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Quantum 2.0 Conference and Exhibition

Towards Entanglement Distribution in a Suburban Quantum Network

Pooja Malik, Tommy Block, Florian Fertig, Yiru Zhou, Harald Weinfurter, Maya Büki, Tobias Frank, Marvin Scholz, Gianvito Chiarella, Pau Farrera, Gerhard Rempe

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We demonstrate first steps toward atom-atom entanglement over 24 km of deployed fiber by distributing entanglement between a Rb-87 atom and a photon using quantum frequency conversion from 780 nm to low-loss telecom S-band.
DOI
10.1364/quantum.2025.qw4a.4

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Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation, it will enable efficient distribution of quantum states. We report on the generation and observation of entanglement between a ^{87}Rb atom and a photon at telecom wavelength transmitted through up to 20 km of optical fiber. For this purpose, we use polarization-preserving quantum frequency conversion to transform the wavelength of a photon entangled with the atomic spin state from 780 nm to the telecom S band at 1522 nm. We achieve an unprecedented external device conversion efficiency of 57% and observe an entanglement fidelity between the atom and telecom photon of ≥78.5±0.9% after transmission through 20 km of optical fiber, mainly limited by decoherence of the atomic state. This result is an important milestone on the road to distribute quantum information on a large scale.
Entanglement distribution is the overarching purpose of quantum networks. While communication over long distances can use deployed fiber infrastructure, it requires photons in the telecom band. However, advanced quantum network nodes do not operate at such wavelengths. Here we overcome this limitation with two tailor-made low-noise quantum-frequency converters to distribute entanglement between a single atom and a resonant photon over 14km line-of-sight via 24km of a deployed commercial fiber. The photon at wavelength 780nm is first entangled with the atom, then converted to the telecom S-band, and finally back-converted after propagation through the fiber. This link enables a photon transfer efficiency of 1.7% while affecting the atom-photon entanglement fidelity by less than 1%. This brings integration of atomic quantum nodes with existing long-distance fiber networks into reach, enabling novel applications in quantum information processing.
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.
Long-distance entanglement distribution is the key task for quantum networks, enabling applications such as secure communication and distributed quantum computing. Here we report on novel developments extending the reach for sharing entanglement between a single 87^{87}87Rb atom and a single photon over long optical fibers. To maintain a high fidelity during the long flight times through such fibers, the coherence time of the single atom is prolonged to 7 ms by applying a long-lived qubit encoding. In addition, the attenuation in the fibers is minimized by converting the photon's wavelength to the telecom S-Band via polarization-preserving quantum frequency conversion. This enables to observe entanglement between the atomic quantum memory and the emitted photon after passing 101 km of optical fiber with a fidelity better than 70.8$\pm$2.4%. The fidelity, however, is no longer reduced due to loss of coherence of the atom or photon but in the current setup rather due to detector dark counts, showing the suitability of our platform to realize city-to-city scale quantum network links.
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.
Quantum nonlocality, typically revealed through entanglement distribution across quantum networks, is a cornerstone of quantum information science. Long-distance distribution of entanglement requires the information carrier, i.e. flying photons, to operate in the minimum-loss telecom band of optical fiber. While extensive efforts have been devoted to the direct generation of entanglement between C-band telecom photons and various stationary spins, the verification of quantum nonlocality remains an outstanding challenge. Here, utilizing a dipole transition in rubidium atoms with a wavelength of 1530 nm and a cavity-assisted protocol, we achieve resonant excitation and direct emission of C-band telecom photons from a single atom, generating spin-photon entanglement with a measured Bell state fidelity exceeding 91.4%. We then verify Bell nonlocality by observing a Bell inequality violation of 2.455(77) > 2 using this high-quality entangled pair. These results extend the wavelength of a single-atom quantum emitter to the telecom C-band, achieving sufficiently high-fidelity spin-photon entanglement to finally verify Bell nonlocality. This work thereby provides a promising building block for a large-scale atom-based quantum network capable of distributed quantum metrology and long-distance quantum communication.