  A game is rigid if a near-optimal score guarantees, under the sole assumption
of the validity of quantum mechanics, that the players are using an
approximately unique quantum strategy. Rigidity has a vital role in quantum
cryptography as it permits a strictly classical user to trust behavior in the
quantum realm. This property can be traced back as far as 1998 (Mayers and Yao)
and has been proved for multiple classes of games. In this paper we prove
ridigity for the magic pentagram game, a simple binary constraint satisfaction
game involving two players, five clauses and ten variables. We show that all
near-optimal strategies for the pentagram game are approximately equivalent to
a unique strategy involving real Pauli measurements on three
maximally-entangled qubit pairs.
