  According to a well-known principle of quantum physics, the statistics of the
outcomes of any quantum experiment are governed by a Positive Operator-Valued
Measure (POVM). In particular, for experiments designed to measure a specific
physical quantity, like the time of a particle's first arrival at a surface,
this principle establishes that if the probability distribution of that
quantity does not arise from a POVM, no such experiment exists. Such is the
case with the arrival time distributions proposed by Das and D\"urr
[arXiv:1802.07141], due to the nature of their spin dependence.
