Rigidity for Monogamy-of-Entanglement Games

TL;DR

This paper establishes a rigidity property for a specific monogamy-of-entanglement game, enabling new cryptographic protocols like weak string erasure, bit commitment, and secure randomness expansion without shared entanglement.

Contribution

It proves a rigidity property for a MoE game, showing optimal strategies are of a specific form, and applies this to develop cryptographic schemes with security based on this rigidity.

Findings

Rigidity property for the MoE game established.

Construction of a weak string erasure scheme based on the game.

Achieved secure randomness expansion without shared entanglement.

Abstract

In a monogamy-of-entanglement (MoE) game, two players who do not communicate try to simultaneously guess a referee's measurement outcome on a shared quantum state they prepared. We study the prototypical example of a game where the referee measures in either the computational or Hadamard basis and informs the players of her choice. We show that this game satisfies a rigidity property similar to what is known for some nonlocal games. That is, in order to win optimally, the players' strategy must be of a specific form, namely a convex combination of four unentangled optimal strategies generated by the Breidbart state. We extend this to show that strategies that win near-optimally must also be near an optimal state of this form. We also show rigidity for multiple copies of the game played in parallel. We give three applications: (1) We construct for the first time a weak string erasure (WSE) scheme where the security does not rely on limitations on the parties' hardware. Instead, we add a prover, which enables security via the rigidity of this MoE game. (2) We show that the WSE scheme can be used to achieve bit commitment in a model where it is impossible classically. (3) We achieve everlasting-secure randomness expansion in the model of trusted but leaky measurement and untrusted preparation and measurements by two isolated devices, while relying only on the temporary assumption of pseudorandom functions. This achieves randomness expansion without the need for shared entanglement.