Photon Addition Boosts Secret Key Rates in Quantum Key Distribution

Quantum key distribution promises secure communication, but its practical implementation requires overcoming the limitations of current technology, and researchers continually seek ways to improve its performance and range. Hao Jeng, Ping Koy Lam, and Syed M. Assad, along with colleagues from the Australian National University and A*STAR, now demonstrate a significant advance by utilising uniquely tailored states of light for quantum key distribution. Their work reveals that adding single photons to specifically prepared ‘squeezed’ light enhances the rate at which secure keys can be generated and extends the distance over which they can be reliably distributed. Importantly, the team developed new analytical methods to assess these non-standard states, bypassing the limitations of conventional approaches and demonstrating that this technique actually protects the system against signal degradation, paving the way for more robust and long-range quantum communication networks.

Centre for Quantum Technologies, Agency for Science, Technology and Research (A*STAR), Fusionopolis Way, Innovis #08-03, Singapore 138634, Republic of Singapore. Adding single photons to entangled light states enhances quantum entanglement, and when implemented in quantum key distribution, promises to increase the rate at which secure keys can be generated. Extracting secure keys from these complex, non-Gaussian entangled states remains a challenge, but this research demonstrates a technique for photon addition and shows how it improves both key rates and the maximum distance over which secure communication is possible.

Non-Gaussian States Enhance Quantum Key Distribution

This research investigates the use of non-Gaussian quantum states, specifically two-mode squeezed vacuum states with added photons, for quantum key distribution. Traditional quantum key distribution often relies on Gaussian states, but non-Gaussian states can offer advantages in security and key rates, particularly against certain types of attacks. The team overcame limitations inherent in Gaussian key distribution, which is vulnerable to attacks that exploit the distribution of photons. The researchers not only developed a theoretical framework but also experimentally generated and characterised these non-Gaussian states, validating their performance in a quantum key distribution protocol.

They successfully generated two-mode squeezed vacuum states and added photons, using a technique called homodyne tomography to reconstruct the quantum states and confirm their properties. The addition of photons demonstrably increases entanglement and negativity, indicating a stronger quantum correlation and potentially enhanced security against eavesdropping attempts. Importantly, the addition of photons initially increases the key rate, demonstrating a practical advantage in communication efficiency. While this increase plateaus after a certain number of photons are added, the research shows that non-Gaussian states can maintain a higher key rate even when signal loss occurs.

Analysis of the bit error rate, a crucial metric for secure communication, confirms the feasibility of this approach. This research suggests that non-Gaussian states offer a path towards more secure quantum key distribution systems. Practical implementation requires balancing the benefits of non-Gaussian states with the challenges of generating and characterising them. Future research could focus on optimising the generation of these states, exploring different types of non-Gaussian states, and developing more robust quantum key distribution protocols. Advanced data processing and error correction techniques could further improve the performance of these systems. In summary, this research provides compelling evidence that non-Gaussian states can be a valuable resource for enhancing the security and performance of quantum key distribution systems.

Photon Addition Strengthens Quantum Entanglement Distillation

Researchers have demonstrated a technique for enhancing quantum communication security by adding single photons to entangled light states, effectively distilling quantum entanglement. This photon addition creates stronger correlations between photons than previously achievable, moving beyond the limitations of traditional Gaussian states used in quantum key distribution. The team’s approach not only increases the potential secret key rate but also extends the maximum distance over which secure communication is possible. The research addresses a fundamental challenge in quantum communication: the fragility of entangled states.

Surprisingly, adding photons does not weaken the entanglement, but actively protects the communication protocol against both signal loss and deliberate attempts to intercept information. This resilience is crucial for practical applications where perfect conditions are rarely met. The method transforms the initial entangled state into a more robust form, capable of withstanding greater levels of noise and interference. A particularly innovative aspect of the work is the implementation of photon addition through a post-processing technique. By combining heterodyne detection with data filtering, the researchers achieved the effects of adding photons without needing actual single-photon sources.

This simplifies the experimental setup and reduces the complexity of the technology, making it more accessible for wider implementation. The team demonstrated that by accepting only specific measurement outcomes, they could accurately mimic the addition of photons. The effectiveness of this approach was validated through analysis of archived data from experiments with entangled light states subjected to varying degrees of signal loss. The results show that the post-processing technique accurately reconstructs the enhanced quantum state, paving the way for more secure and reliable quantum communication networks. This method represents a significant step forward in overcoming the limitations of existing quantum key distribution protocols and realizing the full potential of secure quantum communication.

Non-Gaussian States Enhance Secure Key Distribution

This research demonstrates that adding single photons to specific quantum states, known as two-mode-squeezed-vacuum states, enhances the secure distribution of cryptographic keys. Contrary to predictions based on Gaussian principles, these non-Gaussian states do not inherently create vulnerabilities, but offer advantages for quantum key distribution and appear to perform better than Gaussian states in resisting eavesdropping attempts. The team successfully analysed the properties of these states and showed an increase in both key rates and the maximum distance over which secure keys can be distributed. The study employed quantum state tomography to accurately characterise the complex quantum states involved, a necessity given the limitations of simpler analytical methods for non-Gaussian systems. While acknowledging the simplifying assumption of asymptotic rates and a focus on ideal conditions, the researchers highlight the robustness of their findings against typical experimental imperfections. They note that further research could explore alternative measurement schemes, more advanced encoding procedures, and the use of different non-Gaussian entangled states to further optimise performance and investigate potential vulnerabilities to attacks beyond those considered in this work.