Quantum technologies are developed to overcome classical limits in communication and processing but also in new areas like sensing, imaging and simulations. They will impact all aspects of life by allowing e.g. ultra-secure communications, simulation of complex drug molecules, or new bio-medical imaging techniques.
Many quantum systems are investigated for specific tasks. The next major challenge is to overcome the limits of single systems by associating radically different quantum systems in hybrid architectures, each selected for its specific properties. Interconnection of these systems will also be necessary to further develop functionalities like distributed processing or extremely secure data exchange, in a global 'quantum internet'.
Our vision is that RE nanostructures will play a pivotal role in this scheme by offering a solid-state platform that can be coupled to other quantum systems, while incorporating a coherent spin-photon interface. This essential bridge functionality is expected to have a broad impact, especially since RE ions are extremely versatile systems. They span a wide range of optical transition frequencies from the UV to the infrared, including the 1.5 µm telecom wavelength, and they have electron and nuclear spin transitions. This offers possibilities of coupling between matter and electro-magnetic waves that are unmatched by other quantum systems.
Novelty and ambition
For more than a decade, developments on solid state quantum systems that can be optically addressed have almost exclusively focused on two centers: defects (mainly nitrogen vacancy-NV) in diamonds and semi-conductor quantum dots. Although these systems are currently showing high performance in quantum sensing of fields or broadband single-photon emission, they suffer from a high sensitivity to environmental noise. Especially in nanostructures, this causes strong electron-phonon coupling and spectral diffusion in NV centers, and strong spin dephasing in quantum dots. Moreover, interactions between different centers are difficult to implement because of large center-to-center variations in properties (e.g. in emission efficiency and optical transition frequency) and fabrication issues.
Recently, rare earth ions have been recognized as a promising alternative. Indeed, they behave like isolated atoms but are naturally 'trapped' inside a solid. In bulk single crystals and at liquid helium temperatures, RE ions can reach extremely long coherence lifetimes both on optical and spin transitions. Moreover, these centers are very stable, show consistent properties from center to center and have high emission efficiencies, especially at the 1.5 µm telecom wavelength. These favorable properties have led for example to striking demonstrations of quantum state storage. In nanostructures, single ion detection was also recently reported.
Our ambition is to demonstrate long coherence lifetimes for RE in nanostructures, efficient ion readout by the Purcell effect, strong interactions between RE and graphene plasmons, and spatial homogeneity for efficient coupling with mechanical modes. This will result in new functionalities and solid-state devices with unprecedented performances. This requires a large and coordinated effort which will not only address RE material development but also encompass device fabrication, opto-electronic interfacing and control, theoretical modeling and guidance, and, ultimately, operation of the nano-systems.