Quantum key distribution stands as the most mature branch of quantum cryptography, offering truly unbreakable security for the forthcoming quantum internet. Solid-state quantum light sources, like semiconductor quantum dots (SQDs), have attracted intense interest because they can emit high-quality non-classical photons for quantum communications, promising with higher quantum key rate and enabling quantum repeaters. Meanwhile, the encryption of information on the time degree of freedom of photonic qubits, time-bin encoding, shows strong potential for long-haul quantum communications in practical scenarios. Time-bin qubits are naturally robust against the environmental instabilities that affect deployed fibre links.
In a new journal cover art published in Light: Science & Applications , an international team of researchers from several German and Chinese universities, report the first genuine time-bin QKD demonstration driven by an on-demand telecom semiconductor QD device. In this work, three distinct time-bin qubit states are prepared deterministically and randomly by a self-stabilised time-bin encoder that converts polarized single photons emitted from a telecom C-band QD (Figure 2). At the receiver, the encoded photonic qubits are decoded using an actively stabilized interferometer equipped with a phase shifter, enabling long-term operation without manual tuning. The system achieves a transmission distance >120 km over an optical fibre link between encoder and decoder, while maintaining an exceptional stability for more than 6 hours of continuous operation.
The proof-of-concept experiment results in the highest secure key rate among the time-bin QKDs based on a high-performance QD device . The device delivers bright, high-purity single photons at an operation rate around 76 MHz. The system maintains average quantum bit error rates below 11% at 120 km of standard optical fibre (Figure 3). In a practical finite key regime, an average secure key rate of ~15 bits/s remain stable that is feasible for real-world text message encryption applications. These scientists remark the importance of their work:
“Telecom-band QDs with Purcell enhancement can provide high-brightness photons suitable for intercity fiber communication, making them promising candidates for integration into practical QKD systems.”
“Most existing QD-based QKD systems are vulnerable to changes in the practical quantum channel caused by environmental factors, such as turbulence, temperature and vibrations. This necessitates active compensation. In contrast, time-bin encoding, where qubits are encoded in the temporal position of single photons, offers intrinsic stability against such channel fluctuations even without any complex compensation protocols”
“The system is operated continuously for 6 hours, highlighting the intrinsic robustness of the time-bin scheme enabled by the system including the Sagnac interferometer (SNI), active feedback control, etc”
“This result underscores the feasibility of integrating QD single-photon sources into stable and field-deployable time-bin QKD systems, marking an important step toward scalable, quantum-secure communication networks based on solid-state single-photon emitters.”
Light Science & Applications
Time-bin encoded quantum key distribution over 120 km with a telecom quantum dot source