Narrow basis angle doubles secret key in the BB84 protocol

TL;DR

This paper proposes a modification to the BB84 quantum key distribution protocol by narrowing the basis angle, which significantly increases the secret key length without compromising channel estimation accuracy.

Contribution

The study introduces a modified BB84 protocol with smaller basis angles, demonstrating doubled secret key generation potential compared to the original protocol.

Findings

Narrower basis angles increase the correlation between transmitted and received bits.

The modified protocol can nearly double the secret key length under certain conditions.

Channel parameter estimation remains consistent with the original protocol.

Abstract

We consider a modified version of the BB84 quantum key distribution protocol in which the angle between two different bases are less than $\pi/4$. We show that the channel parameter estimate becomes the same as the original protocol with sufficiently many transmitted qubits. On the other hand, the statistical correlation between bits transmitted in one basis and those received in the other basis becomes stronger as the angle between two bases becomes narrower. If the angle is very small, the statistical correlation between bits transmitted in one basis and those received in the other basis is as strong as those received in the same basis as transmitting basis, which means that the modified protocol can generate almost twice as long secret key as the original protocol, provided that Alice and Bob choose two different bases with almost the same probability. We also point out that the reverse reconciliation often gives different amount of secret key to the direct reconciliation over Pauli channels with our modified protocol.