Quantum Network Secures Data over 100km with Single Atoms

Researchers have demonstrated a significant advance in secure communication with a device-independent quantum key distribution (DI-QKD) system operating over a 100km fibre optic network. Bo-Wei Lu, Chao-Wei Yang, and Run-Qi Wang, from the Hefei National Research Center for Physical Sciences at the Microscale and School of Physical Sciences, University of Science and Technology of China, alongside Bo-Feng Gao from the Jinan Institute of Quantum Technology and Yi-Zheng Zhen et al., achieved high-fidelity entanglement between single atoms, paving the way for practical quantum networks. By leveraging single-photon interference and quantum frequency conversion to minimise fibre loss, the team generated a positive asymptotic key rate over extended distances, culminating in the preparation of 1.2 million heralded Bell pairs at 11km with an estimated secure key rate of 0.112 bits per event. This work represents a crucial step towards bridging the gap between theoretical quantum communication and its implementation in real-world scenarios.

This achievement represents a significant step towards building a secure quantum internet, overcoming limitations present in earlier quantum communication systems.

The research leverages single-photon interference and quantum frequency conversion to enhance entanglement rates and minimise signal loss during transmission. A novel Rydberg-based emission scheme was tailored to suppress the photon recoil effect on the atom, maintaining signal integrity without introducing additional noise into the system.
High-fidelity atom-atom entanglement was successfully achieved, alongside positive asymptotic key rates extending up to 100km in fibre length. Specifically, over a 11km distance, 1.2 million heralded Bell pairs were prepared during 624 hours of operation. This resulted in an estimated extractable finite-size secure key rate of 0.112 bits per event against potential attacks.

The implementation utilises single 87Rb atoms trapped in optical tweezers, serving as long-lived quantum memories with a discrimination fidelity exceeding 99.5 percent. To improve the entangling rate, the system employs single-photon interference, enabling efficient long-distance entanglement distribution.

Quantum frequency conversion down-converts emitted 780-nanometer photons to 1.3-micrometer telecom wavelengths, reducing attenuation within the optical fibres. The tailored Rydberg-based emission process effectively suppresses the photon recoil effect, a common source of noise in similar experiments, without compromising signal quality. These combined advances bridge the gap between theoretical quantum network experiments and practical, real-world applications of secure communication.

Long-distance entanglement distribution via high-fidelity atom-atom coupling

A 72-qubit superconducting processor forms the foundation of this work, enabling the realization of device-independent quantum key distribution (DI-QKD) between two single-atom nodes connected by 100-km optical fibres. To enhance the rate of entanglement, single-photon interference was leveraged for entanglement heralding, a technique where detection of a photon signals the creation of entanglement.

Quantum frequency conversion was then implemented to mitigate fibre loss, improving signal transmission over long distances. A tailored Rydberg-based emission scheme was developed to suppress the photon recoil effect on the atoms, preserving qubit coherence without introducing extraneous noise into the system.

High-fidelity atom-atom entanglement was achieved, with fidelity exceeding 0.9 across all tested fibre lengths up to 100km. Entanglement fidelity was characterised at fibre lengths of 11, 20, 50, 70, and 100km using projective measurements of the atomic qubits in both the XX and ZZ bases. The fidelity was calculated as F = 1/4 [1 + VZZ + 2VXX], where VZZ and VXX represent the measured visibilities in the respective bases.

To preserve qubit coherence during delays inherent in long-distance communication, dynamical decoupling (DD) sequences were applied to each atomic memory, extending coherence to over 300ms by transferring the qubit into magnetically insensitive clock states. At 11km, 1.2 million heralded Bell pairs were prepared over 624 hours, yielding an estimated extractable finite-size secure key rate of 0.112 bits per event against general attacks.

The atomic hyperfine qubits were measured using a push-out method, with the measurement basis set by a Raman pulse following single-photon detection. Fluorescence detection, employing a single aspheric lens with 5ms imaging, was used due to the non-space-like separation of measurement events. Employing an XY-4 dynamical decoupling sequence extended memory coherence for longer fibre lengths, resulting in measured fidelities of 0.947 ±0.005 (11km), 0.933 ±0.006 (20km), 0.931 ±0.008 (50km), 0.921 ±0.009 (70km), and 0.911±0.010 (100km). The success probability for entanglement generation was optimised by balancing excitation probability and overall photon transmission, demonstrating a significant boost in entanglement generation rate compared to two-photon interference schemes.

Rydberg-atom entanglement and key distribution over 100-kilometre fibre links

Device-independent quantum key distribution (DI-QKD) was realised between two single-atom nodes linked by 100-km fibres. To enhance the entangling rate, single-photon interference was leveraged for entanglement heralding, and quantum frequency conversion was employed to mitigate fibre loss. A specialised Rydberg-based emission scheme effectively suppressed the photon recoil effect on the atom without introducing additional noise into the system.

High-fidelity atom-atom entanglement was achieved, alongside positive asymptotic key rates sustained for fibre lengths up to 100 kilometers. Specifically, at a distance of 11 kilometers, 1.2 million heralded Bell pairs were successfully prepared over a period of 624 hours. This preparation yielded an estimated extractable finite-size secure key rate of 0.112 bits per event against general attacks.

The research demonstrates a significant step towards practical quantum communication networks by closing the gap between theoretical quantum network experiments and viable real-world applications. Remote entanglement generation utilised the single-photon interference scheme, which minimised photon loss and facilitated efficient long-distance entanglement distribution.

Fibre attenuation was further reduced by down-converting the emitted 780-nanometer photons to a 1.3-micrometer telecom wavelength. The tailored Rydberg-based single-photon emission process strongly suppressed the photon recoil effect in single-photon interference, without introducing noise. Each quantum node incorporated a single 87Rb atom trapped in optical tweezers, serving as a long-lived quantum memory with a discrimination fidelity exceeding 99.5 percent.

Rydberg atom entanglement and secure key distribution over extended fibre links

Researchers have demonstrated device-independent quantum key distribution (DIQKD) between single-atom nodes connected by 100-kilometre fibre optic cables. This achievement leverages single-photon interference to enhance the rate of entanglement and employs quantum frequency conversion to minimise signal loss within the fibres.

A specialised Rydberg-based emission scheme was implemented to mitigate the effects of photon recoil on the atoms, ensuring high-fidelity entanglement without introducing additional noise. The experiment successfully generated high-fidelity entanglement between atoms and yielded positive asymptotic key rates over distances up to 100km.

Specifically, over 1.2 million heralded Bell pairs were prepared across 11km of fibre over 624 hours, resulting in an estimated extractable finite-size secure key rate of 0.112 bits per event against potential attacks. These results represent a significant step towards bridging the gap between theoretical quantum network experiments and practical, real-world applications of secure communication.

The authors acknowledge limitations related to the complexity of maintaining stable entanglement over extended periods and the challenges of scaling the system to larger networks. Future research will likely focus on improving the efficiency of entanglement generation and distribution, alongside exploring methods for integrating these nodes into existing communication infrastructure. Further development could also address the practical challenges of implementing quantum repeaters to extend the range of DIQKD beyond the current 100km limit, paving the way for a more secure and robust quantum internet.