sometimes, the box formula_44 shared by alice and bob is such that it only admits a unique quantum realization. this means that there exist measurement operators formula_190 and a quantum state formula_191 giving rise to formula_44 such that any other physical realization formula_193 of formula_44 is connected to formula_195 via local unitary transformations. this phenomenon, that can be interpreted as an instance of device-independent quantum tomography, was first pointed out by tsirelson and named self-testing by mayers and yao. self-testing is known to be robust against systematic noise, i.e., if the experimentally measured statistics are close enough to formula_44, one can still determine the underlying state and measurement operators up to error bars.