In an article recently published in the journal Light: Science & Applications, researchers demonstrated the first intercity quantum key distribution (QKD) experiments using deterministic telecom single photon sources (SPSs).
Study: Semiconductor Quantum Dots in Intercity Quantum Key Distribution. Image Credit: Summit Art Creations/Shutterstock.com
Importance of Semiconductor SPSs
QKD has gained significant attention worldwide due to its ability to provide security based on quantum mechanics principles, surpassing classical cryptography's capabilities. Specifically, it securely enables information transmission by preventing general attacks by eavesdroppers.
In QKD protocols, the utilization of on-demand quantum light sources can improve maximum tolerable loss and security. Semiconductor SPSs can revolutionize large-scale quantum communication. Semiconductor quantum dots (QDs) can serve as a foundation for quantum communication applications due to the deterministic single-photon emission with low multiphoton contribution and high brightness.
Specifically, these QDs can emit indistinguishable single photons on demand with exceptional purity and efficiency, thereby providing significant advantages for QKD. A scheme involving QDs can enhance the key rate significantly for measurement-device-independent QKD, which requires high Hong-Ou-Mandel interference visibility between two single-photon sources.
Quantum repeaters can also be realized using QDs as they can emit photonic cluster states and allow inherent quantum information storage. Quantum communication studies using QDs have displayed their ability to connect metropolitan areas and university campuses.
However, the progress to intercity distances has been hindered due to the absence of bright single-photon signals within the telecommunication bands. Recently, a breakthrough has been achieved by researchers that enabled bright single-photon emission with high emission rates, due to Purcell enhancement, directly from a QD device in the telecommunication C-band.
The Study
In this work, researchers performed the first intercity QKD experiment using a bright, deterministic single-photon source. A semiconductor QD embedded in a circular Bragg grating structure efficiently emitted high-rate single photons in the telecommunication C-band and was used along with polarization encoding based on the standard BB84 protocol.
The experimental setups included the superconducting nanowire detector (SNSPD), receiver, fiber spools, and the transmitter. The clock-variable pulsed fiber laser was coupled into free space in the transmitter to excite the QD after passing through a beam splitter.
Additionally, the Attodry1100 system was equipped with a Thorlabs aspheric lens with a 0.7 numerical aperture to collect the single photons from the device. Thorlabs polarizer, quarter-waveplate, and zero-order half-waveplate were employed to encode and purify the polarization states. These components were controlled by the Thorlabs electronic stages.
The encoded single-photon qubits were coupled into Corning SMF-28® Ultra fiber spools, with each of them spanning 40 km.
A fiber-based Bragg grating was utilized by the receiver to split the single-photon signals and reference laser. The encoded single photons were detected by the SNSPDs after passing through two plate polarizing beam splitters and a 50:50 beam splitter.
Eventually, the time tagger registered the photon arrival times. The average photon number per pulse of the circularly polarised light’s linear component from the device was determined using the SNSPD.
Significance of this Work
Researchers successfully showed the first intercity QKD experiments using a deterministic telecom SPS by harnessing single-photon emissions from a semiconductor QD emitting within the telecom C-band and having excitation rates up to the GHz range.
Using the 79 km long link with 25.49 ± 0.02 dB loss (equivalent to 130.32 km for the direct-connected optical fiber/standard telecom fiber) between the German cities of Braunschweig and Hannover, a record-high secret key bits per pulse of 4.8 × 10-5 with a ~ 0.65% average quantum bit error ratio were demonstrated.
The low quantum bit error ratio of ~ 0.65% and high-rate secret key transmission were obtained for 35 h. More than 2 × 10-5 average secret key bits per pulse could be attained in the finite-key regime over 30 min of acquisition time.
Additionally, an asymptotic maximum tolerable loss of 28.11 dB was observed, corresponding to a 144 km channel length in repeaterless quantum communication with standard fiber-optic networks, indicating the competitiveness of semiconductor SPSs for quantum communication applications.
A comparative analysis with the current QKD systems involving SPS showed that the secret key rate realized in this study surpassed all existing SPS-based implementations.
It approached the levels achieved by established decoy-state weak coherent pulse-based QKD protocols even without further optimization of the setup performance and the source, which indicated the viability of integrating semiconductor SPSs seamlessly into large-scale, high-capacity, and realistic quantum communication networks.
To summarize, the findings of this work demonstrated that deterministic semiconductor sources possess the potential to excel in quantum repeater applications and measurement-device-independent protocols.
Journal Reference
Yang, J. et al. (2024). High-rate intercity quantum key distribution with a semiconductor single-photon source. Light: Science & Applications, 13(1), 1-10. DOI: 10.1038/s41377-024-01488-0, https://www.nature.com/articles/s41377-024-01488-0
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