Flexible entanglement-distribution network with an AlGaAs chip for secure communications

Large-scale quantum communication networks promise revolutionary advancements in secure communication, clock synchronization, and distributed quantum computing. The scalability of these networks hinges on miniaturized, flexible, and cost-effective resources. Existing entanglement distribution architectures, while demonstrating feasibility, face challenges in scalability due to the quadratic increase in required frequency channels with the number of users and the high losses associated with cascaded passive elements like dense wavelength division multiplexers (DWDM) and beamsplitters. This work addresses these challenges by combining a broadband entangled photon source on an AlGaAs chip with industry-standard flexible-grid wavelength division multiplexing (WDM) techniques. The AlGaAs chip offers advantages due to its direct bandgap, strong electro-optical effect, and low birefringence, enabling the generation of polarization-entangled states without additional optical components. The use of WSS technology allows for straightforward reconfigurability in terms of central frequency and channel bandwidth, displaying attractive features for flexible entanglement distribution.

Previous research explored entanglement distribution networks using various methods, such as wavelength-multiplexed distribution of entangled photon pairs and active routing of entanglement. While these demonstrated successful entanglement distribution among a few users, limitations existed in scalability due to narrow bandwidths and high losses from cascaded passive elements. The use of a fully connected architecture (where each user shares entanglement with every other user) was explored, but the quadratic scaling of required frequency channels posed a significant obstacle for larger networks. Recent work improved scaling, but narrowband sources still restricted the number of users. The use of advanced multiplexing techniques based on wavelength-selective switches (WSS) was investigated, offering potential for flexible entanglement distribution, yet again was limited by relatively narrowband entangled photon sources. This study builds upon these advancements by using a broadband source and WSS technology to overcome the scalability limitations.

The experimental setup consists of three stages: entanglement generation using an AlGaAs chip, frequency demultiplexing/multiplexing using either a WSS (in the C-band) or a CWDM unit followed by a tunable filter (in the L-band), and a distribution stage. The AlGaAs chip, employing type-II spontaneous parametric down-conversion (SPDC), produces time-energy and polarization-entangled photons in a |ψ⁺⟩ Bell state. The broadband nature of the source allows multiplexing the photons into numerous wavelength channels. The biphoton bandwidth was measured using a fibered beam splitter and tunable filter, revealing a 60 nm bandwidth, roughly equivalent to 72 ITU 100 GHz channels. Entanglement fidelity was assessed by measuring the fidelity to the |ψ⁺⟩ Bell state as a function of channel number, exceeding 95% over a 26 nm range and remaining above 85% over a 60 nm range. A theoretical model, incorporating cavity effects from facet reflectivity, accurately predicted the experimental fidelity. For QKD, the BBM92 protocol was implemented. The asymptotic secret key rate (Rkey) and quantum bit error rate (QBER) were calculated based on coincidence counts. Long-distance performance was evaluated by adding 25 km SMF28 fiber spools and variable attenuation, assessing both symmetric and asymmetric configurations. Multi-user networks (4, 5, and 8 users) were implemented by reconfiguring the WSS to adjust channel bandwidths, demonstrating the network's flexibility and scalability. Finally, bandwidth reallocation was used to optimize an unbalanced network with one distant user, showcasing the system's compatibility with elastic network configurations.

The AlGaAs chip demonstrated high-quality polarization entanglement over a broad (60 nm) spectral range, with fidelity exceeding 95% near degeneracy and staying above 85% across the entire range. The BBM92 QKD protocol yielded QBERs below 2% and high asymptotic key rates (28-39 bits/s) across 13 ITU 100 GHz channels. Long-distance tests with 25 km SMF28 fibers maintained positive key rates, even considering finite-key effects, up to 75 km in both symmetric and asymmetric configurations. Multi-user networks with up to 8 users were successfully demonstrated with 50 GHz channels, showcasing the scalability of the system. The fidelity remained largely insensitive to channel bandwidth changes, ensuring high QKD performance even with bandwidth adjustments. Bandwidth reallocation effectively balanced the signal across an unbalanced network with one distant user, highlighting the system's adaptability to real-world network constraints.

The results address the scalability challenges of quantum networks by demonstrating high-fidelity entanglement distribution across a broad spectral range, enabling multiple users to share entangled states concurrently. The use of an AlGaAs chip provides a compact, potentially cost-effective, and easily integrable source. The flexible-grid WDM technology ensures the network's adaptability to changing network demands, allowing for dynamic bandwidth allocation and optimization. The demonstration of high-performance QKD over metropolitan distances in both symmetric and asymmetric configurations proves the system's practicality for real-world scenarios. The successful implementation of multi-user networks and the ability to balance an unbalanced network significantly advance the feasibility of deploying large-scale quantum communication networks.

This study demonstrates a highly scalable and flexible entanglement distribution network using a broadband AlGaAs photon source and WSS technology. The achieved high fidelity, low QBER, and high key rates across multiple users and long distances pave the way for practical deployment of metropolitan-scale quantum networks. Future work could focus on further miniaturizing the source, optimizing the chip design to broaden the bandwidth further, and exploring different network topologies to accommodate even more users. Implementing the architecture in a real-world metropolitan network would provide valuable insights into its long-term performance and robustness.

The current system utilized manual adjustment of the polarization analysis stage to compensate for polarization rotation in the fibers. Future systems should incorporate active polarization controllers for automated compensation. The multi-user experiments did not perform real-time QKD sessions for all users in parallel due to detector limitations. A more comprehensive analysis of the impact of cavity effects on entanglement fidelity at larger detunings from degeneracy would enhance the understanding of the system's performance limitations.