Researchers have taken a significant step towards building secure, intercity quantum communication networks with the first demonstration of quantum key distribution (QKD) using deterministic single-photon sources (SPSs) based on semiconductor quantum dots (QDs).
The Challenge of Secure Communication
Quantum key distribution (QKD) offers a revolutionary approach to secure communication. It leverages the principles of quantum mechanics to guarantee secure key exchange, making it impossible for eavesdroppers to intercept or decode the transmitted information. While QKD has shown promise in shorter distances, scaling it up to intercity communication has been a significant challenge.
Semiconductor Quantum Dots: The Key to Intercity QKD
Semiconductor QDs are nanoscale materials that exhibit exceptional properties for quantum communication. They can emit single photons on demand with high efficiency and purity, making them ideal for QKD.
However, traditional QDs lacked the brightness and wavelength necessary for long-distance transmission. A recent breakthrough achieved by researchers involved developing QDs that emit bright single photons directly within the telecommunication C-band. This crucial development has paved the way for intercity QKD.
The Experiment: Linking Braunschweig and Hannover
In this groundbreaking study, researchers successfully performed a QKD experiment over a 79 km link between the German cities of Braunschweig and Hannover. The experiment employed a QD-based SPS that emitted high-rate single photons in the telecommunication C-band.
The setup involved a transmitter, receiver, fiber spools, and a superconducting nanowire detector (SNSPD). The transmitter used a pulsed laser to excite the QD, which emitted single photons. These photons were then encoded with polarization information and sent through a fiber optic cable to the receiver. The receiver detected the encoded photons and used them to generate a shared secret key.
Breaking Records: Key Rate and Tolerable Loss
The intercity QKD experiment achieved a record-high secret key bit rate of 4.8 Ã 10<sup>-5</sup> per pulse, with an exceptionally low quantum bit error ratio of 0.65%. The secret key transmission was maintained for 35 hours, demonstrating the reliability of the system.
Moreover, the experiment showed an asymptotic maximum tolerable loss of 28.11 dB, equivalent to a 144 km channel length using standard fiber optic networks. This signifies the high potential of semiconductor SPSs for future quantum communication networks.
A Promising Future for Quantum Communication
This study marks a significant milestone in the development of large-scale quantum communication networks. The high key rate, low error ratio, and long achievable distance demonstrated in the experiment highlight the potential of semiconductor SPSs for secure intercity communication.
Furthermore, the ability to seamlessly integrate these SPSs into existing fiber optic networks makes them a practical solution for future quantum communication infrastructure.
Looking Ahead: Quantum Repeaters and Beyond
The research also paves the way for the development of quantum repeaters. Quantum repeaters are essential for enabling long-distance quantum communication by overcoming the limitations of signal attenuation in optical fibers. The deterministic nature of semiconductor QDs makes them ideal candidates for developing quantum repeaters.
The successful implementation of intercity QKD with semiconductor SPSs marks a critical step towards a future where secure and ultra-fast communication is possible across vast distances, revolutionizing how we share and protect information.
