A quantum event occurs when an excited source photon is in resonance with and therefore goes on to excite an observer with that same photon. While science approximates such quantum transitions or jumps as instantaneous, that approximation is not really true even though it is often quite useful. In other words, getting a photon from here to there does take time and there are no instantaneous photon transfers.
One very common classical approximation of a quantum event is to have a excited source photon excite a classical observer in a completely separate second event long after the photon travels for a period through space following a first and separate source emission event. This is only an approximation and for a quantum observer, the same photon excites a quantum observer during the same event as the source emission.
A second classical approximation occurs when both source and observer are excited with very long wavelength photons. For the very special case of very long wavelength gravity biphotons, the two complementary gravity excited states remain in phase coherence because gravity phase coherence decays very, very slowly.
For single photons, an excited quantum source and observer are coupled by both phase as well as amplitude as the figure shows. Quantum photon travel is then simply a matter of phase between source and observer and a photon event creates a transient resonant bond between the observer and source. It is not really the photon that journeys through space and time, it is the action of the photon event that exchanges mass between source and observer during the same event just with different phases.
Quantum gravity between the two hydrogen atoms shown involves the complementary exchange of the biphoton excitations that exist in each atom. Unlike the relatively short wavelength of the Rydberg photon at 13.6 eV, the very long wavelengths of complementary gravity biphoton excitations mean that phase decay is very slow. Thus the very slow phase decay of quantum gravity means that classical gravity does not need to include phase for precise predictions of quantum action.
A photon event can be over in a few nanoseconds and nanometers or a photon event can last the age and radius of the universe. Now to be sure, a source can dephase from a photon event long before the photon excites an observer. However, phase decay is simply a part of how the universe points the direction of time and does not change the fact that there is some period of phase coherence between source and observer. For the very slow phase decay of quantum gravity means that until very large scale, classical gravity works very well.
Thus a classical photon excites an observer but does not retain any of the phase coherence of the excited source emission or never loses phase coherence while phase coherence between excited source and observer quantum photons necessarily decays. Indeed a quantum resonance can actually end up with the excitation largely back at the source and not lost to the observer at all. Even such a failed photon transmission has still generated phase coherence between the source and observer and therefore has changed source and observer entropy. In this realm, entropy alone drives quantum information transfer instead of total free energy transfer. Only a very small fraction of the photon free energy is in its entropy.
In a classical approximation for a quantum state-to-state transition, there must be a series of vacuum states that span the gap between two states. In aethertime, the density of states of quantum gravity biphotons in space is very large and more than provides the needed laddering for filling the gap. Similar to phonon decay in the solid state, gravity vacuum modes provide the mechanism to bridge the gap. These high order quantum gravity states are then what carry photon amplitude and phase and replace the vacuum oscillator modes of QED.
In quantum gravity, both source and observer exchange complementary biphoton excitations with each other. So a quantum gravity resonance always involves the exchange of complementary phase coherence between observer and source. This means that quantum gravity phase coherence between a source and observer always decays very, very slowly.