Scalable Continuous Variable Cluster State Quantum Technologies

Project Information

CLUSTEC

Grant agreement ID: 101080173

DOI

10.3030/101080173

EC signature date 14 September 2022

Start date 1 November 2022

End date 31 October 2026

Funded under

• Digital, Industry and Space

Total cost

€ 2 994 543,75

EU contribution

€ 2 994 543,75

Coordinated by

DANMARKS TEKNISKE UNIVERSITET
Denmark

Periodic Reporting for period 2 - CLUSTEC (Scalable Continuous Variable Cluster State Quantum Technologies)

Reporting period: 2024-05-01 to 2025-10-31

Summary of the context and overall objectives of the project

CLUSTEC is a pioneering EU Horizon Europe initiative aimed at unlocking the potential of optical cluster states for scalable quantum technologies. Despite rapid advancements in quantum computing, scalability remains a key challenge. CLUSTEC addresses this by leveraging continuous variable (CV) cluster state technology to build a universal, fault-tolerant, and network-compatible quantum computer. The project seeks to answer fundamental questions related to scalability, computational universality, quantum error-correction, and quantum advantage certification.

The initiative brings together the expertise of eight international partners from Germany, France, Switzerland, the Czech Republic, and Denmark, uniting leading researchers and industry partners in quantum technology, photonic integration, mathematics, computer science, and quantum information theory. CLUSTEC aims to demonstrate critical steps towards CV quantum computing and quantum networking. To achieve this, CLUSTEC has set four primary objectives:

(i) Developing experimental platforms for CV cluster state generation based on fiber optics and the lithium niobate on insulator platform.
(ii) Exploring CV cluster states of different network topologies to understand the role of entanglement structures in practical quantum technologies.
(iii) Developing and executing quantum protocols and near-term quantum algorithms with certified quantum advantage using CV cluster states.
(iv) Exploring new quantum error correction schemes towards fault-tolerant CV cluster state quantum computing and networking.

The project's outcomes are expected to significantly advance quantum computing by overcoming current limitations and providing a clear pathway towards scalable, fault-tolerant quantum technologies. By addressing these critical challenges, CLUSTEC aims to pave the way for the next generation of quantum computing and networking, with substantial impacts on the quantum technology landscape and society at large.

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

The project made significant progress toward fulfilling the four primary project objectives, resulting in 57 articles (31 published). Notably, during RP2, the project delivered major new results across all objectives (O):

Experimentally (O1), the fiber-based platform was upgraded to enable stable generation of large-scale temporal cluster states, and a key milestone was achieved with the first two-mode squeezing demonstrated on LNOI. In parallel, chip–fiber coupling was significantly improved, with losses reduced to below 1 dB per facet, enabling scalable integrated implementations.

In topology and entanglement (O2), a new four-dimensional cluster-state architecture, the Octo-Rail Lattice, was introduced. Significant advances were also made in entanglement certification, nonlinear squeezing theory, multimode entanglement classification, and self-testing via CV Bell inequalities, strengthening validation of large-scale quantum resources.

For algorithms and quantum advantage (O3), the project achieved the first verified continuous-variable quantum advantage and developed a new Gaussian Boson Sampling algorithm with exponential speed-up. These advances enable concrete progress toward real-world use cases in finance, molecular modelling, and machine learning.

In fault tolerance (O4), new surface-code-inspired continuous-variable constructions compatible with GKP encodings were developed, alongside ongoing threshold analyses and noise-suppression strategies, laying the groundwork for scalable and fault-tolerant photonic quantum computing.

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 progressed beyond state of the art by enhancing the technical capabilities of the experimental setup and advancing numerous theoretical aspects. These key results have been disseminated through 57 articles (31 published). Several highlights are summarized under "work performed and main achievements".