  Quantum bits (qubits) are at the heart of quantum information processing
schemes. Currently, solid-state qubits, and in particular the superconducting
ones, seem to satisfy the requirements for being the building blocks of viable
quantum computers, since they exhibit relatively long coherence times,
extremely low dissipation, and scalability. The possibility of achieving
quantum coherence in macroscopic circuits comprising Josephson junctions,
envisioned by Legett in the 1980's, was demonstrated for the first time in a
charge qubit; since then, the exploitation of macroscopic quantum effects in
low-capacitance Josephson junction circuits allowed for the realization of
several kinds of superconducting qubits. Furthermore, coupling between qubits
has been successfully achieved that was followed by the construction of
multiple-qubit logic gates and the implementation of several algorithms. Here
it is demonstrated that induced qubit lattice coherence as well as two
remarkable quantum coherent optical phenomena, i.e., self-induced transparency
and Dicke-type superradiance, may occur during light-pulse propagation in
quantum metamaterials comprising superconducting charge qubits. The generated
qubit lattice pulse forms a compound "quantum breather" that propagates in
synchrony with the electromagnetic pulse. The experimental confirmation of such
effects in superconducting quantum metamaterials may open a new pathway to
potentially powerful quantum computing.
