  The Einstein Equivalence Principle (EEP) is of crucial importance to test the
foundations of general relativity. When the particles involved in the test
exhibit quantum properties, it is unknown whether this principle still holds. A
violation of the EEP would have drastic consequences for physics. A more
conservative possibility is that the EEP holds in a generalised form for
delocalised quantum particles. Here we formulate such a generalised EEP by
extending one of its paradigmatic tests with clocks to quantum clocks that are
in a quantum superposition of positions and velocities. We show that the
validity of such a generalised version of the EEP is equivalent to the
possibility of transforming to the perspective of an arbitrary Quantum
Reference Frame (QRF), namely a reference frame associated to the quantum state
of the clock. We further show that this generalised EEP can be verified by
measuring the proper time of entangled clocks in a quantum superposition of
positions in the Earth gravitational field. The violation of the generalised
EEP corresponds to the impossibility of defining dynamical evolution in the
frame of each clock, and results in a modification to the probabilities of
measurements calculated in the laboratory frame. Hence, it can be verified
experimentally, for instance in an atom interferometer.
