Quantum weak coin flipping with a single photon
Mathieu Bozzio, Ulysse Chabaud, Iordanis Kerenidis, Eleni Diamanti
Abstract
Weak coin flipping is among the fundamental cryptographic primitives which ensure the security of modern communication networks. It allows two mistrustful parties to remotely agree on a random bit when they favor opposite outcomes. Unlike other two-party computations, one can achieve information-theoretic security using quantum mechanics only: both parties are prevented from biasing the flip with probability higher than $1/2+\ensuremath{\epsilon}$, where $\ensuremath{\epsilon}$ is arbitrarily low. Classically, the dishonest party can always cheat with probability 1 unless computational assumptions are used. Despite its importance, no physical implementation has been proposed for quantum weak coin flipping. Here, we present a practical protocol that requires a single photon and linear optics only. We show that it is fair and balanced even when threshold single-photon detectors are used, and reaches a bias as low as $\ensuremath{\epsilon}=1/\sqrt{2}\ensuremath{-}1/2\ensuremath{\approx}0.207$. We further show that the protocol may display a quantum advantage over a few-hundred meters with state-of-the-art technology.