Scalable Rare Earth Ion Quantum Computing Nodes

Project Information

SQUARE

Grant agreement ID: 820391

DOI

10.3030/820391

Project closed

EC signature date 4 September 2018

Start date 1 October 2018

End date 31 March 2022

Funded under

• EXCELLENT SCIENCE - Future and Emerging Technologies (FET)

Total cost

€ 2 990 277,50

EU contribution

€ 2 990 277,50

Coordinated by

KARLSRUHER INSTITUT FUER TECHNOLOGIE
Germany

Periodic Reporting for period 2 - SQUARE (Scalable Rare Earth Ion Quantum Computing Nodes)

Reporting period: 2020-04-01 to 2022-03-31

Summary of the context and overall objectives of the project

Quantum technologies rely on materials that offer the central resource of quantum coherence, that allow one to control this resource, and that provide suitable interactions to create entanglement. Rare-earth ions (REI) doped into solids have outstanding potential in this context and could serve as a scalable, multi-functional quantum material. REI provide a unique physical system enabling a quantum register with a large number of qubits, strong dipolar interactions between the qubits allowing fast quantum gates, and coupling to optical photons – including telecom wavelengths – opening the door to connect quantum processors in a quantum network. The SQUARE project aimed at establishing individually addressable REI as a fundamental building block of a quantum computer with a photonic interface, and to overcome the basic roadblocks on the way towards scalable quantum hardware. The goal was to realize the fundamental elements of a multifunctional quantum processor node, where multiple qubits can be used for quantum storage, quantum gates, and for coherent spin-photon quantum state mapping. Within SQUARE, we worked to experimentally demonstrate key elements therefore, including efficient readout and coherent qubit control of single REI, generation of telecom single photons, and demonstrations of two-qubit gates. To guide the way towards a scalable technology, detailed simulations and refined theoretical descriptions were needed. Technology development was the second focus of SQUARE to enable scalable REI quantum computing: The proposed scheme requires highly tunable and coherent lasers to address and control multiple REI qubits, quantum-grade materials, tunable microcavities, and ultra-stable cryogenic nanopositioning systems. These elements will be directly useful for a variety of applications, ranging from other quantum technologies such as quantum communication and quantum sensing, to more general applications such as microscopy, cryogenics, and advanced spectroscopy. An overarching goal was finally to combine all the results into a roamap to guide the field. Working towards these results contributes to the development of future quantum technologies, and strengthens the European high-tech industry. At the same time, it will help to expand the leading role of European quantum research and contribute to advance quantum science.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Within SQUARE, we have achieved the following key results:
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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project has several important achievements which go beyond the current state of the art: E.g. sensing of a single nuclear spin via a single Cerium ion for the first time was an important step towards harnessing external nuclear spins as quantum memories for REI, promising for high-density qubit implementation. Dynamic switching of Purcell enhancement of Erbium and Ytterbium ions adds a new method for on-demand photon emission and enables optical addressing of large numbers of qubits separated in frequency space. The discovered molecular REI represent a novel material class with outstanding prospects for quantum computing and networking. The novel formalism for the description of traveling quantum pulses fills a gap for the realistic treatment of elementary processes in quantum networks and can be generalized to a broad range of questions. The various devices developed within SQUARE include so far commercially unavailable technology that satisfies the demands across several fields.
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.

Schematic of a tunable fiber cavity coupling to rare earth ions in a thin film