Quantum Key Distribution (QKD) is widely considered to be the most advanced form of quantum cryptography, providing a path to virtually unbreakable security for the future quantum internet. One of the promising technologies behind these secure systems is semiconductor quantum dots (SQDs), small solid-state light sources that can generate high-quality single photons for quantum communications. These devices could increase the speed of secure key generation while also supporting future quantum repeaters needed for large-scale quantum networks.
Another important development is time-bin encoding, a technique that stores information in the time of arrival of a photon. This method is particularly attractive for long-distance quantum communications because it is naturally resistant to many environmental perturbations that can disrupt fiber-optic networks.
Quantum encryption stable over 120 kilometers
An international research team from universities in Germany and China has demonstrated the first true time-bin QKD system using semiconductor quantum dot devices for on-demand communications. Their results were published as a magazine cover art. Light: Science and Applications.
In the experiment, the scientists used a self-stabilizing time-bin encoder to generate three distinct time-bin qubit states, both deterministically and randomly. This setup converts polarized single photons produced by C-band quantum dots into encoded quantum signals for communication. On the receiving side, the photonic qubits were decoded using an actively stabilized interferometer with a phase shifter, allowing the system to operate for long periods of time without manual adjustments.
Researchers have successfully transmitted quantum signals over more than 120 kilometers of fiber optic links between encoders and decoders. The system also maintained excellent stability during more than 6 hours of continuous operation.
High secure key rate with quantum dots
In proof-of-concept experiments, we achieved the highest secure key rate reported to date for a time-bin QKD system based on high-performance quantum dot devices. The quantum dot source produced bright, highly pure single photons at an operating speed of approximately 76 MHz.
Even after 120 kilometers of transmission over standard optical fiber, the system achieved an average qubit error rate of less than 11%. Under real-world finite-key conditions, the setup maintained an average secure key rate of approximately 15 bits/second. This is a level that is considered suitable for real-world encrypted text messaging applications.
The researchers emphasized the importance of this progress:
“Communication-band quantum dots with parcel enhancement can provide high-brightness photons suitable for intercity fiber communications, making them promising candidates for integration into practical quantum dot systems.”
Time bin encoding improves real-world stability
The research team also highlighted the advantages of time-bin encoding compared to many existing quantum dot-based QKD systems, which are highly susceptible to environmental damage.
“Most existing QD-based QKD systems are vulnerable to changes in the actual quantum channel caused by environmental factors such as turbulence, temperature, and vibrations, which require active compensation. In contrast, time-bin encoding, where qubits are encoded with the temporal position of a single photon, provides inherent stability against such channel fluctuations without complex compensation protocols.”
According to the scientists, the system’s long uninterrupted execution time demonstrates the robustness of the approach.
“The system operates continuously for six hours, highlighting the inherent robustness of the time-bin scheme achieved by the system, which includes the Sagnac Interferometer (SNI), active feedback control, and more.”
The researchers say this work represents an important step toward practical, scalable quantum communications systems that can ultimately support secure quantum networks in real-world environments.
“These results highlight the feasibility of integrating quantum dot single-photon sources into stable and field-deployable time-bin quantum dot systems and represent an important step toward scalable and quantum-secure communication networks based on solid-state single-photon emitters.”
