  In physics, it is crucial to identify operational measurement procedures to
give physical meaning to abstract quantities. There has been significant effort
to define time operationally using quantum systems, but the same has not been
achieved for space. Developing an operational procedure to obtain information
about the location of a quantum system is particularly important for a theory
combining general relativity and quantum theory, which cannot rest on the
classical notion of spacetime. Here, we take a first step towards this goal,
and introduce a model to describe an extended material quantum system working
as a position measurement device. Such a "quantum ruler" is composed of $N$
harmonically interacting dipoles and serves as a (quantum) reference system for
the position of another quantum system. We show that we can define a quantum
measurement procedure corresponding to the "superposition of positions", and
that by performing this measurement we can distinguish when the quantum system
is in a coherent or incoherent superposition in the position basis. The model
is fully relational, because the only meaningful variables are the relative
positions between the ruler and the system, and the measurement is expressed in
terms of an interaction between the measurement device and the measured system.
