  In recent experiments, electromagnetically induced transparency (EIT) were
observed with giant atoms, but nothing unconventional were found from the
transmission spectra. In this letter, we show that unconventional EIT does
exist in giant atoms, and indicate why it has not been observed so far.
Different from these existing works, this letter presents a consistent theory
including a real space method and a time delayed master equation for observing
unconventional EIT. We discover that this phenomenon is a quantum effect which
cannot be correctly described in a semi-classical way as those in recent works.
Our theory shows that it can be observed when the time delay between two
neighboring coupling points is comparable to the relaxation time of the atom,
which is crucial for a future experimental observation. This new phenomenon
results from inherent non-locality of the giant atom, which physically forces
propagating fields to be standing waves in space and the atom exhibiting
retardations in time. Our theory establishes a framework for application of
nonlocal systems to quantum information processing.
