Stoichiometric rare-earth crystals for novel integrated quantum memories

Quantum information science is the field of research that studies the information present in a quantum system. A number of new technological applications can be envisaged thanks to exquisitely quantum phenomena. While classical information encoding relies on bits, which can be either 0s and 1s, the quantum bits (or qubits) are associated to the state of quantum objects, e.g. single atoms, single spins, or single photons. Because of the quantum superposition principle, the qubits can then be 0s, 1s, or coherent superposition of both, thus giving access to an exceptionally richer alphabet. Quantum information science also exploits quantum entanglement, i.e. strong correlation between quantum objects, as a resource for fast and secure quantum communication protocols.
In view of realising networks for quantum communication, quantum memories are fundamental devices as they act as interfaces between the photons, used as information carriers, and atoms, exploited for information storage and processing. To be useful in quantum networks, the quantum memories must fulfil specific requirements, as on-demand read-out, high efficiency and fidelity, long storage time, and multimodality. While atomic gases enabled the first remarkable quantum storage experiments, solid-state systems also offer interesting perspectives.
Among these, the rare-earth doped crystals recently emerged as attractive candidates because they are ensembles of optically active ions naturally trapped in inert media, which do not require external trapping fields and ultra-high vacuum chambers. They have already featured performances equalising or overcoming those of trapped atoms or cold atomic ensembles in terms of efficiency and storage times. These crystals exhibit transitions both in the optical and in the radio- and micro-wave range, thus they could serve as photonic or microwave memories, but also as interfaces between optical and microwave frequencies, thus opening the way to hybrid systems employing superconducting devices.
Despite their very promising performances and the milestone experiments realised in the last decade, a unique rare-earth doped crystal that fulfils all the requirements of an ideal photonic quantum memory does not yet exist.
This project exactly tackles this problem and aims at developing a novel platform for telecom-compatible integrated quantum devices, containing solid-state quantum memories with unprecedented functionalities. The central idea is to employ not rare-earth doped crystals but stoichiometric crystals, i.e. where the rare-earth ions fully substitute one element of the crystal matrix, with the two-fold aim of increasing the absorption of light and narrowing the inhomogeneous linewidth of the electronic transitions, thanks to a lower local mechanical stress.
The challenges addressed are:
- the optimisation of the coherence properties of bulk crystals that will enable the implementation of quantum storage protocols, never demonstrated in these kind of materials;
- the exploration of confined environment, i.e. laser written waveguides, for the realisation of integrated quantum memories.
We expect the waveguide fabrication to facilitate the realisation of fibre-coupled devices and the efficient manipulation of the atomic transitions by means of electric fields, and to boost the interaction strength between the light and the rare-earth ions. This might give access to the storage of telecom light exploiting optical transitions that in diluted bulk samples would be too weak. Therefore, the proposed platform might permit the simultaneous demonstration of efficient, long-lived and multiplexed storage devices, which are also compatible with existing telecom fibre network. Such quantum memories would outperform the existing quantum storage devices, and their demonstration would open new avenues for the use of solid-state technologies for real quantum information applications.

The material platforms proposed in this project have the potential to be converted in solid-state quantum storage devices with unprecedented capabilities. The perspective of waveguide fabrication will open the way to compact and scalable quantum memories readily compatible with the current telecom fibre network. The demonstration of such quantum memories would open new avenues for the use of solid-state technologies for real quantum information applications. The proposed research plan will thus be beneficial to different people and entities, e.g., society, industry, and students.
For example, a short-term impact of this project will be on the crystal growth industry. This project will improve the understanding of the mechanisms affecting the optical properties of the crystals characterised, thus our continuous feedback will contribute optimising the growth processes and to maximising the quality of the final products.
In the medium term, one application of this project is the so-called quantum internet, i.e. a large-scale network for the secure transfer of massive amount of data. Its implementation will be beneficial to whoever deals with the transfer of huge quantities of vulnerable information, e.g. governments, businesses and individuals. By helping the UK consolidate its unique world leading position in quantum technology, this project will contribute to the future UK economic and industrial development. The commercialisation of the final research output of this project, e.g. fibre pigtailed waveguides, or accessory tools developed in the lab, as fast and reliable mechanical switches, can have great impact not only on quantum communication but also in the classical telecommunication and photonics industry.
State-of-the-art scientific research as that outlined in this project can drive industrial development in the construction of software and hardware that can boost the progresses of other fields. For example:
- biomedical industry could benefit from more efficient and compact cryogenics systems for the cryo-preservation of biological tissues,
- ultra-low noise photon counters will be fundamental in pre-clinical and clinical bio-medicine for the non-invasive optical monitoring and tomography of deep tissues,
- sophisticated arbitrary waveform generators can be employed in the classical telecommunication industry for efficient and fast manipulation and transfer of data through light pulses.
For the first time, we will complement fs-laser waveguides for light with electrical contacts at the micro-scale and optical fibres. This will require facing several technical challenges and finding affordable and reliable solutions that will find applications in classical photonics and micro-electronics.
One important impact of this work will be the training of the PDRA and the PhD and MSc students involved in it in important scientific fields like quantum technology, solid-state physics, and photonics. At the same time, they will develop additional skills in complementary fields like optical and radio-frequency engineering, cryogenics and vacuum technology, electronics and programming, important not only in academia but for high-tech industry. They will also be exposed to frontier science and inspired by world-leading scientists. Now that the time has come to move from proof-of-principle demonstrations to real world realisation of quantum technologies, it is of paramount importance the training of the next generation of scientists and technologists who will make this new quantum revolution happen. Lastly, the award of this grant would have a crucial impact on my professional development, as it would provide me the necessary support to establish an independent activity and a leading research group in quantum photonics at Heriot-Watt University.