  Thomas Young's slit experiment lies at the heart of classical interference
and quantum mechanics. Over the last fifty years, it has been shown that
particles (e.g. photons, electrons, large molecules), even individual
particles, generate an interference pattern at a distant screen after passage
through a double slit, thereby demonstrating wave-particle duality. We revisit
this famous experiment by replacing both slits with single-mode fibre inputs to
two independent quantum memories that are capable of storing the incident
electromagnetic field's amplitude and phase as a function of time. At a later
time, the action is reversed: the quantum memories are read out in synchrony
and the single-mode fibre outputs are allowed to interact consistent with the
original observation. In contrast to any classical memory device, the write and
read processes of a quantum memory are non-destructive and hence, preserve the
photonic quantum states. In principle, with sufficiently long storage times and
sufficiently high photonic storage capacity, quantum memories operating at
widely separated telescopes can be brought together to achieve optical
interferometry over arbitrarily long baselines.
