Periodic Reporting for period 2 - SQUARE (Scalable Rare Earth Ion Quantum Computing Nodes)
Berichtszeitraum: 2020-04-01 bis 2022-03-31
Scientific advancements
1) Demonstration of basic qubit functionality of a single REI by the detection and quantum control of a single Cerium spin. We have further used it to sense a single nuclear spin as a possible quantum memory.
2) Realization of three different tunable microcavity types for emission enhancement and efficient addressing of REI: fiber-based microcavities, LiNbO3 disc and photonic crystal cavities.
3) Demonstration of dynamic tuning of strong Purcell enhancement of Erbium ions.
4) Cavity-enhanced spectroscopy of single Erbium ions and generation of telecom-band single photons.
5) High-speed tunable emission enhancement of single Yb ions in a tunable disc cavity.
6) Preparation of epitaxial REI thin films with long optical coherence times.
7) Demonstration of ultra-narrow optical linewidths in molecular REI complexes. We could show efficient optical spin intialization, a coherent photon memory, and the presence of ion-ion interactions useful for quantum gate operations.
8) Development of a theoretical framework to realistically describe traveling quantum pulses, which is key to understand the features and limitations of quantum networks.
9) Realistic modeling to assess the limits of single-, two- and multi-qubit gate fidelities, and the detailed modelling of a nano-scale computing node with up to 100 qubits, where each one is connected to up to 50 other qubits.
10) Composition of a roadmap that summarizes all the relevant parts of a REI quantum processing node.
Technology development
11) A cryogenic nanopositioning platform and a vibration isolation platform for operating open-access microcavities were realized at the prototype level, with first commercial distribution.
12) Fiber cavities were advanced towards TRL8 as an enabling tool for optical quantum technologies, and commercialization via a start-up company was started.
13) An ultra-stable, cryogenic scanning cavity platform was developed and is prepared for commercialization via a start-up company.
14) Epitaxial REI-doped thin films with encouraging optical coherence properties were achieved.
15) A scheme for a scalable laser source that can coherently address up to 100 qubits was devised in full detail, and a prototype was built and used in an experiment on addressing single Erbium ions.
16) A concept for an all-European closed-cycle cryocooler tailored for the operation of REI-based quantum nodes and other optically addressable quantum materials was worked out.
Our results were published in 34 publications, including Nature, Physical Review Letters, and others. SQUARE Researchers presented the work at more than 150 events, including 13 events targeting the general public and policy makers. We organized 6 events, including a 3-day summer school, a 4-day international workshop with 145 participants, and an industry workshop uniting 7 companies. We disseminated our work also via social media like twitter, youtube, and a website.
We have taken important steps to evidence the potential of REI for scalable quantum computing nodes and combined our results into a technologically relevant roadmap to direct future efforts. Our work has a direct connection to companies in all stages of development, and thus contributes to strengthen the near future high-tech industry sector in the EU.
On a long-term scale, REI technology is promising e.g. for distributed quantum computing across small computing nodes of ~100 qubits, and 3rd generation quantum repeaters, which require error correction capability and thus 10 – 100 qubits with quantum logic at each node connected to telecom photons. It is also a promising approach to connect and interface superconducting quantum computers. Realization of REI quantum computing with a large qubit number would enable technologically relevant computations with disruptive impact, and quantum repeaters could enable secure communication and quantum networks with unconditional security.