Tight bounds on the eavesdropping of a continuous-variable quantum cryptographic protocol with no basis switching

The Gaussian continuous-variable quantum key distribution protocol based on coherent states and hetero-dyne detection [Phys. Rev. Lett. 93, 170504 (2004)] has the advantage that no active random basis switching is needed on the receiver’s side. Its security is, however, not very satisfyingly understood today because the bounds on the secret key rate that have been derived from Heisenberg relations are not attained by any known scheme. Here, we address the problem of the optimal Gaussian individual attack against this protocol, and derive tight upper bounds on the information accessible to an eavesdropper. Interestingly, this protocol is proven to be even more resistant to individual attacks than originally thought. Optical schemes achieving these bounds are also exhibited, which concludes the security analysis of Gaussian protocols against individual attacks.