We present the implementation of a time synchronization protocol as a part of an experimentally deployed entanglement-based quantum key distribution (QKD) link. The system is deployed over 48 km of optical fibers across the Métropole Côte d'Azur and enables secret cryptographic key exchange between two remote users, with an average rate of 7 kbps. We exploit the time correlation of paired photons generated by a high-quality source of energy-time entanglement implemented in the QKD link to synchronize two rubidium clocks located at the end stations. The level of stability achieved guarantees a time offset between the clocks under 12 ps at all time. We also show that this protocol requires less hardware than a typical synchronization protocol for QKD that would distribute a reference clock signal between the users.
We present the implementation of a time synchronization protocol as a part of an experimentally deployed entanglement-based quantum key distribution (QKD) link. The system is deployed over 48 km of optical fibers across the Métropole Côte d'Azur and enables secret cryptographic key exchange between two remote users, with an average rate of 7 kbps. We exploit the time correlation of paired photons generated by a high-quality source of energy-time entanglement implemented in the QKD link to synchronize two rubidium clocks located at the end stations. The level of stability achieved guarantees a time offset between the clocks under 12 ps at all time. We also show that this protocol requires less hardware than a typical synchronization protocol for QKD that would distribute a reference clock signal between the users.
Abstract The quantum entanglement distribution network, serviced as the communication infrastructure which distributes quantum information among remote users, enables many applications beyond the reach of classical networks. Recently, the applications such as quantum key distribution and quantum secure direct communication, have been successfully demonstrated in the quantum entanglement distribution network. In this article, we propose a multi-user round-trip quantum clock synchronization (QCS) scheme in the quantum network, which can be implemented with one single entangled photon source located at the server. The server distributes the entangled photons to remote multiple users with the wavelength division multiplexing strategy, and each user feeds partial received photons back to the server. The clock difference between the server and each user is calculated from the one-way and round-trip propagation times, which are determined according to the time correlation of entangled photons. Afterwards, the demonstration has been conducted between the server and a user over a 75-km-long fiber link, where the measured clock difference uncertainty is 4.45 ps, and the time deviation is 426 fs with an average time of 4000 s. Furthermore, the proposed QCS scheme is linearly scalable to many users, with respect to user hardware and number of deployed fibers.
Entanglement-based quantum links are the backbone of future quantum internet networks, enabling secure communication between distant cities. Realizing such networks requires addressing multiple practical challenges in long-distance quantum key distribution : time synchronisation, interferometer stabilisation and automation. Here, we report several advances. First, we maintained an operational QKD link continuously for 325hours over 50km between two remotes locations, demonstrating the feasibility of long-duration key generation. We further extended secure key distribution up to a 100km operational link connecting the University of Nice to a ground-based optical station, a setup compatible with future quantum satellite connections. Finally, by employing wavelength demultiplexing to separate photons of entangled pairs, we performed QKD across multiple ITU channels, achieving secure key exchange via the BBM92 protocol and time-energy observables.
We present a field-deployed quantum key distribution (QKD) link based on energy-time entanglement. Three nodes are connected, bridging the cities of Nice and Sophia Antipolis, by means of commercial grade optical fibers, spanning over a total distance of 50 km. We have built a packaged and resilient source of energy-time entangled photon pairs and implemented remote interferometer stabilization. Moreover, the clocks of the two end users are synchronized exploiting photon-counting techniques only, therefore requiring no dedicated channel. The system runs over the ITU 100-GHz standard telecom grid, through which a secure key rate of 7.0 kbit/s is extracted for one pair of channels. A homemade software performs all the necessary postprocessing procedures enabling to establish secret keys in real time. We present the first fully operational entanglement-based metropolitan QKD system being totally self-sufficient, i.e., requiring no additional overheads to be operated on a conventional metropolitan fiber network.
Quantum key distribution (QKD) enables the generation of information-theoretic secure keys between two remote parties. A critical component of successful QKD implementation is the effective synchronization of clocks between these users. Traditional methods of locking to a reference clock or transmitting a synchronization signal rely on auxiliary equipment, which increases the complexity and cost, thus hindering the widespread deployment of QKD. Here, we propose a method to achieve clock and pulse synchronization by exploiting a subset of the transmitted qubits. We demonstrate our scheme with a time-bin phase-coding BB84 QKD system and verify the realization of long-term stable key generation over distances of 50 km and 100 km. Our method simplifies the implementation of QKD systems and introduces new possibilities for synchronization algorithms in other quantum communication protocols.
We present a real field quantum key distribution link based on energy-time entanglement. Three nodes are connected over the city of Nice by means of optical fibers with a total distance of 50\,km. We have implemented a high-quality source of energy-time entangled photon pairs and actively stabilized analysers to project the quantum states, associated with an innovative remote synchronization method of the end stations' clocks which does not require any dedicated channel. The system is compatible with the ITU 100\,GHz standard telecom-grid, through which a raw key rate of 40\,kbps per pair of channels is obtained. A post-treatment software performs all the necessary post-processing procedures enabling to establish secret keys in real time. All of those embedded systems and achieved performance make this network the first fully operational entanglement based metropolitan quantum network to be implemented in real field.
Abstract Entanglement-based quantum key distribution (QKD) is an essential ingredient in quantum communication, owing to the property of source-independent security and the potential on constructing large-scale quantum communication networks. However, implementation of entanglement-based QKD over long-distance optical fiber links is still challenging, especially over deployed fibers. In this work, we report an experimental QKD using energy-time entangled photon pairs that transmit over optical fibers of 242 km (including a section of 19 km deployed fibers). The QKD is realized through the protocol of dispersive-optics QKD (DO-QKD) with high-dimensional encoding to utilize coincidence counts more efficiently. A reliable, high-accuracy time synchronization technology for long-distance entanglement-based QKD is developed based on the distribution of optical pulses in quantum channels. Our system operates continuously for more than 7 d without active polarization or phase calibration. We ultimately generate secure keys with secure key rates of 0.22 bps and 0.06 bps in the asymptotic and finite-size regimes, respectively. It shows that entanglement-based DO-QKD is reliable for long-distance realization in the field if its high requirement on time synchronization is satisfied.