  A chiral photonic interface is a quantum system that has different
probabilities for emitting photons to the left and right. An on-chip compatible
chiral interface is attractive for both fundamental studies of light-matter
interactions and applications to quantum information processing. We propose
such a chiral interface based on superconducting circuits, which has wide
bandwidth, rich tunability, and high tolerance to fabrication variations. The
proposed interface consists of a core that uses Cooper-pair boxes (CPBs) to
break time-reversal symmetry, and two superconducting transmons that connect
the core to a waveguide in the manner reminiscent of a ``giant atom.'' The
transmons form a state decoupled from the core, akin to dark states of atomic
physics, rendering the whole interface insensitive to the CPB charge noise. The
proposed interface can be extended to realize a broadband fully passive on-chip
circulator for microwave photons.
