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Continuous-variable quantum cryptography using two-way quantum communication

<01>Uncertainty principle allows distant parties to share secret correlations by detecting and filtering the interference of a third malicious party. Standard protocols of quantum key distribution (QKD) involve unidirectional transmission of systems. They are secure as long as the noise is below a certain threshold. Here we prove that a double quantum communication, where quantum systems are bounced back and forth between the honest parties, remarkably improves the security thresholds. As a result, two–way QKD is much more robust to noise and can be implemented in noiser and longer communication lines, with long-term impact on practical implementation of QKD.