  The coherence of quantum systems is crucial to quantum information
processing. While it has been demonstrated that superconducting qubits can
process quantum information at microelectronics rates, it remains a challenge
to preserve the coherence and therefore the quantum character of the
information in these systems. An alternative is to share the tasks between
different quantum platforms, e.g. cold atoms storing the quantum information
processed by superconducting circuits. In our experiment, we characterize the
coherence of superposition states of 87Rb atoms magnetically trapped on a
superconducting atom-chip. We load atoms into a persistent-current trap
engineered in the vicinity of an off-resonance coplanar resonator, and observe
that the coherence of hyperfine ground states is preserved for several seconds.
We show that large ensembles of a million of thermal atoms below 350 nK
temperature and pure Bose-Einstein condensates with 3.5 x 10^5 atoms can be
prepared and manipulated at the superconducting interface. This opens the path
towards the rich dynamics of strong collective coupling regimes.
