A Tuneable Telecom Wavelength Entangled Light Emitting Diode Deployed in an Installed Fibre Network

The integration of quantum light sources with existing telecommunication infrastructure is crucial for advancing quantum network applications. Current quantum key distribution (QKD) systems often rely on weak coherent laser pulses, but sources based on semiconductor quantum dots (QDs) offer advantages such as a natural lack of multi-photon pair generation, leading to improved security against photon number splitting attacks. For practical network deployment, these sources need to operate at telecom wavelengths and possess wavelength tuneability, ideally matching the coarse wavelength division multiplexing (CWDM) standards (20 nm channel spacing) for multiplexing with classical data. This allows for efficient integration with existing fiber optic networks and minimizes interference with conventional communication signals. Existing QD-based sources often rely on optical excitation, requiring complex and bulky setups unsuitable for field deployment. This work focuses on developing and deploying a fully electrically operated, tuneable ELED that addresses these challenges, enabling true network integration of entangled photon sources.

Entanglement, a fundamental resource for quantum communication, offers advantages over classical methods in various applications. Semiconductor QDs embedded in p-i-n diodes are a promising platform for generating entangled photon pairs via the biexciton cascade. While resonant excitation schemes achieve high photon indistinguishability, they require sophisticated pulsed laser systems, hindering field deployment. Non-resonant excitation offers simpler integration but typically results in lower efficiency and purity. Recent advancements have yielded QDs emitting in telecom wavelength bands, including successful transmission over deployed fiber networks. However, integrating these sources with classical data traffic in the telecom C-band (favoring O-band operation for quantum light due to reduced Raman scattering) and achieving the desired 20 nm tuneability for CWDM compatibility remained a challenge. Previous attempts at tuning QD emission using magnetic fields or strain lacked the practicality required for network deployment. Static electric fields offer a more suitable approach for long-term stable operation, but achieving sufficient tuning range remained an issue.

The researchers designed a tuneable ELED with a ring-circle structure. The central circular region contains QDs, optically excited by a surrounding "pumping diode." A reverse bias applied to the inner "tuning diode" allows for static field tuning of QD emission. The device utilizes InAs/GaAs QDs grown using the bimodal growth mode, emitting in the telecom O-band. A 5 nm InGaAs layer serves as an optical pump and absorption layer, improving on-chip optical excitation efficiency. Stacked distributed Bragg reflector (DBR) mirrors enhance emission in the O-band. The AlGaAs barriers around the QD layer prevent charge carrier escape, enabling large electric field application. The quantum confined Stark effect, enhanced by the InGaAs QW, enables significant wavelength tuneability. A single QD was selected based on brightness and entanglement fidelity. Wavelength tuneability was characterized by varying the reverse bias on the tuning diode. Fine structure splitting (FSS) was measured via time-resolved photon-pair correlations. For the field deployment, the ELED, housed in a cryostat, was connected to a metropolitan fiber network in Cambridge. Entangled photon transmission was evaluated using a spectral filter, polarisation control and single-photon detectors. An innovative method based on time-resolved photon-pair correlations in randomly oriented detection bases was used for precise calibration of the detection basis. Entanglement fidelity was measured by recording photon-pair correlations in different bases (HV, DA, RL). Classical data traffic was multiplexed with the entangled qubits using CWDM. The second-order autocorrelation function (g⁽²⁾(0)) was measured to assess single-photon purity before and after transmission. The experiment ran continuously for 40 hours to assess long-term stability.

The tuneable ELED demonstrated a wavelength shift of over 25 nm with low-voltage operation, exceeding the tuneability of short-wavelength devices and satisfying CWDM requirements. The FSS remained below the 10 µeV threshold over the entire tuning range, ensuring entangled photon-pair emission. In the field deployment, high entanglement fidelity (over 94%) was achieved even after multiplexing with classical data traffic over a 15 km installed fiber link. The single-photon purity remained significantly anti-bunched (g⁽²⁾(0) = 0.26) after transmission. The entanglement fidelity remained stable at around 94.4 ± 0.3% over 40 hours of continuous operation. The experiment demonstrated successful multiplexing of entangled qubits from a sub-Poissonian source with classical data traffic over a real-world network.

The results demonstrate the feasibility of integrating electrically driven, tuneable ELEDs into existing fiber networks for quantum communication applications. The high entanglement fidelity achieved in the field deployment validates the practicality of the technology. The large tuneability enables flexible wavelength division multiplexing with classical channels and other quantum light sources. The stable long-term operation highlights the robustness of the system for practical deployment. The successful multiplexing of entangled qubits with classical data demonstrates a key step towards building practical quantum networks.

This work presents the first electrically operated, tuneable ELED emitting in the telecom O-band suitable for multiplexing with classical communication. The demonstrated high single-photon purity, entanglement fidelity, and long-term stability, along with low-voltage operation and CWDM compatibility, pave the way for integrating this technology into real-world quantum network infrastructure. Further research could focus on improving photon generation efficiency for deterministic operation and exploring different QD growth methods for enhanced coherence properties.

The current system's photon rates are relatively low (33 kHz for XX photons after transmission). The high loss in one of the fibers (12.5 dB + 2.5 dB filtering) may have affected the overall fidelity. The photon indistinguishability of the source, while sufficient for this proof-of-principle demonstration, is not optimal for advanced scalable quantum networks. Improvements in photon extraction efficiency and coherence times are needed for higher-level quantum network applications.