Ainhoa Agulleiro

We present a system implementing a three-state BB84 protocol with time-bin encoding, one decoy and a simplified measurement scheme that uses passive basis choice. Our implementation simplifies the state characterization with respect to previous iterations. We also adapt the loss-tolerant method to our protocol, thus dealing with the measured state preparation flaws. We compare the obtained phase error rate and secret key rate when including the state imperfections and when assuming perfect states. Our results highlight the importance of characterization and implementation security.

Bandwidth-limited devices in the transmitter of fast QKD implementations cause pulse correlations that leak information about previous setting choices. To take them into account in the existing security proofs, a measure of their strengths is needed. This is experimentally challenging, especially for long-range correlations, which are not experimentally accessible. In this work, we propose a new characterization method that exploits a linear model of the modulation devices. We show that this model predicts an upper bound for arbitrary order correlations that makes their characterization possible. We also present experimental results using the proposed method. In doing so, we can retrieve security even in the presence of arbitrary long correlations, with similar performance to classical security proofs.

Quantum key distribution (QKD) has raised as an attractive alternative to classical cryptography due to its security being provided by quantum mechanics rather than relying on algorithms that could potentially be broken in the future, rendering current communications insecure. However, many of the security proofs rely on assumptions that may not agree with reality, for instance, device imperfections can open loopholes that could potentially be exploited by a malicious party in order to extract part, if not all, of the secret key.