Continuous Variable Quantum Cryptography: Beating the 3 dB Loss Limit

We demonstrate that secure quantum key distribution systems based on continuous variable implementations can operate beyond the apparent 3 dB loss limit that is implied by the beam splitting attack. The loss limit was established for standard minimum uncertainty states such as coherent states. We show that, by an appropriate postselection mechanism, we can enter a region where Eve’s knowledge on Alice’s key falls behind the information shared between Alice and Bob, even in the presence of substantial losses.

Figure 1

Comparison of the mutual information between Alice and Bob and the information they share with Eve for different state preparations with effective amplitude $E=\cos (\alpha )$ and measured outcomes $x$ of Bob. Positive areas, colored bright, indicate effective information channels that contribute to secure key exchange since they satisfy $I_{AB}(x,E)>I_{AE}(x,E)$.