Stability of isometries of quantum states

We prove dimension-independent Hyers-Ulam stability for surjective approximate isometries of the positive trace-class cone in both the trace and Bures metrics. Here $\epsilon \ge 0$ is a uniform upper bound on the additive error in preserving distances. Such an approximate isometry is uniformly approximated by a Wigner symmetry (a unitary or antiunitary conjugation), with error at most $9\epsilon \mathrm{/}2$ in the trace metric and $2\sqrt{2}\epsilon$ in the Bures metric, even if it does not fix zero. For maps fixing zero, the trace bound improves to $3\epsilon$ even under the hypothesis of $\delta$-surjectivity, independently of $\delta$. In infinite dimensions, we construct bijections of the full state space, including mixed states, whose distortions tend to zero in both metrics but which stay a fixed uniform distance from every Wigner symmetry. For state spaces, stability holds in each fixed finite dimension with a dimension-dependent modulus tending to zero as $\epsilon \to 0$, but no such modulus can be chosen independently of dimension.