The first part of the work carried out during NanOQTech deals with material development and characterization. We synthesized yttrium oxide (Y2O3) particles between 400 and 60 nm by chemical methods and measured, at liquid helium temperatures (1.5-5 K), the lifetime of optical quantum states. The longest lifetimes in europium doped samples (Eu3+:Y2O3) reached about 6 µs in 100 nm particles and 3 µs in 60 nm particles. We also measured spin quantum state lifetimes in the ms range. These values of quantum state lifetimes and size are sufficient for coupling particles to micro-cavities and perform a number of initial experiments on single ion quantum storage, quantum gates and single photon emission. Similar results were obtained in praseodymium and erbium Y2O3 nano-particles. Thin films were obtained by atomic layer deposition, a technique that allows a high control of the films' thickness. Although the films were not suitable for experiments based on quantum states, fluorescence lifetimes could be optimized in erbium doped ultra-thin films (10 nm thickness) to reach values similar to bulk crystals.
Second, we investigated fiber-based optical microcavities for coupling to different rare earth ions and reach the strong Purcell regime, i.e. a large increase in fluorescence intensity. We have realized cavities at different wavelengths that can be operated at low temperatures with high stability. They were used to perform spectroscopy of single nanocrystals and observe Purcell factors up to 150. This allowed detection of about 10 ions for europium and erbium. Fast tuning of the cavity in the kHz range was also demonstrated, highlighting a key advantage of the fiber-based design.
Suitable RE candidates were also selected and experimentally studied to act as ions performing quantum processing (qubits) and ions reading out quantum states. These processing schemes were theoretically analyzed and a powerful modelling toolbox produced.
Software tools have been developed to allow for real-time control of the cavity length, as well as a hardware programmatic interface. The latter is especially suited to large scale quantum processing experiments.
The third activity in NanOQTech focused on hybrid systems. On one hand, erbium doped ultra-thin films were used in RE-graphene devices that showed very good coupling. This enabled efficient, electrically controlled, launching of plasmons with Purcell factor >10. Fast (>103 times erbium radiative decay rate) temporal control of ion-plasmon interactions for ion-ion coupling has also been achieved. Theory has focused on the interaction between ensembles of RE and plasmons.
On the other hand, fabrication of mechanical resonators of various designs for investigating RE coupling through strain has been achieved in Y2SiO5 crystals by using a focused ion beam. The tunability via mechanical strain of ultra-narrow spectral holes, and quantitative measurement of uniaxial strain sensitivities have also been demonstrated. Protocols for efficient resonator cooling have been developed, and their application as force sensors studied.
The project activities and results have been disseminated thanks to a website (www.nanoqtech.eu) and logo, flyers, newsletters, participation in conferences, general public events and social media. The project led so far to 23 publications in high-quality journals. The latter are deposited on an open access repository (www.zenodo.org 'NanOQTech H2020 Project' community) together with the corresponding data. Besides several consortium meetings, we organized a mini summer school to train the students and post-doc researchers hired on the project. A report on quantum technologies, available on the project's website has also been produced.
Exploitation resulted in two patent extensions and one in preparation. A company that will sell fiber micro-cavities is also being launched at the time of the project’s end.
