[Paper Review] Continuous variable B92 quantum key distribution protocol using single photon added and subtracted coherent states

S. Srikara, Kishore Thapliyal+1 more|arXiv (Cornell University)|Jun 18, 2019

Quantum Information and Cryptography2 references4 citations

TL;DR

This paper proposes a continuous variable B92 quantum key distribution protocol using single photon-added and subtracted coherent states (PASCS), which achieves intrinsic robustness against unambiguous state discrimination (USD) and beam splitter attacks without requiring uninformative states or entanglement. The scheme ensures a low bit-error rate and high key rate, outperforming discrete-variable B92 and similar CV-QKD protocols in security and practicality for lossy channels.

ABSTRACT

In this paper, a continuous variable B92 quantum key distribution (QKD) protocol is proposed using single photon added and subtracted coherent states, which are prepared by adding and subsequently subtracting a single photon on a coherent state. It is established that in contrast to the traditional discrete variable B92 protocol, this protocol for QKD is intrinsically robust against the unambiguous state discrimination attack, which circumvents the requirement for any uninformative states or entanglement used in corresponding discrete variable case as a remedy for this attack. Further, it is shown that the proposed protocol is intrinsically robust against the eavesdropping strategies exploiting classical communication during basis reconciliation, such as beam splitter attack. Security against some individual attacks, key rate, and bit-error rate estimation for the proposed scheme are also provided. Specifically, the proposed scheme ensures very small bit-error rate due to properties of the states used. Thus, the proposed scheme is shown to be preferable over the corresponding discrete variable B92 protocol as well as some similar continuous variable quantum key distribution schemes.

Motivation & Objective

• To develop a continuous variable B92 QKD protocol that overcomes the vulnerability of discrete-variable B92 to unambiguous state discrimination (USD) attacks.
• To eliminate the need for uninformative states or entanglement in securing the B92 protocol, as required in previous DV-based solutions.
• To enhance security against eavesdropping strategies exploiting classical communication during basis reconciliation, such as beam splitter attacks.
• To achieve high key rate and low bit-error rate using practical continuous-variable states like PASCS.
• To provide a secure, practical alternative to existing CV-QKD schemes by leveraging the intrinsic properties of PASCS.

Proposed method

• The protocol uses single photon-added and subtracted coherent states (PASCS) as the quantum states for encoding information, prepared by adding and then subtracting a single photon from a coherent state.
• Alice prepares and sends PASCS states corresponding to bit values 0 and 1, which are non-orthogonal and exhibit distinct phase-space distributions with regions where one quadrature dominates over the other.
• Bob performs homodyne measurements on the incoming states and selects only conclusive outcomes based on a post-selection threshold ζc, discarding inconclusive results.
• The protocol leverages the fact that any attempt by Eve to perform unambiguous state discrimination (USD) will result in vacuum-like signals, which are detectable due to the absence of expected signal power.
• Security against intercept-and-resend and beam splitter attacks is analyzed by computing the success probability P_corr of Eve’s state discrimination, showing that low P_corr increases detection likelihood.
• The key rate and bit-error rate are estimated using the joint probability distribution of quadrature measurements, with optimization over the coherent amplitude α for a fixed bit-error rate δ.

Experimental results

Research questions

• RQ1 Can a continuous variable B92 protocol be constructed using PASCS that is intrinsically robust against unambiguous state discrimination (USD) attacks?
• RQ2 Does the proposed protocol eliminate the need for uninformative states or entanglement to counter USD attacks, unlike the original discrete-variable B92 protocol?
• RQ3 How does the protocol perform against beam splitter attacks that exploit classical communication during basis reconciliation?
• RQ4 What is the achievable key rate and bit-error rate of the proposed CV B92 scheme using PASCS?
• RQ5 Can the protocol maintain low bit-error rate and high security under individual attacks without requiring additional classical communication for reconciliation?

Key findings

• The proposed CV B92 protocol using PASCS is intrinsically robust against unambiguous state discrimination (USD) attacks because any inconclusive measurement by Eve results in vacuum-like signals, which are detectable by Bob due to the absence of expected signal power.
• The protocol does not require uninformative states or entanglement to counter USD attacks, unlike previous discrete-variable modifications of B92, thus simplifying implementation.
• The protocol is robust against beam splitter attacks that exploit classical communication during basis reconciliation, as Eve cannot reliably infer the correct state without introducing detectable disturbances.
• The bit-error rate is very low due to the distinct phase-space distributions of the PASCS states, which minimize overlap and enhance state distinguishability.
• The key rate remains high and is estimated to be comparable to or better than similar CV-QKD schemes, with security established against individual attacks including intercept-and-resend and simultaneous quadrature measurement.
• The success probability P_corr of Eve’s state discrimination is minimized, increasing the likelihood of detection, which enhances overall protocol security.