Semi-counterfactual Cryptography

TL;DR

This paper introduces a semi-counterfactual quantum key distribution protocol that encodes bits through particle blocking, leveraging single-particle non-locality for security, and proves its security against general attacks.

Contribution

It presents a novel semi-counterfactual QKD scheme with orthogonal encoding states, differing from existing protocols, and provides a comprehensive security proof.

Findings

Protocol is secure against photon-number-preserving incoherent attacks

Secret bits are maximally indeterminate until joint action of parties

Security relies on single-particle non-locality

Abstract

In counterfactual quantum key distribution (QKD), two remote parties can securely share random polarization-encoded bits through the blocking rather than the transmission of particles. We propose a semi-counterfactual QKD, i.e., one where the secret bit is shared, and also encoded, based on the blocking or non-blocking of a particle. The scheme is thus semi-counterfactual and not based on polarization encoding. As with other counterfactual schemes and the Goldenberg-Vaidman protocol, but unlike BB84, the encoding states are orthogonal and security arises ultimately from single-particle non-locality. Unlike any of them, however, the secret bit generated is maximally indeterminate until the joint action of Alice and Bob. We prove the general security of the protocol, and study the most general photon-number-preserving incoherent attack in detail.