  Metamaterials are artificially engineered periodic structures with
exceptional optical properties that are not found in conventional materials.
However, this definition of metamaterials can be extended if we introduce a
quantum degree of freedom by adding some quantum elements (e.g quantum dots,
cold atoms, Josephson junctions, molecules). Quantum metamaterials can then be
defined as artificially engineered nanostructures made up of quantum elements.
Furthermore, they exhibit controllable quantum states, maintain quantum
coherence for times much higher than the transversal time of the
electromagnetic signal. Metamaterials have been used to realised invisibility
cloaking, super-resolution, energy harvesting, and sensing. Most of these
applications are performed in the classical regime. Of recent, metamaterials
have gradually found their way into the quantum regime, particularly to quantum
sensing and quantum information processing. The use of quantum metamaterials
for quantum information processing is still new and rapidly growing. In quantum
information processing, quantum metamaterials have enabled the control and
manipulation of quantum states, single photon generation, creating quantum
entanglement, quantum states switching, quantum search algorithm, quantum state
engineering tasks, and many more. In this work, we briefly review the theory,
fabrication and applications of quantum metamaterials to quantum information
processing.
