  Collective quantum states, such as subradiant and superradiant states, are
useful for controlling optical responses in many-body quantum systems. In this
work, we study novel collective quantum phenomena in waveguide-coupled Bragg
atom arrays with inhomogeneous frequencies. For atoms without free-space
dissipation, collectively induced transparency is produced by destructive
quantum interference between subradiant and superradiant states. In a large
Bragg atom array, multi-frequency photon transparency can be obtained by
considering atoms with different frequencies. Interestingly, we find
collectively induced absorption (CIA) by studying the influence of free-space
dissipation on photon transport. Tunable atomic frequencies nontrivially modify
decay rates of subradiant states. When the decay rate of a subradiant state
equals to the free-space dissipation, photon absorption can reach a limit at a
certain frequency. In other words, photon absorption is enhanced with low
free-space dissipation, distinct from previous photon detection schemes. We
also show multi-frequency CIA by properly adjusting atomic frequencies. Our
work presents a way to manipulate collective quantum states and exotic optical
properties in waveguide QED systems.
