High-quality, distributed quantum entanglement is the distinctive resource for quantum communication and forms the foundation for the unequalled level of security that can be assured in quantum key distribution. While the entanglement provider does not need to be trusted, the secure key rate drops to zero if the entanglement used is too noisy. In this paper, we show experimentally that QPA is able to increase the secure key rate achievable with QKD by improving the quality of distributed entanglement, thus increasing the quantum advantage in QKD. Beyond that, we show that QPA enables key generation at noise levels that previously prevented key generation. These remarkable results were only made possible by the efficient implementation exploiting hyperentanglement in the polarisation and energy-time degrees of freedom. We provide a detailed characterisation of the gain in secure key rate achieved in our proof-of-principle experiment at different noise levels. The results are paramount for the implementation of a global quantum network linking quantum processors and ensuring future-proof data security.
High-quality, distributed quantum entanglement is the distinctive resource for quantum communication and forms the foundation for the unequalled level of security that can be assured in quantum key distribution. While the entanglement provider does not need to be trusted, the secure key rate drops to zero if the entanglement used is too noisy. In this paper, we show experimentally that QPA is able to increase the secure key rate achievable with QKD by improving the quality of distributed entanglement, thus increasing the quantum advantage in QKD. Beyond that, we show that QPA enables key generation at noise levels that previously prevented key generation. These remarkable results were only made possible by the efficient implementation exploiting hyperentanglement in the polarisation and energy-time degrees of freedom. We provide a detailed characterisation of the gain in secure key rate achieved in our proof-of-principle experiment at different noise levels. The results are paramount for the implementation of a global quantum network linking quantum processors and ensuring future-proof data security.
High-quality, distributed entanglement forms the foundation for the unequaled level of security that can be assured in quantum key distribution (QKD), but its susceptibility to noise hinders practical implementations. This study experimentally demonstrates that enhancing quantum resources with quantum privacy amplification (QPA) increases the noise resilience of QKD beyond classical limits. Leveraging hyperentanglement in different field-tested degrees of freedom of a photon pair increases the efficiency of QPA, thereby unlocking its advantage for QKD. Here is a method to generate secure keys under noisy conditions, which was previously impossible, paving the way for robust QKD.
Integrating quantum communication into existing fiber infrastructures requires coexisting with high-power classical traffic, which generates noise in the quantum channel. Spontaneous Raman Scattering (SpRS) is the dominant noise source, generating a broadband photon background that degrades quantum states and, in Quantum Key Distribution (QKD), can entirely suppress secure key generation. In this work, we implement single-copy entanglement distillation utilizing energy-time and frequency hyperentanglement, and we exploit it to perform Quantum Privacy Amplification (QPA) directly at a physical layer. Strict frequency anti-correlations of spontaneous parametric down-conversion are used to selectively discard uncorrelated Raman photons while preserving genuine entangled pairs, suppressing accidental coincidences by a factor 1/N, where N is the number of frequency channel pairs. Using passive borosilicate-glass photonic integrated circuits and standard DWDM demultiplexers over a 20~km single-mode fiber testbed, we demonstrate a 4~dB improvement in tolerance to classical laser launch power PC when QPA is performed, extending the secure key threshold from PC=−13.3 dBm to PC=−9.3 dBm. The result shows that off-the-shelf telecom components can be repurposed as physical-layer quantum privacy amplifiers.
Entanglement-based quantum communication offers an increased level of security in practical secret shared key distribution. One of the fundamental principles enabling this security-the fact that interfering with one photon will destroy entanglement and thus be detectable-is also the greatest obstacle. Random encounters of traveling photons, losses, and technical imperfections make noise an inevitable part of any quantum communication scheme, severely limiting distance, key rate, and environmental conditions in which quantum key distribution can be employed. Using photons entangled in their spatial degree of freedom, we show that the increased noise resistance of high-dimensional entanglement can indeed be harnessed for practical key distribution schemes. We perform quantum key distribution in eight entangled paths at various levels of environmental noise and show key rates that, even after error correction and privacy amplification, still exceed 1 bit per photon pair and furthermore certify a secure key at noise levels that would prohibit comparable qubit based schemes from working.
High-dimensional entanglement not only offers a high security level for quantum communication but also promises improved information capacity and noise resistance of the system. However, due to various constraints on different high-dimensional degrees of freedom, whether these advantages can bring improvement to the actual implementation is still not well proven. Here we present a scheme to fully utilize these advantages over long-distance noisy fiber channels. We exploit polarization and time-bin hyperentanglement to achieve high-dimensional coding, and observe significant enhancements in secure key rates and noise tolerance that surpass the capabilities of qubit systems. Moreover, the demonstration achieves a distribution up to 50 km, which is the longest distance for high-dimensional entanglement-based quantum key distribution up to date, to our knowledge. Our demonstration validates the potential of high-dimensional entanglement for quantum communications over long-distance noisy channels, paving the way for a resilient and resource-efficient quantum network.
Abstract The security of quantum key distribution (QKD) usually relies on that the users’s devices are well characterized according to the security models made in the security proofs. In contrast,device-independent QKD an entanglement-based protocol permits the security even without any knowledge of the underlying devices. Despite its beauty in theory, device-independent QKD is elusive to realize with current technology. This is because a faithful realization requires ahigh-quality violation of Bell inequality without the fair-sampling assumption. Particularly, in a photonic realization, a rather high detection efficiency is needed where the threshold values depend on the security proofs; this efficiency is far beyond the current reach. Here, both theoretical and experimental innovations yield the realization of device-independent QKD based on a photonic setup. On the theory side, to relax the threshold efficiency for practical deviceindependent QKD, we exploit the random post-selection combined with adding noise for preprocessing, and compute the entropy with complete nonlocal correlations. On the experiment side, we develop a high-quality polarization-entangled photonic source and achieve state-of-theart (heralded) detection efficiency of 87.49%, which outperforms previous experiments and satisfies the threshold efficiency for the first time. Together, we demonstrate device-independent QKD at a secret key rate of 466 bits/s over 20 m standard fiber in the asymptotic limit against collective attacks. Besides, we show the feasibility of generating secret keys at a fiber length of 220 meters. Importantly, our photonic implementation can generate entangled photons at a high rate and in the telecom wavelength, which is desirable for high-speed key generation over long distances. The results not only prove the feasibility of device-independent QKD with realistic devices, but also push the security of communication to an unprecedented level.
High-quality long-distance entanglement is essential for both quantum communication and scalable quantum networks. Entanglement purification is to distill high-quality entanglement from low-quality entanglement in a noisy environment and it plays a key role in quantum repeaters. The previous significant entanglement purification experiments require two pairs of low-quality entangled states and were demonstrated in tabletop. Here we propose and report a high-efficiency and long-distance entanglement purification using only one pair of hyperentangled state. We also demonstrate its practical application in entanglement-based quantum key distribution (QKD). One pair of polarization spatial-mode hyperentanglement was distributed over 11 km multicore fiber (noisy channel). After purification, the fidelity of polarization entanglement arises from 0.771 to 0.887 and the effective key rate in entanglement-based QKD increases from 0 to 0.332. The values of Clauser-Horne-Shimony-Holt inequality of polarization entanglement arises from 1.829 to 2.128. Moreover, by using one pair of hyperentanglement and deterministic controlled-NOT gates, the total purification efficiency can be estimated as 6.6×10^{3} times than the experiment using two pairs of entangled states with spontaneous parametric down-conversion sources. Our results offer the potential to be implemented as part of a full quantum repeater and large-scale quantum network.