Robustness against noise and efficiency of two-way quantum key distribution protocols

• Grant number: 13/21066-0
• Support Opportunities: Scholarships in Brazil - Master
• Start date: March 01, 2014
• End date: August 31, 2014
• Field of knowledge: Physical Sciences and Mathematics - Physics - General Physics
• Principal Investigator: Roberto Menezes Serra
• Grantee: Carlos Iván Henao Osorio
• Host Institution: Centro de Ciências Naturais e Humanas (CCNH). Universidade Federal do ABC (UFABC). Ministério da Educação (Brasil). Santo André , SP, Brazil

Abstract

We present new results concerning security of Two-Way Quantum Key Distribution (TWQKD) protocols that employ non orthogonal states (e.g. LM05). We work within the scenario where the classical post-processing is performed with one-way classical communication and there is no pre-processing. The security analysis is carried out with techniques that allow to cover quite broad sets of protocols. In this way, we give a new security proof for a recently proposed TWQKD protocol and investigate the possibility of out- performing it. This protocol is deterministic, meaning that the receiver can decode the encoded bits with probability one. We show that there is only one TWQKD protocol of this kind (non reported in the literature as far as we know) with the potential to have a larger secret fraction and consequently be more robust against noise. Furthermore, it is argued that the corresponding secret fraction could be the same of the 6-State protocol [9]. On the other hand, we analyze the performance of TWQKD protocols when the noise of the communication channel may depend of the particular polarization states being transmitted. We find that, for certain families of protocols and a given amount of total noise, the potentially leaked information is upper bounded by the associated to a depolarizing channel. Moreover, such protocols include "pre-encoding" operations that could contribute to this enhancement in the asymmetric noise case. We also define a parameter to characterize the Efficiency of QKD protocols, referring to the fraction of qubits that the receiver is able to decode; accordingly, this parameter achieves the maximum value for deterministic protocols. It is discussed how a high efficiency can in practice improve factors like the secret key rate and the quality of the noise (or error) estimation.