Quantum networks provide conduits capable of transmitting quantum information that connect to nodes at remote locations where the quantum information can be stored or processed. Fiber-based transmission of quantum information over long distances may be achieved using quantum memory elements and quantum repeater protocols. However, atombased quantum memories typically involve interactions with light fields outside the telecom window needed to minimize absorption in transmission by optical fibers. We report on progress towards a quantum memory based on the generation of 795 nm spontaneously emitted single photons by a write-laser beam interacting with a cold 87Rb ensemble. The single photons are then frequency-converted into (out of) the telecomm band via difference (sum) frequency generation in a PPLN crystal. Finally, the atomic state is read out via the interaction of a read-pulse with the quantum memory. With such a system, it will be possible to realize a long-lived quantum memory that will allow transmission of quantum information over many kilometers with high fidelity, essential for a scalable, long-distance quantum network.
Quantum networks provide conduits capable of transmitting quantum information that connect to nodes at remote locations where the quantum information can be stored or processed. Fiber-based transmission of quantum information over long distances may be achieved using quantum memory elements and quantum repeater protocols. However, atombased quantum memories typically involve interactions with light fields outside the telecom window needed to minimize absorption in transmission by optical fibers. We report on progress towards a quantum memory based on the generation of 795 nm spontaneously emitted single photons by a write-laser beam interacting with a cold 87Rb ensemble. The single photons are then frequency-converted into (out of) the telecomm band via difference (sum) frequency generation in a PPLN crystal. Finally, the atomic state is read out via the interaction of a read-pulse with the quantum memory. With such a system, it will be possible to realize a long-lived quantum memory that will allow transmission of quantum information over many kilometers with high fidelity, essential for a scalable, long-distance quantum network.
Constructing a quantum memory node with the ability of long-distance atom-photon distribution is the essential task for future quantum networks, enabling distributed quantum computing, quantum cryptography, and remote sensing. Here we report the demonstration of a quantum-network node with a simple cavity-free cold atomic ensemble. This node gives an initial retrieval efficiency of approximately 55% and memory lifetime of 160 μs for atomic qubits. With the aid of a high-efficiency and polarization-independent quantum frequency conversion (QFC) module, the generated entangled photon in the node at 780-nm wavelength is converted to telecom S band at 1522 nm, enabling atom-photon distribution over long distance. We observe an entanglement fidelity between the atoms and telecom photon exceeding 80% after photon transmission over 20-km fiber with an end excitation probability of 0.2% and repetition of 1.7 kHz, the remaining infidelity being dominated by atomic decoherence. The low-noise QFC with an external efficiency up to 48.5% gives a signal-to-noise ratio of 6.9 for transmitted photons with fiber length up to 100 km, laying the cornerstone for entanglement distribution at a hundred-km level. This result provides a new platform toward the realization of a long-distance quantum network.
Realization of long-distance quantum communication through the quantum repeater network requires a combination of some key elements, including a multiplexed quantum memory for storage of entanglement and a telecom photon for propagation of information through the fiber.Although impressive experimental advances have demonstrated the individual elements, combining these key capabilities together and realizing them in a single experimental system remains a significant challenge.Here, we report an experimental realization of long-distance entanglement between a multiplexed quantum memory with 49 individually accessible memory cells and a telecom photon after transmission in a 10-km optical fiber.Excitation of an atomic ensemble generates narrow-band polarization entanglement between a telecom photon of 1530-nm wavelength and another photon of 780-nm wavelength, which is then stored into a memory cell of a multiplexed atomic quantum memory and read out after a controllable delay.The entanglement is verified through quantum-state tomography after quantum storage in the atomic memory and fiber transmission of the telecom photon.This experiment demonstrates an important step towards realization of long-distance quantum communication networks.
In quantum repeater schemes for long distance quantum communication, quantum memories (QMs) play a vital role. For these applications, QMs need to be connected to the fiber optics network. However most QMs operate at wavelengths where the absorption in optical fibers is high. Therefore, their connection to a quantum frequency converter (QFC) to telecom wavelengths is required. In this work we convert an heralded single photon emitted by a rubidium atomic ensemble QM, using a QFC implemented with a non-linear waveguide. The main advantages of this converter setup are its compactness, relative simplicity and wavelength flexibility. We show that after this process the non-classical correlations between the heralding photons and converted heralded photons generated in the QM are preserved. This is the first time that frequency conversion of non-classical light emitted by an atomic QM is performed with a solid state device.
The realization of a large-scale quantum network of interconnected quantum nodes is one of the most promising directions in quantum technology. Such a network distributes entanglement between remote matter qubits via single photons in optical fibers, enabling quantum repeaters and long-distance secure communication and supporting emerging applications such as distributed quantum computing and network-enhanced metrology. To realize this vision, proposed architectures require quantum nodes that provide an efficient quantum light-matter interface for entanglement generation, serve as a quantum memory allowing long-lived storage of quantum states, and have the possibility to connect to telecommunication fiber-based photonic channels. This thesis presents the development and experimental realization of a quantum network node based on a single Rb-87 atom. A single atom is confined in a tightly focused optical dipole trap, and entanglement is generated between the polarization of an emitted photon and the Zeeman state of the atom. To enable long-lived quantum storage, we coherently map the atomic qubit to a basis less sensitive to magnetic fields using Zeeman-state-selective Raman transfer, reducing magnetic field sensitivity by a factor of 500. Such hyperfine qubit control is implemented via a two-photon Raman process driven by a laser field that is amplitude modulated at the qubit splitting using a fiber-based Mach-Zehnder interferometer. With this approach, we achieve a 1/e storage lifetime of 10 ms, extending coherence by a factor of 30 compared with the initial qubit encoding. Residual dephasing arises primarily from position-dependent effective magnetic fields caused by the tightly focused optical dipole trap beam together with atomic motion, and the differential AC-Stark shifts come from the trap detuning. While the first can be fully canceled via a standing-wave dipole trap, which needs to change the trap setup. We mitigate the second effects with a spin-echo pulse, extending coherence by a factor of two. Nonetheless, the significantly longer coherence time already allows high-fidelity entanglement distribution over fiber links with a length of a few hundred kilometers. To reduce fiber attenuation for long-distance distribution, we convert the emitted 780 nm single photons to the low-loss telecom S band using difference-frequency generation in a periodically poled lithium niobate waveguide, pumped at 1607 nm to produce 1517 nm photons. By combining long-lived quantum memory with a telecom interface, we demonstrated long-distance atom-photon entanglement distribution through 50 km and 101 km of spooled fiber with fidelities of 85% and 71%, respectively, establishing a key component for scalable quantum communication. These results establish neutral atom nodes as promising building blocks for metropolitan-scale quantum networking and outline a scalable path toward repeater protocols based on multiplexing memories and Rydberg-mediated interactions.
Long-distance entanglement distribution is the key task for quantum networks, enabling applications such as secure communication and distributed quantum computing.In this work, we take a crucial step toward this task by sharing entanglement over long optical fibers between a single 87 Rb atom and a single photon.High fidelity of the atomic state could be maintained during long flight times through such fibers by prolonging the coherence time of the single atom to 10 ms based on encoding in long-lived states.In addition, the attenuation in the fibers is minimized by converting the wavelength of the photon to the telecom S band via polarization-preserving quantum frequency conversion.These improvements enable us to observe entanglement between the atomic quantum memory and the emitted photons transmitted through standard spooled telecom fibers with a length of 101 km with a fidelity of 70.8 ± 2.4%.This fidelity is comparable to recent demonstrations over 20 km, despite the channel loss now significantly exceeding 20 dB.In fact, now the reduction in fidelity is due to detector dark counts rather than loss of coherence of the atom or photon, proving the suitability of our platform to realize city-to-city-scale quantum network links.
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.