  Magic-angle twisted bilayer graphene (MATBG) can host an intriguing variety
of gate-tunable correlated states, including superconducting and correlated
insulator states. Junction-based superconducting devices, such as Josephson
junctions and SQUIDs, have been introduced recently and enable the exploration
of the charge, spin, and orbital nature of superconductivity and the coherence
of moir\'e electrons in MATBG. However, complementary fundamental coherence
effects - in particular, the Little-Parks effect in a superconducting and the
Aharonov-Bohm effect in a normal conducting ring - remained to be observed.
Here, we report the observation of both these phenomena in a single
gate-defined ring device where we can embed a superconducting or normal
conducting ring in a correlated or band insulator. We directly observe the
Little-Parks effect in the superconducting phase diagram as a function of
density and magnetic field, confirming the effective charge of $2e$. By
measuring the Aharonov-Bohm effect, we find that in our device, the coherence
length of normal conducting moir\'e electrons exceeds a few microns at 50 mK.
Surprisingly, we also identify a regime characterized by $h/e$-periodic
oscillations but with superconductor-like nonlinear transport. Taken together,
these experiments establish a novel device platform in MATBG, and more
generally in tunable 2D materials, to unravel the nature of superconductivity
and other correlated quantum states in these materials.
