 10 while the mean signal count is larger than 0.2. Our work may be combined with quantum memories based on cold 85 Rb ensemble at 795 nm and serve for long-distance quantum networks.
In a fiber-based quantum network, the utilization of the telecom band is crucial for long-distance quantum information (QI) transmission between quantum nodes. However, the near-infrared wavelength is identified as optimal for storing and processing QI through alkaline atoms. Recognizing the challenge of efficiently bridging the frequency gap between atomic quantum devices and telecom fibers while maintaining the QI carried by photons, quantum frequency conversion (QFC) serves as a pivotal quantum interface. In this study, we explore an efficient telecom-band QFC mechanism based on diamond-type four-wave mixing (FWM) with rubidium energy levels. The mechanism enables the conversion of photons between the near-infrared wavelength of 795 nm and the telecom band of 1367 or 1529 nm. Using the Heisenberg-Langevin approach, we optimize conversion efficiency (CE) across varying optical depths while considering quantum noises and present corresponding experimental parameters. Unlike previous works neglecting the applied field absorption loss, our results are more relevant to practical scenarios. Moreover, by employing the reduced-density-operator theory to construct a theoretical framework, we demonstrate that this diamond-type FWM scheme can maintain the quantum characteristics of input photons with high fidelity, such as quadrature variances and photon statistics. Importantly, these properties remain unaffected by vacuum field noise, enabling the system to achieve high-purity QFC. Another significant contribution lies in examining how this scheme impacts QI encoded in photon-number, path, and polarization degrees of freedom. These encoded qubits exhibit remarkable entanglement retention under sufficiently high CE. In the case of perfect CE, the scheme can achieve unity fidelity. This comprehensive exploration establishes a theoretical foundation for the application of the diamond-type QFC scheme based on atomic ensembles in quantum networks, laying essential groundwork for advancing the scheme in distributed quantum computing and long-distance quantum communication.
Signal photons emitted by quantum nodes typically fall outside the low-loss telecom window of optical fibers, leading to severe transmission losses. Quantum frequency conversion (QFC) offers an effective optical interface that bridges quantum nodes with telecom-band channels, enabling long-distance quantum communication. In this work, we demonstrate a compact, fiber-integrated QFC system with low noise and a high signal-to-noise ratio (SNR). Using a periodically poled lithium niobate (PPLN) waveguide, input photons at 637.2 nm are down-converted to telecom photons at 1588.3 nm. Our system achieves a total conversion efficiency of approximately 9%, with pump-induced noise suppressed to 154 Hz. For input photon rates of 32.7, 118.0, and 327.7 kHz, the corresponding SNRs are 12.3, 43.9, and 117.8, respectively. We further develop a theoretical model to simulate the entanglement fidelity between nitrogen-vacancy (NV) center spins and the frequency-converted telecom photons. At the emission rate of an NV center, our QFC system maintains an expected fidelity exceeding 52% over a transmission distance of 100 km. These findings highlight the potential of our QFC system for scalable, long-distance quantum networking.
Efficient frequency conversion of photons has important applications in optical quantum technology because the frequency range suitable for photon manipulation and communication usually varies widely. Recently, an efficient frequency conversion system using a double- Λ four-wave mixing (FWM) process based on electromagnetically induced transparency (EIT) has attracted considerable attention because of its potential to achieve a nearly 100% conversion efficiency (CE). To obtain such a high CE, the spontaneous emission loss in this resonant-type FWM system must be suppressed considerably. A simple solution is to arrange the applied laser fields in a backward configuration. However, the phase mismatch due to this configuration can cause a significant decrease in CE. Here, we demonstrate that the phase mismatch can be effectively compensated by introducing the phase shift obtained by two-photon detuning. Under optimal conditions, we observe a wavelength conversion from 780 to 795 nm with a maximum CE of 91.2 % ± 0.6 % by using this backward FWM system at an optical depth of 130 in cold 87 Rb atoms. The current work represents an important step toward achieving low-loss, high-fidelity quantum frequency conversion based on EIT.
In a typical quantum repeater scenario, remote quantum memories have to be interconnected by flying qubits transmitted in quantum channels. e.g. single photons in photonic fiber links. To establish long-distance communication, photons in the low-loss telecom wavlength regime are desirable. However, the majority of the candidates for quantum memories such as trapped atoms or ions, rare earth ensembles, color centers in diamond, atomic ensembles etc. possess optical transitions merely in the visible or near-infrared spectral regime below 1000 nm. One possible solution is the transduction of these wavelengths to the telecom region via quantum frequency conversion (QFC) based on three-wave-mixing processes in a nonlinear χ(2)-medium. Technical achievements in recent years enabled high conversion efficiencies and low conversion-induced noise contributions resulting in signal-to-background ratios high enough to demonstrate the preservation of several classical and quantum properties of the photons, e.g. first- and second-order coherence [1], time-bin entanglement [2], polarization entanglement [3] or indistinguishability [4]. QFC thus provides a powerful toolbox to interface quantum systems to the telecom bands and beyond that yet to interconnect dissimilar quantum systems.
Abstract Entanglement between a stationary quantum system and a flying qubit is an essential ingredient of a quantum-repeater network. It has been demonstrated for trapped ions, trapped atoms, color centers in diamond, or quantum dots. These systems have transition wavelengths in the blue, red or near-infrared spectral regions, whereas long-range fiber-communication requires wavelengths in the low-loss, low-dispersion telecom regime. A proven tool to interconnect flying qubits at visible/NIR wavelengths to the telecom bands is quantum frequency conversion. Here we use an efficient polarization-preserving frequency converter connecting 854 nm to the telecom O-band at 1310 nm to demonstrate entanglement between a trapped 40Ca+ ion and the polarization state of a telecom photon with a high fidelity of 98.2 ± 0.2%. The unique combination of 99.75 ± 0.18% process fidelity in the polarization-state conversion, 26.5% external frequency conversion efficiency and only 11.4 photons/s conversion-induced unconditional background makes the converter a powerful ion–telecom quantum interface.