  We show that the weak equivalence principle (WEP) is violated for a quantum
particle in a gravitational wave (GW) background, in the sense that extra mass
information can be extracted in the presence of the GW. We quantify the degree
of violation with the Fisher information of mass. This provides a precise
characterisation of WEP violation by quantum systems in a GW, that should be
useful in formalising other works that have argued for such violations
heuristically.
